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Electrolytic receivers in wireless telegraphy
JOURNAL OF T H E
FRANKLININSTITUTE OF THE STATE OF PENNSYLVANIA,
FOR T H E PROMOTION OF THE MECHANIC ARTS.
VOL. C L V I I I , No. 4.
79TH Y E A R .
OCTOBER, I9O4
THE Franklin I n s t i t u t e is not responsible for the state. ments and opinions advanced by contributors to thefaurnal.
ELECTRICAL
SECTION.
Stated Meeting held Thursday, March 3], ]904.
Electrolytic Receivers in Wireless Telegraphy. B Y DR. L E E DE FOREST.
W i t h the form and nature of the ordinary filings-tube coherer as applied to-day in wireless telegraphy the public has become fairly familiar. Branley, the distinguished French physicist, discovered in i89i that the effect of electrical oscillations upon a body of metallic filings, or dust, was to produce a sudden increase in the conductivity of the mass, a conductivity which persisted until the particles were broken apart again by mechanical jar. A l t h o u g h the American, Varley, the English scientist Hughes, and especially Onesti, the Italian, had previously noted this phenomenon, none of these investigators had fully appreciated the causes involved, or given to the world of science the benefit of their researches in t h o r o u g h published reports. VOL. CLVIII.
No. 946.
I6
::42
.de F o r e s t ."
[J. F. I.,
T h e discovery of the " c o h e r e r " therefore is r i g h t f u l l y a t t r i b u t e d to M. B r a n l e y ; b u t to Professor Popoff, of St. P e t e r s b u r g , and to Sir Oliver Lodge, of England, m u s t be given the credit of a p p l y i n g the relay and mechanical t a p p e r to the filings tube, the addition of a n t e n n a wings, of u p r i g h t wire and earth connection, and the first steps towards refinem e n t and an appreciation of t h e possibilities of the new detector as applied to an uiaborn b r a n c h of h u m a n enterprise and progress. W i t h the n o t a b l e work of Marconi in still f u r t h e r adapting the Popoff-Lodge a r r a n g e m e n t to commercial requirements, the gradual increase of distance on which aerographic c o m m u n i c a t i o n w a s established, the ingenious improvem e n t s in coherers b y such m e n a b r o a d as Tissot, Ducretet, Castelli, then b y L o d g e and Branley themselves, and in this c o u n t r y b y Shoemaker, t h e audience is more or less familiar. E n o r m o u s is t h e b u l k of research m a t t e r p u b l i s h e d r e g a r d i n g the coherer. T h e o r i e s of the action involved have b e e n n u m e r o u s , b u t divided chiefly into two schools : First, that of Lodge, w h o d e m o n s t r a t e d an actual w e l d i n g t o g e t h e r of metallic filings, after the s a m e h a d b e e n d r a w n into contact b y the electro-static attraction of the opposite charges induced upon the faces of the filings ; and, second, that of Branley, w h o ascribed t h e action to a b r e a k i n g d o w n or p u n c t u r i n g b y m i n u t e sparks of the dielectric film (either gaseous or a metallic oxide), w h i c h he held m u s t exist between opposing surfaces of the filings. T h e result of the years of i n v e s t i g a t i o n w h i c h have followed the early work of t h e s e two pioneers proves that in a m e a s u r e b o t h were right. T h e Branley effect undoubtedly exists, and affords a satisfactory explanation of the self-decohering p h e n o m e n a o b s e r v e d in the microphone type of auto-coherers. T h e s e i n s u l a t i n g oxide or gaseous films inevitably form and cling to the surface of any metallic particles not enelosed in a b s o l u t e v a c u u m , and w h e r e strictest care is not t a k e n to prevent. T h e y are, moreover, elastic, self-healing; and unless the electric impulses are of excessive violence, so t h a t a d j a c e n t particles are actually welded firmly together, the original high resistance is a u t o m a t i c a l l y
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Electrolytic Receivers bz Wireless Telegrap]O/.
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restored upon cessation of the electric impulse. This is notably the case when the elements of th~ radio-conductor are of carbon, hardened steel, carbon-mercury, aluminum, etc., where the formation of this resisting film is easily accomplished, or where amid the minute roughnesses of the carbon surface the adhesion of a gaseous film is made more easy. If, however, the coherer consists of filings of some soft metal of relative low fusing point--silver, gold, l e a d - - t h e Lodge cohering effect through actual welding follows upon the Branley action; making the final fall of resistance far more marked, and necessitating a vigorous mechanical jar to break up the cohered filings and restore sensitiveness. It is on account of this enormous change in conductivity of the filings-tube coherer that this form has been the standard used almost exclusively in the coherer systems of wireless telegraphy. For the filings-tube frequently shows a change from hundreds of thousands of ohms down to a few ohms under the influence of exceedingly weak electrical impulses. This readily allows the use of a relay, operating a" Morse inker and the tapping-back device ; and in the old country, where the coherer systems have been chiefly developed, reading by sight has ever been deemed a necessity, especially in the navies, where, until recently, wireless telegraphy has enjoyed its most cordial reception. The result of all this has been necessarily a system employing delicate relays, tapping-back adjuncts, complications of apparatus, requiring frequent and careful adjustments, a skill and delicacy on the part of the receiving operator seldom found outside of the physics laboratory, least of all upon a man-of-war, where the hands of Jack Tar, of the wigwag and gundeek school, are all thumbs and of Cyclopic dimensions. Despite many extravagant claims for high speeds of word transmission by coherer systems, I have yet to see proof of anything exceeding twelve to fifteen words the minute in ~enuine commercial working, while the actual every-day apeed attained seldom exceeds half that amount.
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de ]~'orest .'
[J. F. I.,
For commercial applications of wireless telegraphy this pathetic speed limitation of itself renders the filings-tube systems to-day an impossibility; regartlless of any other considerations, such as uncertainty of their action, ill-adapt. ability to electrical tuning, liability to interference (notably from atmospheric electricity), harmful effects from mechau. ical vibrations, etc., etc. It has been stated by its early champions, and more recently by those prominently interested in w i r e telegraphs, that wireless telegraphy is destined to fill a place where existing methods of wire communication are impossible, such as from ship to ship and ship to shore. And their point was wel! taken; for, with the best tape-recording coherer systems developed to-day, after eight years of elaborate effort, no sane man would think of employing wireless telegraphy, with its six words and as m a n y guesses per minute, its treacherous a d j u s t m e n t s ' a n d its moody dispositions, where a wire system could possibly be used. W i t h the auto-coherer, or so-called microphonic contact, the case is better. Here, since no tapping-back is required, a relay is superfluous, although, to speak frankly, the changes of resistance in the auto-coherer are generally so slight and the n o r m a l resistance such a shifting quantity t h a t no relay capable of operating reliably with the autocoherer has been devised. A telephone, the most sensitive indicating instrument known, is invariably used to-day in place of the relay and inker; and, inasmuch as the autocoherer is usually a quantitative instrument, possessed of no critical voltage, its combination with the telephone affords an extremely sensitive wave detector, the range of which is limited by the sensitiveness of the h u m a n ear. With this arrangement one obtains a speed of word reception limited only by the ability of the operators t o send dots and dashes with the transmitter spark, and to~ translate these into words. Consequently, in America one finds the alternating current of relatively high frequene# and excellent regularity of action employed, with an ordi~ nary Morse key obtaining a speed of twenty-five to thirty, five words per minute, instead of the antiquated inductiott'
Oct., ,9o4.]
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coil with its spluttering sticking h a m m e r interrupter, or its messy, explosive mercury-brake a n d the pump-handle Zeichengeber, which is considered abroad as a fitting accompaniment to the coherer receiver, and is so well designed to hold down the impatient sender to a sloth sufficient for that form of receiver. As we frequently see in the daily press, the spark.coil coherer systems are excellently adapted to the trying requirements of a "chess game by wireless telegraphy !" The a u t o - o r microphonic coherer is, however, poorly adapted to the nice requirements of close syntony or electrical tuning. This is on account of its variable normal resistance and its normal capacity. Moreover, this device is liable to "close u p " under the effects of severe static discharges, requiring r e a d j u s t m e n t by tapping, and rendering difficult its employment during time of severe atmospheric disturbances and lightning storms. Furthermore, its inability to reliably operate by a relay, a bell for calling purposes often necessitates the addition of a filings-tube for that purpose. I feel justified in saying while on this subject that the type of auto,coherer invented by Mr. H a r r y Shoemaker, until recently of this city, together with the special antenna which he employs therewith, is the most practical and satisfactory form which has come to m y attention. Having briefly reviewed the field of cohering indicators of electric waves, I wish now to call attention to an entirely distinct line of investigation leading up to forms of receivers which I believe may well be set in a class by themselves as regards sensitiveness, simplicity and general adaptability to most of the demands which can be made to.day in the field of wireless telegraphy. I refer to the electrolytic principle, one which has until recently attracted little investigation from experimenters generally, but which, notwithstanding, has to its credit performances which are rapidly Compelling study and research in various sources here and abroad. In i898 the German, Neugschwender, performed the fol.owing experiment : The silver plating of a strip of mirror
246
de F o r e s t ."
[J. F. I.,
glass was divided into two parts b y a sharp razor cut, leaving a narrow gap b e t w e e n t w o silver edges, c o m p l e t e l y i n s u l a t i n g the t w o sections. Each of the sections was now connected to the terminals of a dry battery, and a telephone and g a l v a n o m e t e r inserted in series in the circuit. lXTo current was o b s e r v e d to pass until a film of m o i s t u r e was deposited upon the slit, either b y b l o w i n g the b r e a t h thereon or b y placing a s a t u r a t e d sponge n e a r b y or b y placing a drop of w a t e r directly upon the slit. A f t e r a brief period the g a l v a n o m e t e r b e g a n to show violent and irregu. lar deflections. In the telephone a scratching, b u b b l i n g s o u n d was heard, followed shortly b y c o m p a r a t i v e q u i e t n e s s and a stable position of the g a l v a n o m e t e r needle, indicating t h a t the resistance of the g a p in the silver mirror h a d been broken d o w n until it m e a s u r e d b u t a few ohms. If, now, electric w a v e s were g e n e r a t e d in the neighborhood, as from t h e spark of an induction coil, the galvanometer s h o w e d an increase of resistance in the circuit, while the listener in the t e l e p h o n e heard a purring sound, r e p r o d u c i n g w i t h singular fidelity t h e sound of the induction-coil spark. U p o n cessation of the electrical impulses the conductivity of the gap in the mirror i n s t a n t l y reasserted itself; the sound in the telephone ceased. A year after N e n g s c h w e n d e r discovered this s t r a n g e action of the H e r t z i a n waves, A s c h k i n a s s i n d e p e n d e n t l y noted the same phenomena, the publication of which in W i e d e m a n n ' s A n n a l e n led the former to f u r t h e r pursue his researches, and to examine the action at the moist edges of t h e silver electrode u n d e r a p o w e r f u l microscope. H i s observations thereon and m y own i n v e s t i g a t i o n s in 1899 coincide in all essentials, and clearly d e m o n s t r a t e that the p h e n o m e n o n is one of electrolysis, due to the combined action of the Hertzinn and local currents. T h e y d e m o n s t r a t e , however, s e v eral features n e w to the generally accepted idea of electrOlytic action; and open up lines of speculation and research of u n u s u a l interest to the physicist, and rife with suggestion. T h e p h e n o m e n o n n o t e d a b o v e exist to a more prominent degree with tinfoil electrodes in place of silver, and these I shall now a t t e m p t to describe.
Oct.. 19o4.]
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W i t h the telephone to the ear and the eye at the microscope the action thus d o u b l y observed affords in fact one of the most fascinating, most b e a u t i f u l " p a s t i m e s (as I m a y well term it) ever g r a n t e d to the i n v e s t i g a t o r in these fields. W h e n the local E.M.F. is first applied to the gap, m i n u t e s t metallic particles, all b u t invisible even with a i,ooo-power lens, are seen torn off from the anode, u n d e r the stress of the electric forces, a p p a r e n t l y mechanical in action; and these dust-like particles, floating in the fluid, m o v e across to the cathode, s o m e rapidly, some slowly, b y s t r a n g e and g r o t e s q u e p a t h w a y s , or directly to their goal. T i n y ferry-boats, each laden with its little electric charge, and u n l o a d i n g its invisible cargo at the opposite electrode, retrace their j o u r n e y i n g s , or, c a u g h t b y a cohesive force, build up little bridges, trees, with branches of q u a i n t and crystalline patterns. D u r i n g this f o r m a t i v e period (lasting p e r h a p s for half a minute) the ear hears an irregular boiling sound, and the average deflection of the g a l v a n o m e t e r indicates a g r a d u a l decrease of resistance, until one or more of these tin trees or tentacles has been b u i l t c o m p l e t e l y across t h e gap. T h e n silence ensues until the current across this b r i d g e is s u d d e n l y increased as b y the H e r t z i a n oscillations from an electric spark m a d e in the neighborhood, or even from a source of so low f r e q u e n c y as the ordinary 6o-cycle alternating current. I n s t a n t l y all is c o m m o t i o n and c h a n g e a m o n g the tentacles, and especially w h e r e these join the cathode. T i n y b u b b l e s of h y d r o g e n gas appear, and enlarging s u d d e n l y break or b u r s t apart the bridges, while the click in the telephone indicates the r u p t u r e of the curreflt's path. Vet t h e y are persevering, these little p o n t o o n ferrymen, and instantly re-form, locking hands and h a s t e n i n g from their s u d d e n r o u t back to build n e w paths and chains. So the process continues, the local current re-establishing, the electric oscillations b r e a k i n g up, its h i g h w a y s of passage, with furious b u b b l i n g s and a g i t a t i o n s - - a v e r i t a b l e t e m p e s t in a microscopic tea-pot.
248
de F o r e s t ."
[J. F. I,
T h e h y d r o g e n gas, having, of course, twice the v o l u m e of the oxygen, is m o s t in evidence, and therefore the r u p t u r e of the tentacle occurs chiefly at its cathodic terminal. A n d w h e r e s e g r e g a t e d b r a n c h e s of the tin trees are broken off, the b u b b l e s of gas are generally noticed at the cathodes. Moreover, the o x y g e n to a large e x t e n t enters into chemical c o m b i n a t i o n w i t h the tin, and after the slit has b e e n used for some time a greyish deposit of s t a n n o u s oxide m a y be scraped from the anode. A peculiar f e a t u r e of this p h e n o m e n o n is the rapid disappearance of the gas b u b b l e s . T h e h y d r o g e n , and w h a t oxygen does not c o m b i n e with the tin, s e e m s to be a b s o r b e d b y the liquid ; or the two, b e i n g nascent, m a y r e c o m b i n e to form water. One fact m u s t be b o r n e in mind, t h a t the fine tentacles (whose diameter, b y the way, is of t h e order of some hund r e d - t h o u s a n d t h s of an inch) do not come into actual metallic c o n t a c t with t h e anode terminal. A film of electrolyte of almost m o l e c u l a r thickness m u s t exist b e t w e e n the two, c o n d u c t i n g n o r m a l l y b y electrolytic ionization and conduction, y e t easily d e c o m p o s e d and t r a n s f o r m e d b y a sudden increase of c u r r e n t into an i n s u l a t i n g g a s e o u s film, the expansion of w h i c h still f u r t h e r increases the resistance of the gap. T h e character of the electrolytic action w h e n soft metals, such as tin, silver, lead, are used as anode, and w h e n t h e distance to the cathode is decreased to the order of T~0-inch or less, is quite surprising. F o r example, the electro-motive forces n e e d e d for electro-deposition from anode to c a t h o d e are e x t r e m e l y small, n o w h e r e a p p r o a c h i n g the critical E.M.F. of polarization of the electrolyte. Moreover, distilled water, so-called chemically-pure glycerine, oils, etc., contain e n o u g h of i m p u r i t i e s (such as acid traces) to enable t h e m to act here like our ordinary electrolytic solution. I h a v e in ten m i n u t e s plated a firm deposit of tin on a gold cathode, u s i n g distilled w a t e r only, and a potential difference not e x c e e d i n g ~ of a volt. F9r the s a m e reasons it is possible to d e c o m p o s e the w a t e r films interposed b e t w e e n b r a n c h e s of these tin trees
Oct., 19o4.]
Electrolytic Receivers zn Wireless Telegraphy.
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by a v e r y m i n u t e difference of potential there-between. T h e result is t h a t this electrolytic reslvonder does not neceasarily possess a "critical potential," as is c o m m o n to a coherer ; and it does not, therefore, cease to respond to electric impulses the potential of w h i c h is less t h a n the E.M.F. of polarization of the electrolyte, as c o m m o n l y understood. This receiver is in fact a current.operated device, as d i s t i n g u i s h e d from potentially-operated forms such as the coher~r, as I h a v e proved in a v a r i e t y of ways. R e t u r n i n g to the silver-mirror slit e x p e r i m e n t of Neugschwender, m y investigations soon showed me t h a t the device was a l t o g e t h e r unreliable as a practical w a v e - r e s p o n d i n g detector. In the slit form above described t h e t e a r i n g off of metal from the anode and the b u i l d i n g up of c o n d u c t i n g bridges and tentacles across the gap was found to continue u n t i l t h e whole gap b e c a m e clogged with an a g g l o m e r a t i o n of crystalline formations, affording so m a n y short-circuiting paths in parallel t h a t v e r y soon the auto-cohering could be obtained only in response to powerful sparks. Again, an a c c u m u l a t i o n of h y d r o g e n gas m a y obtain, completely p r e v e n t i n g the r e f o r m i n g of a w a t e r film between tentacles and electrodes or b e t w e e n sections of the tentacles. T h e r e results t h e r e u p o n the r e g u l a r coherer a r r a n g e m e n t , and at the first electric impulse p e r m a n e n t welding of the chain ensues, so that the device becomes inoperative until cleaned out or readjusted. S y s t e m a t i c and p a i n s t a k i n g e x p e r i m e n t s e x t e n d i n g over two years were required before the electrolytic " r e s p o n d e r , " as I have t e r m e d it, a s s u m e d a commercial form. As described in U. S. P a t e n t 716,oo% a viscous supporting m e d i u m , such as glycerine, h e a v y oil, or one mixed with an absorbent such as lycopodium powder, m a y be inserted between the electrodes to p r e v e n t the a g g l o m e r a t i o n of conducting paths, to confine the action to a few only and to prevent the p e r m a n e n t formation of gas a m o n g the tentacles. In addition a depolarizer was added to chemically dispose of the h y d r o g e n bubbles. This last factor has proved of greatest value, r e n d e r i n g the responder of e x t r e m e sensitiveness and reliability.
250
de F o r e s t :
kJ F . t .
Other electrolytes, such as dilute acids, alkalies, alcohols, a m m o n i a , etc., were early tried, b u t I found generally t h a t such lead to a too violent d i s i n t e g r a t i o n of the m e t a l s ill the responder. It has f r e q u e n t l y been asserted t h a t the Marconi coherer when connected directly b e t w e e n the foot of an u p r i g h t aerial conductor, of a b o u t 6o m e t e r s height, and the earth, failed to respond to impulses from a typical I2-inch induction-coil t r a n s m i t t e r set at distances exceeding t~renty miles. For longer distances it was necessary to insert a " j i g g e r " or transformer, by m e a n s of which the potential difference across t h e t e r m i n a l s of the coherer could be stepped up to t h a t required by the so-called " c r i t i c a l potent i a l " of the coherer. A carbon m i c r o p h o n e auto-coherer is a current-operated device. It can, therefore, be i n s e r t e d w i t h o u t a step.up t r a n s f o r m e r directly at the foot of the aerial wire, and shows no such failure to respond for the distances above mentioned. If h i g h - f r e q u e n c y electric oscillations from a d i s t a n t source be impressed upon a vertical conductor g r o u n d e d at its lower end, curr~nt oscillations will be i n d u c e d in the surface of the conductor, t r a v e r s i n g it from top to b o t t o m . A t the upper e x t r e m i t y the electro-static c o m p o n e n t s of the oscillations will be reflected w i t h o u t c h a n g e of sign, and at the base reflected w i t h c h a n g e of sign. T h e exact converse holds for the e l e c t r o . m a g n e t i c or c u r r e n t c o m p o n e n t s of the oscillation. T h e result is a loop of electro-static force at the upper end of the wire, and a c u r r e n t loop at its base. A current-operated device, therefore, will r6spond best w h e n inserted in the base of the aerial at a point where the coherer is at its g r e a t e s t d i s a d v a n t a g e . T h e e x t r e m e distances f r o m which messages have been received on the electrolytic responder of the types j u s t described, as compared to records with the coherer u n d e r similar conditions, afford a practical proof t h a t it is a current-operated" device, in a d d i t i o n to the consideration of t h e electrolytic principles involved in its action. As an i n t e r e s t i n g i n s t a n c e of the work done by the re-
Oct., i9o4.]
Electrolytic Receivers in Wireless Te/egrap/O,.
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sponder u n d e r such conditions, I m a y cite the following a e r o g r a m received at the Coney Island S t a t i o n of the de F o r e s t Company, March I4, I9o3, and addressed to P r e s i d e n t Schurman, of Cornell University. T h e m e s s a g e was sent b y Charles De Garnio, professor of p e d a g o g y in that institution, from on board the S. S. " Coamo," b o u n d to',Porto Rico, u s i n g a to-inch coil and an a n t e n n a of only 9 ° feet, the distance b e i n g t io miles: " G r e e t i n g s from Old Ocean, who no longer s u n d e r s friends. C o u r t e s y de F o r e s t wireless telegraph system." Signed. I call y o u r attention n o w to a r e m a r k a b l e fact which I have discovered in connection with the electrolytic action of H e r t z i a n waves. T h e sensitive cell, as a b o v e described, s h o w s n o t h i n g of a c o u n t e r electro-motive force of polarization, a l t h o u g h the electrodes m a y be of a g r e a t v a r i e t y of shape and materials. T h e locally applied electro-motive force m a y r a n g e from a few millivolts to 3 volts or more, b u t always with a p r o p o r t i o n a t e increase of indicated current-flow. If, however, the electrodes be slowly s e p a r a t e d from one another, with one or more of the m i n u t e tentacles clinging thereto, a c o u n t e r electro-motive force of polarization m a y be o b s e r v e d w h e n the distance b e t w e e n the n e a r e s t electrodes exceeds a certain small limit. This c o u n t e r E.M.F. exists w h e t h e r the electrodes be of like or unlike material or shape. Now, upon the first application of the local b a t t e r y to the terminals of this cell, a t e m p o r a r y flow of c u r r e n t is observed, the resistance of the cell b e i n g at first slight, especially if a dilute alkali or acid form the electrolyte. Immediately thereafter, however, the current flow falls almost to n o t h i n g ; the c o u n t e r E.M.F. of the cell asserts itself. This, as is well k n o w n to the science of electro-chemistry, is due to the formation of a layer of gas i n s u l a t i n g the faces of the electrodes. This requires that, unless some depolarizing m e a n s b e added, the local applied electro-motive force m u s t be raised a b o v e the opposing electro-motive force of the cell; that is to say, a b o v e the critical v o l t a g e required for the d e c o m p o s i t i o n of the electrolyte.
252
de Forest :
[J. F. I.,
T h i s c o u n t e r E.M.F. of the cell exists w h e t h e r the fine tentacles be a t t a c h e d to the anode or cathode. N o w , w h e n the cell is placed in the p a t h of the h i g h f r e q u e n c y electrical oscillations t h e effect is f o u n d to be a temporary, more or less complete, a n n u l m e n t of the c o u n t e r electro-motive force of polarization. T h i s effect is scarcely noticeable w h e n the tentacles are m a d e the c a t h o d e ; b u t when m a d e the anode, so t h a t o x y g e n is t h e gas s u r r o u n d i n g and i n s u l a t i n g the fine tentacles, the effect o f the Hertzialx oscillation is to decrease to a m a r k e d degree the a p p a r e n t resistance of the cell. T h e sensitiveness of the electrolytic cell to this second action of the H e r t z i a n oscillations is n e a r l y inversely pro. portional to the exposed area of the anode. It can be obtained even for such coarse anodes as are f u r n i s h e d by a i m i l l i m e t e r p l a t i n u m wire ; b u t the d i a m e t e r of these is I00 times t h a t of the fine tentacles or trees previously described. L a s t year a G e r m a n experimenter, Schloemilch, r e p o r t e d the i n d e p e n d e n t discovery of this same effect in w h a t h e called " p o l a r i z a t i o n cells." A n o d e s h a v i n g d i a m e t e r s sufficiently small to give g r e a t s e n s i t i v e n e s s to the r e s p o n d e r are now o b t a i n e d by m e c h a n i c a l means, u s i n g the W a l l a s ton wire h a v i n g gold or p l a t i n u m core a n d a silver s h e a t h ; wherewith, after d r a w i n g the whole to a fineness of 2 or 3 millimeters, the silver s h e a t h is dissolved in c o n c e n t r a t e d acid in the m a n n e r long k n o w n to the arts. T h e order of d i a m e t e r t h u s o b t a i n e d is c o m p a r a b l e to t h a t of the metallic trees built up by electrolytic d e p o s i t ; b u t the wire a n o d e s are more stable and b e t t e r suited to the work in this second type of the electrolytic responder. To r e c a p i t u l a t e these points : Electrolytic effects of the H e r t z i a n oscillations are available in a practical d e t e c t o r w h e n e v e r e x t r e m e l y fine t e n t a c l e s or wires are used for one of the electrodes: (i) W h e n such tentacle is b r o u g h t into close p r o x i m i t y of the other e.lectrode, preferably the anode. T h e n o r m a l resistance of the cell t h e n becomes very small, g e n e r a l l y a few h u n d r e d ohms. T h e cell offers no c o u n t e r E M.F. of polarization. A m i n u t e film of liquid intervenes, conduct-
Oct., 19o4. ]
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i n g by ionization. T h e passage of the I-Iertzian oscillation will now decompose this film, insulatfng the fine electrode, and c a u s i n g a t e m p o r a r y and m a r k e d decrease in the local c u r r e n t flow. T h i s is a current-operated device, and is ext r e m e l y sensitive w h e n inserted as a potential node, directly at the base of the receiving antenna. (2) W h e n the anodic tentacle is w i t h d r a w n from p r o x i m i t y w i t h the o t h e r electrode, of w h a t e v e r shape or material. T h e n o r m a l a p p a r e n t resistance of the cell now becomes v e r y great, 3o,ooo or 5o,ooo ohms, due to the c o u n t e r E.M.F. of polarization. A m i n u t e film of o x y g e n gas normally envelopes the s u b m e r g e d surface of the fine cathode tentacle, practically i n s u l a t i n g it. T h e passage of the Hertzian oscillation will now cause a t e m p o r a r y dissipation of this gas, a p p a r e n t l y into the liquid, causing a m a r k e d increase in the local curr e n t flow. T h i s .is a potential-operated device, and is best located w h e r e can exist potential loops of opposite sign at its two electrodes. One investigator, p r o m i n e n t for his inventions in the wireless field, m a i n t a i n s t h a t the p h e n o m e n o n observed in this second form of t h e electrolytic responder is a heat effect. I h a v e been unable, however, to find a single fact warranting such a view. First of all, the fact that the device is a valve-effect, not indifferent to t h e direction of applied local potential, b u t practically i n o p e r a t i v e if the tentacle be m a d e negative, shows t h a t the results obtained are not chiefly C2R effects, as would be required were the action a h e a t p h e n o m e n o n due to t h e a m o u n t of c u r r e n t flowing. Second, t h a t it is a potential-operated and not a current-operated device, possessing a capacity, and normally insulating the fine anode. Third, t h a t its action is sensibly unaffected by e x t r e m e s of t e m p e r a t u r e , h i g h or low. M a n y other considerations; based upon theoretical calculations as well as practical experiment, upon which I c a n n o t dwell at this time, point to the correctness of t h e explanation of the action involved, based entirely upon electrolytic p h e n o m e n a . T h e electrolytic responders which I have described seem
254
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[J. F. I.,
to possess in the h i g h e s t degree the qualities necessary to place the art of a e r o g r a p h y on a basis to c o m p e t e with existing telegraphic service, b y land as well as by sea. T h e y are extraordinarily sensitive, r e g u l a r in response, strictly quantitative, a u t o m a t i c in action, h a v i n g a p p r o x i m a t e l y constant normal factors as r e g a r d s resistance and capacity, allowing thus a speed of word-transmission limited only b y the ability of operators to send and receive. By such m e a n s the problems of electrical tuning, or syntony, are e n o r m o u s l y simplified ; for heretofore the erratic n a t u r e of the coherer has rendered it impossible to closely r e g u l a t e the c o n s t a n t s of the t u n e d electrical circuits in which t h e y are placed; t h u s m a k i n g syntonization b y their e m p l o y at best crude and uncertain. In r e g u l a r i t y of action the r e s p o n d e r is strictly comparable with the R u t h e r f o r d m a g n e t i c d e t e e t o r as d e v e l o p e d b y Wilson, Shoemaker, Marconi and Ewing, while as regards relative sensitiveness there is to-day no comparison b e t w e e n that of the m a g n e t i c d e t e c t o r and the electrolytic receiver. Moreover, the device is practically indestructible, while its syntonizing qualities enable me to so c u t out foreign signals and electric d i s t u r b a n c e s as to render wireless communication by this means practically i m m u n e from interruption. In corroboration of these s t a t e m e n t s allow me to call attention to the service w h i c h the A m e r i c a n De F o r e s t Company has now.'established b e t w e e n Cleveland and Buffalo. T h e distance here is I8o miles, extending, as the two stations are located, almost entirely overland. D u r i n g the recent severe weather, when this land was frozen to a depth of several feet, and Lake Erie a m a s s of ice e x t e n d i n g from the shore for miles (conditions which afford the g r e a t e s t difficulties for aerographic transmission), the daily e x c h a n g e of m e s s a g e s at high rates of s p e e d has been continuous. T h e loudness and clearness of the signals are most surprising, originating, one m i g h t suppose, from some station less than 30 miles distant. T u n i n g here is so close that the addition or s u b t r a c t i o n of 6 to 8 inches of wire in the syntonizing coils cuts o u t the signals a l t o g e t h e r and enables the operator to dispose entirely of a t m o s p h e r i c d i s t u r b a n c e s
Oct., 19o4. ]
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arising from severe l i g h t n i n g storms in the close neighborhood of the stations. D u r i n g a recent test to Lockport, 36 miles overland from the Buffalo station, m e s s a g e s were received on two small wires e x t e n d i n g to the roof of a b u i l d i n g b u t 85 feet a b o v e the earth, with the u t m o s t clearness and accuracy. Again, the recent excellent record which the U n i t e d States Signal Corps has m a d e b e t w e e n F o r t S c h u y l e r and Port W r i g h t , 98 miles apart, concerning which so m u c h has recently a p p e a r e d in the daily press, was m a d e with receivers, t u n i n g devices and oscillator ~ets p u r c h a s e d from the de F o r e s t Companies. A s a result of this work, following upon the failure of several coherer s y s t e m s to cover the distance, the a p p a r a t u s is to be installed next s u m m e r in A l a s k a b e t w e e n N o m e and St. Michaels, over a r a n g e which the Signal Corps has s o u g h t in vain for several years to cover.*
T h e exact n a t u r e of the electro.magnetic w a v e s b y which wireless signals are t r a n s m i t t e d has afforded field for a g r e a t a m o u n t of speculation and t h e o r y a m o n g scientists and laymen. Considerable interest has a t t a c h e d to the discussion of this subject, which has recently been r u n n i n g in the c o l u m n s of the Electrical World and Engineer. U n f o r t u n a t e l y the classic work in exploration of the field of force and the n a t u r e of p r o p a g a t i o n from the dumb-bell oscillator, b y which Professor H e r t z immortalized his name, is not entirely applicable to the case; b u t the a n a l o g y is very close. W h e n the lower half of the H e r t z oscillator is s u b s t i t u t e d for a connection of t h a t side of the spark-gap to a c o n d u c t i n g or s e m i - c o n d u c t i n g m e d i u m of u n l i m i t e d extent (such as sea w a t e r or the earth's surface), the laws of electrical i m a g e s do not strictly hold. T h i s was, however, the view taken b y A b r a h a m in his m a t h e m a t i c a l i n v e s t i g a t i o n of the s u b j e c t ; b y Blondel, at least until recently, and others. Mr. J. E. Taylor, engineer of the British General P o s t * Since this paper was written these Alaskan stations have been opened. Cat:t. W i l d m a n reports the system as giving complete satisfaction.
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de F o r e s t :
[J. F. I.,
Office, and a zealous pioneer in wireless work, was, I believe, first to apply in a clear and iucid m a n n e r the t h e o r y and d i a g r a m s of H e r t z to the ease of the g r o u n d e d oscillator. T a y l o r pointed o u t that we had in practiee the sliding wave, following in general the shape worked o u t b y H e r t z ; b u t with the feet of the lines of force w h i c h form the polarized wave-front gliding over the surface of the earth or sea, following the c o n t o u r of hill and dale; and scaling such obstacles as do not a b s o r b portions of the w a v e e n e r g y and east t h e r e b y m o r e or less s e r i o u s shadows. T h e Hertzian fielcl of force does not call for approxim a t e l y spherical radiation of energy, as s e e m s to be taken for g r a n t e d b y s o m e ; and m y o w n e x p e r i e n c e goes to s h o w that the falling off in the s t r e n g t h of received e n e r g y does not follow a n y t h i n g like t h e law of the inverse s q u a r e s of the distance, as this a s s u m p t i o n w o u l d require. T h e electrolytic r e s p o n d e r is q u i t e exactly a q u a n t i t a t i v e instrument, and with it I find, at distances far from the course of oscillations, a decrease in s t r e n g t h of r e c e i v e d signals a p p r o x i m a t e l y proportional to the first p o w e r of the distance. One fact is certaiu, t h a t the p r o p a g a t i o n is not by hemispherical waves. One searches in vain the writings of H e r t z for a n y t h i n g s u g g e s t i n g such a t h e o r y there. H e r t z points out t h a t even with the free polarized w a v e in space the force d i m i n i s h e s c o n t i n u o u s l y w i t h the inverse distance; at first rapidly, as the inverse cube, b u t later slowly, as the inverse of the distance itself. So at g r e a t distances will the effect of oscillation be marked only in the equatorial plane ; that is, in our case, in the region nearest the earth's surface. On the other hand, the electric force in the direction of the axis of the oscillator, or vertically a b o v e the r a d i a t i n g antenna, falls off v e r y rapidly, at first as the cube, later as the square of the distance. T h u s , at g r e a t distances, an appreciable effect from the H e r t z i a n w a v e s sliding over the earth's surface need n o t be expected at distances far a b o v e the surface. This does n o t require an h y p o t h e s i s of the reflecting p o w e r of a highlyc o n d u c t i n g s t r a t u m of tariffed air, at an elevation of several
Oct., 19o4.]
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257
miles, as has been i n g e n i o u s l y s u g g e s t e d to account for the d e p a r t u r e of the p h e n o m e n a from the law of the invet'se squares. P o s s i b l y such strata m a y influence the radiation outwardly into space to some extent. But an exact k n o w l e d g e of the actual field of force, b o t h on the earth's surface and in the e t h e r s u r r o u n d i n g the t r a n s m i t t i n g station, can only be had b y a t h o r o u g h exploration with d e t e c t i n g oscillators, and u n d e r conditions such as obtain in practical, long-distance w o r k i n g ; an exploration such as H e r t z c o n d u c t e d in the l a b o r a t o r y at Karlsruhe, in that m a s t e r l y w a y which so characterized his work. Such work m u s t o b v i o u s l y be u n d e r t a k e n in a captive balloon or air-ship, and will be a t t e n d e d with o b v i o u s and costly difficulties. A b e g i n n i n g was m a d e several years ago by Captain Ferrie, of the F r e n c h Artillery, b u t has never been pursued. W i t h the u n e q u a l l e d facilities offered this s u m m e r at the World's Exposition at St. Louis, b o t h as regards a transm i t t i n g station of unusual power, or others of lesser range, and as regards vehicles for aerial navigation, it is to be earnestly h o p e d that more light m a y then be thrown upon this subject, so full of academic and practical interest. T h e results of y e a r s of p a t i e n t experiment, careful research, a c c o m p a n i e d often with d i s h e a r t e n i n g failures, on the part of the investigators in the wireless field, f r e q u e n t l y fail to meet the full expectations of the public. T h e difficulties to be overcome can of necessity not be realized b y those u n a c q u a i n t e d with the exact details of development. Such, alas, has been the h i s t o r y of all invention, all progress. It is g r a t i f y i n g to know that the f u n d a m e n t a l obstacles at least are in so large a m e a s u r e overcome, that develop. ment in this, the field possessing perhaps the most popular interest e x h i b i t e d in any age t o w a r d s any invention, any discovery, has y e t been steady, a l t h o u g h at m a n y times slow and faltering. T h e difficulties r e m a i n i n g are manifold and great. T h e development of the past is an e a r n e s t of the g r e a t e r things VOL. CLVIII. No. 946. ~7
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de F o r e s t :
[J. F. I.,
which are so surely in store for this n e w and h u m a n i t a r i a n art. DISCUSSION.
PROF. GEO. A. FIOADLE¥ :--I u n d e r s t a n d t h a t the Hertzian waves decrease t h e resistance of t h e coherer in the Marconi s y s t e m to such an e x t e n t t h a t it is possible to use a local circuit a n d Morse i n s t r u m e n t as a receiver. In this system, since the H e r t z i a n waves increase the resistance of the electrolyte, such an i n s t r u m e n t c a n n o t be employed. W h a t is to be used, a telephone receiver? DR. DE FOREST : - - I n r e a d i n g we always e m p l o y the telephone w i t h the electrolytic receiver. T h i s c o m b i n a t i o n is the s i m p l e s t a n d m o s t sensitive possible and requires no a t t e n t i o n or a d j u s t m e n t from the operator. T h e result is a speed and accuracy of r e a d i n g impossible by r e l a y ; and the trained ear of the o p e r a t o r enables h i m to i n s t i n c t i v e l y discard false or i n t e r f e r i n g signals, a t m o s p h e r i c or otherwise, and to a t t e n d exclusively to the sigr~als h a v i n g the distinctive sound of his t r a n s m i t t e r spark. W e can thus, w i t h two telephones in series, and two operators, read two distinct messages w i t h o u t a n y o t h e r t u n i n g . A relay can, however, be used with the electrolytic receiver. On account of its sensitiveness and r e g u l a r i t y of normal, I use this m e r e l y to ring a calling signal. But the records w h i c h m y s y s t e m has to its credit, often in competition w i t h tape-register systems, justify, I think, this policy of s i m p l i f y i n g the a p p a r a t u s to the g r e a t e s t possible extent. MR. E. A. SCOTT:--In consideration of the fact t h a t the earth, as a mass, is a c o n d u c t o r w i t h o u t resistance, would it not be practicable to m a k e a g r o u n d connection w i t h the two s t a t i o n s of sufficiently low resistance to use the earth as a c u r r e n t c h u t e as the w a t e r is now used ? DR. DE FORESw:--While the c o n d u c t i v i t y of the earth as a mass m a y be considered as infinite, y e t only t h a t of the surface l y i n g in the zone of influence from a n y t r a n s m i t t e r station can e n t e r into the question. T h i s c o n d u c t i v i t y of course varies w i t h the locality, and the q u a l i t y and dampness of the soil i n t e r v e n i n g b e t w e e a the two stations. If this chanced to be especially b e t t e r t h a n the conductivity=
Oct., ,9o4.]
L'lectrolytic Receivers m Wireless 7?leffraphy.
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of the s u r r o u n d i n g c o u n t r y we would find the c o n d u c t i n g strip a c t i n g in a m e a s u r e as a " w a v e - c h u t e , " t e a d i n f f a g r e a t e r portion of the wave e n e r g y in t h a t direction t h a n was p r o p a g a t e d in o t h e r directions of the compass. Such a condition would be offered in an arid c o u n t r y where the c o n d u c t i n g surface was confined l a r g e l y to the n e i g h b o r h o o d of a river or its valley, l y i n g in the line between the two a e r o g r a p h stations. A s y s t e m of railroad tracks would act in this m a n n e r , and if b o t h t r a n s m i t t e r and receiver i n s t r u m e n t s were connected to these tracks t r a n s m i s s i o n along the line would be especially good. MR. CARL HERING : - - W o u l d not the presence of large quantities of telegraph wires, or large m e t a l masses, interfere with t r a n s m i s s i o n of y o u r messages ? W o u l d not the s u b m a r i n e cables play a part in transatlantic wireless ? DR. DE FOREST : - - A mas:; of horizontally e x t e n d i n g telegraph wires would have little effect on the transmission of " wireless " signals if the wires e x t e n d e d at r i g h t angles to the direction of propagation. T h e lines of static displacement in the wave are vertical, and hence it is only the vertical components in the telegraph wires which would cause a reflection or absorption of the wave energy. In the case, however, of large vertical masses of metal, such as elevators, g a s r e s e r v o i r s , steel-framed buildings, etc., a v e r y considerable shadow effect of the waves is observed. T h i s is in accordance with the e x p e r i m e n t s of H e r t z and Lodge on the polarized properties of the H e r t z i a n waves. On the o t h e r hand, if the line of telegraph wires e x t e n d i n t h e direction of propagation, t h e y afford a c o n d u c t i n g path for the waves, and we find the propagation a c t u a l l y aided, with a t e n d e n c y to confine or concentrate the e n e r g y propagated along t h a t line. The same t h i n g is noticed in the laboratory, where a simple wire l y i n g along the floor, and not connected e i t h e r to t r a n s m i t t e r or receiver, will g r e a t l y aid the t r a n s m i s s i o n , if it lie in the proper directions.
260
de Forest.
[J. F. I.,
I h a v e noticed the same t h i n g to a m a r k e d degree at one of m y stations, where a single t e l e p h o n e wire l e a d i n g into s t a t i o n s e x t e n d e d for several miles in a s t r a i g h t line t o w a r d s the f r a n s m i t t i n g station. It was possible to o b t a i n the m e s s a g e s by c o n n e c t i n g the receiver b e t w e e n this wire and e a r t h f u l l y as well as from the I8o feet a n t e n n a , a l t h o u g h t h e d i s t a n c e to the trans. m i t t e r was some 15 miles. If this wire were e x t e n d e d at r i g h t angles to the line of propagation, we would expect no such action from it. DR. E. GOLDSMITH asked t h e lecturer w h e t h e r the earth or the water was not necessary for the t r a n s m i s s i o n of aerial m e s s a g e s ; a n d also w h e t h e r the h i g h poles built into the g r o u n d did not assist to help the t r a n s m i s s i o n ; how h i g h h a v e the poles to be in order to send m e s s a g e s across the A t l a n t i c Ocean ? It s e e m e d to h i m t h a t iron ships flqating on the w a t e r helped in a m e a s u r e the t r a n s m i s s i o n of the waves.
DR. DE FOREST : - - T h e c o n d u c t i v i t y of the e a r t h or w a t e r surfaces over which the sliding waves are p r o p a g a t e d e n t e r s very g r e a t l y into the p h e n o m e n a . T h e r e f o r e is it t h a t we are enabled to operate over g r e a t e r distances by sea t h a n b y l a n d w i t h a given q u a n t i t y of t r a n s m i t t e d energy, o t h e r conditions b e i n g the same. Moreover, o v e r l a n d the vertical wave-fronts are more or less cut into by obstacles which t h e y there m e e t - - f o r e s t s , precipitous hill-sides, b u i l d i n g s , m e t a l structures, etc. E v e r y such obstacle absorbs or refleets a g r e a t e r or less a m o u n t of the e n e r g y of the waves, d e p e n d i n g on the dimensions, c o n d u c t i v i t y and shape of the obstruction. It is necessary to e x t e n d the vertical t r a n s m i t t i n g a n d receiving wires to a c e r t a i n h e i g h t , r o u g h l y d e p e n d i n g u p o n the distances to be covered. T h e c o m m o n l y - a c c e p t e d law is t h a t w i t h a p a r i t y of o t h e r conditions, the distances a t t a i n a b ] e are p r o p o r t i o n a t e to the s q u a r e of t h e h e i g h t of t h e aerial conductors. But to-day a g r e a t n u m b e r of o t h e r conditions and a r r a n g e m e n t s m o d i f y this rule. Signals have been sent across the A t l a n t i c u s i n g ant e n n a 225 feet in the vertical, a l t h o u g h the l e n g t h of t h e s e
Oct., 19o4.]
Notes and Comments.
26I
wires is m u c h greater. T h a t h e i g h t is a b o u t the limit to be o b t a i n e d in practice. Obviously it is not necessary t h a t the tops of the two masts, or towers, should lie in a line not cut by the curvature of the earth, for reasons already explained. T h e distances a t t a i n a b l e are n e a r l y proportionate to the a m o u n t of e n e r g y t r a n s m i t t e d , a l t h o u g h the l e n g t h of wave used plays a considerable part. R o u g h l y speaking, the g r e a t e r the e n e r g y the longer the period of o s c i l l a t i o n of the r a d i a t o r circuit, and the g r e a t e r m u s t be the l e n g t h and the capacity of the a n t e n n a to be in resonance with such periods. No b e t t e r g r o u n d i n g conditions can be afforded t h a n those f o u n d on an iron-hull vessel floating in sea w a t e r . H e n c e the long distances which are often a t t a i n e d u n d e r such circumstances.
E L E C T R I C A L T R A N S M I S S I O N OF P I C T U R E S AND SCRIPT. A correspondent of ( L o n d o n ) N a t u r e refers as follows to a method and apparatus for this purpose lately described by Dr. Arthur Korn: The problem of distant electrical vision is one to which much speculation and e x p e r i m e n t i n g have been devoted. Before this problem can be attempted with any hope of success, however, the preliminary one of the electrical transmission of photographs over a distance has to be solved. This problem, it may be stated at once, has been mastered, and it is now possible to transmit photographs in this manner, and successful results have been obtained over telegraph and telephone lines 800 kilometers long. It does not need much consideration to see how important such a process would be for journalistic and police work if it could be industrially exploited, and it were possible simply to band a sketch or photograph in at the telegraph office and send the same as one now sends an ordinary telegram. The evening papers would be able then to publish photographs taken at the seat of war in Korea on t h e same day. Unfortunately, with the apparatus at present to be had, the time taken to transmit a half-plate photograph is half an hour. The cost of the use of a telegraph line of any length for half an hour would be, it is needless to point out, prohibitive. The lessening of the required time of transmission is, however, simply a matter of further development, and 11o good reason can be seen why in a few years' time the l:rocess should not be an adjunct to every existing telegraph line. The m e t h o d shortly consists of the following : A ray of light is made to pass systematically all over the transparent film to be transmitted. After passing t h r o u g h the film it impinges upon a selenium cell the resistance of which varies p r o p o r t i o , a t e l y to the amount of light which passes lhrough the pho-
FRANKLININSTITUTE OF THE STATE OF PENNSYLVANIA,
FOR T H E PROMOTION OF THE MECHANIC ARTS.
VOL. C L V I I I , No. 4.
79TH Y E A R .
OCTOBER, I9O4
THE Franklin I n s t i t u t e is not responsible for the state. ments and opinions advanced by contributors to thefaurnal.
ELECTRICAL
SECTION.
Stated Meeting held Thursday, March 3], ]904.
Electrolytic Receivers in Wireless Telegraphy. B Y DR. L E E DE FOREST.
W i t h the form and nature of the ordinary filings-tube coherer as applied to-day in wireless telegraphy the public has become fairly familiar. Branley, the distinguished French physicist, discovered in i89i that the effect of electrical oscillations upon a body of metallic filings, or dust, was to produce a sudden increase in the conductivity of the mass, a conductivity which persisted until the particles were broken apart again by mechanical jar. A l t h o u g h the American, Varley, the English scientist Hughes, and especially Onesti, the Italian, had previously noted this phenomenon, none of these investigators had fully appreciated the causes involved, or given to the world of science the benefit of their researches in t h o r o u g h published reports. VOL. CLVIII.
No. 946.
I6
::42
.de F o r e s t ."
[J. F. I.,
T h e discovery of the " c o h e r e r " therefore is r i g h t f u l l y a t t r i b u t e d to M. B r a n l e y ; b u t to Professor Popoff, of St. P e t e r s b u r g , and to Sir Oliver Lodge, of England, m u s t be given the credit of a p p l y i n g the relay and mechanical t a p p e r to the filings tube, the addition of a n t e n n a wings, of u p r i g h t wire and earth connection, and the first steps towards refinem e n t and an appreciation of t h e possibilities of the new detector as applied to an uiaborn b r a n c h of h u m a n enterprise and progress. W i t h the n o t a b l e work of Marconi in still f u r t h e r adapting the Popoff-Lodge a r r a n g e m e n t to commercial requirements, the gradual increase of distance on which aerographic c o m m u n i c a t i o n w a s established, the ingenious improvem e n t s in coherers b y such m e n a b r o a d as Tissot, Ducretet, Castelli, then b y L o d g e and Branley themselves, and in this c o u n t r y b y Shoemaker, t h e audience is more or less familiar. E n o r m o u s is t h e b u l k of research m a t t e r p u b l i s h e d r e g a r d i n g the coherer. T h e o r i e s of the action involved have b e e n n u m e r o u s , b u t divided chiefly into two schools : First, that of Lodge, w h o d e m o n s t r a t e d an actual w e l d i n g t o g e t h e r of metallic filings, after the s a m e h a d b e e n d r a w n into contact b y the electro-static attraction of the opposite charges induced upon the faces of the filings ; and, second, that of Branley, w h o ascribed t h e action to a b r e a k i n g d o w n or p u n c t u r i n g b y m i n u t e sparks of the dielectric film (either gaseous or a metallic oxide), w h i c h he held m u s t exist between opposing surfaces of the filings. T h e result of the years of i n v e s t i g a t i o n w h i c h have followed the early work of t h e s e two pioneers proves that in a m e a s u r e b o t h were right. T h e Branley effect undoubtedly exists, and affords a satisfactory explanation of the self-decohering p h e n o m e n a o b s e r v e d in the microphone type of auto-coherers. T h e s e i n s u l a t i n g oxide or gaseous films inevitably form and cling to the surface of any metallic particles not enelosed in a b s o l u t e v a c u u m , and w h e r e strictest care is not t a k e n to prevent. T h e y are, moreover, elastic, self-healing; and unless the electric impulses are of excessive violence, so t h a t a d j a c e n t particles are actually welded firmly together, the original high resistance is a u t o m a t i c a l l y
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Electrolytic Receivers bz Wireless Telegrap]O/.
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restored upon cessation of the electric impulse. This is notably the case when the elements of th~ radio-conductor are of carbon, hardened steel, carbon-mercury, aluminum, etc., where the formation of this resisting film is easily accomplished, or where amid the minute roughnesses of the carbon surface the adhesion of a gaseous film is made more easy. If, however, the coherer consists of filings of some soft metal of relative low fusing point--silver, gold, l e a d - - t h e Lodge cohering effect through actual welding follows upon the Branley action; making the final fall of resistance far more marked, and necessitating a vigorous mechanical jar to break up the cohered filings and restore sensitiveness. It is on account of this enormous change in conductivity of the filings-tube coherer that this form has been the standard used almost exclusively in the coherer systems of wireless telegraphy. For the filings-tube frequently shows a change from hundreds of thousands of ohms down to a few ohms under the influence of exceedingly weak electrical impulses. This readily allows the use of a relay, operating a" Morse inker and the tapping-back device ; and in the old country, where the coherer systems have been chiefly developed, reading by sight has ever been deemed a necessity, especially in the navies, where, until recently, wireless telegraphy has enjoyed its most cordial reception. The result of all this has been necessarily a system employing delicate relays, tapping-back adjuncts, complications of apparatus, requiring frequent and careful adjustments, a skill and delicacy on the part of the receiving operator seldom found outside of the physics laboratory, least of all upon a man-of-war, where the hands of Jack Tar, of the wigwag and gundeek school, are all thumbs and of Cyclopic dimensions. Despite many extravagant claims for high speeds of word transmission by coherer systems, I have yet to see proof of anything exceeding twelve to fifteen words the minute in ~enuine commercial working, while the actual every-day apeed attained seldom exceeds half that amount.
244
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[J. F. I.,
For commercial applications of wireless telegraphy this pathetic speed limitation of itself renders the filings-tube systems to-day an impossibility; regartlless of any other considerations, such as uncertainty of their action, ill-adapt. ability to electrical tuning, liability to interference (notably from atmospheric electricity), harmful effects from mechau. ical vibrations, etc., etc. It has been stated by its early champions, and more recently by those prominently interested in w i r e telegraphs, that wireless telegraphy is destined to fill a place where existing methods of wire communication are impossible, such as from ship to ship and ship to shore. And their point was wel! taken; for, with the best tape-recording coherer systems developed to-day, after eight years of elaborate effort, no sane man would think of employing wireless telegraphy, with its six words and as m a n y guesses per minute, its treacherous a d j u s t m e n t s ' a n d its moody dispositions, where a wire system could possibly be used. W i t h the auto-coherer, or so-called microphonic contact, the case is better. Here, since no tapping-back is required, a relay is superfluous, although, to speak frankly, the changes of resistance in the auto-coherer are generally so slight and the n o r m a l resistance such a shifting quantity t h a t no relay capable of operating reliably with the autocoherer has been devised. A telephone, the most sensitive indicating instrument known, is invariably used to-day in place of the relay and inker; and, inasmuch as the autocoherer is usually a quantitative instrument, possessed of no critical voltage, its combination with the telephone affords an extremely sensitive wave detector, the range of which is limited by the sensitiveness of the h u m a n ear. With this arrangement one obtains a speed of word reception limited only by the ability of the operators t o send dots and dashes with the transmitter spark, and to~ translate these into words. Consequently, in America one finds the alternating current of relatively high frequene# and excellent regularity of action employed, with an ordi~ nary Morse key obtaining a speed of twenty-five to thirty, five words per minute, instead of the antiquated inductiott'
Oct., ,9o4.]
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coil with its spluttering sticking h a m m e r interrupter, or its messy, explosive mercury-brake a n d the pump-handle Zeichengeber, which is considered abroad as a fitting accompaniment to the coherer receiver, and is so well designed to hold down the impatient sender to a sloth sufficient for that form of receiver. As we frequently see in the daily press, the spark.coil coherer systems are excellently adapted to the trying requirements of a "chess game by wireless telegraphy !" The a u t o - o r microphonic coherer is, however, poorly adapted to the nice requirements of close syntony or electrical tuning. This is on account of its variable normal resistance and its normal capacity. Moreover, this device is liable to "close u p " under the effects of severe static discharges, requiring r e a d j u s t m e n t by tapping, and rendering difficult its employment during time of severe atmospheric disturbances and lightning storms. Furthermore, its inability to reliably operate by a relay, a bell for calling purposes often necessitates the addition of a filings-tube for that purpose. I feel justified in saying while on this subject that the type of auto,coherer invented by Mr. H a r r y Shoemaker, until recently of this city, together with the special antenna which he employs therewith, is the most practical and satisfactory form which has come to m y attention. Having briefly reviewed the field of cohering indicators of electric waves, I wish now to call attention to an entirely distinct line of investigation leading up to forms of receivers which I believe may well be set in a class by themselves as regards sensitiveness, simplicity and general adaptability to most of the demands which can be made to.day in the field of wireless telegraphy. I refer to the electrolytic principle, one which has until recently attracted little investigation from experimenters generally, but which, notwithstanding, has to its credit performances which are rapidly Compelling study and research in various sources here and abroad. In i898 the German, Neugschwender, performed the fol.owing experiment : The silver plating of a strip of mirror
246
de F o r e s t ."
[J. F. I.,
glass was divided into two parts b y a sharp razor cut, leaving a narrow gap b e t w e e n t w o silver edges, c o m p l e t e l y i n s u l a t i n g the t w o sections. Each of the sections was now connected to the terminals of a dry battery, and a telephone and g a l v a n o m e t e r inserted in series in the circuit. lXTo current was o b s e r v e d to pass until a film of m o i s t u r e was deposited upon the slit, either b y b l o w i n g the b r e a t h thereon or b y placing a s a t u r a t e d sponge n e a r b y or b y placing a drop of w a t e r directly upon the slit. A f t e r a brief period the g a l v a n o m e t e r b e g a n to show violent and irregu. lar deflections. In the telephone a scratching, b u b b l i n g s o u n d was heard, followed shortly b y c o m p a r a t i v e q u i e t n e s s and a stable position of the g a l v a n o m e t e r needle, indicating t h a t the resistance of the g a p in the silver mirror h a d been broken d o w n until it m e a s u r e d b u t a few ohms. If, now, electric w a v e s were g e n e r a t e d in the neighborhood, as from t h e spark of an induction coil, the galvanometer s h o w e d an increase of resistance in the circuit, while the listener in the t e l e p h o n e heard a purring sound, r e p r o d u c i n g w i t h singular fidelity t h e sound of the induction-coil spark. U p o n cessation of the electrical impulses the conductivity of the gap in the mirror i n s t a n t l y reasserted itself; the sound in the telephone ceased. A year after N e n g s c h w e n d e r discovered this s t r a n g e action of the H e r t z i a n waves, A s c h k i n a s s i n d e p e n d e n t l y noted the same phenomena, the publication of which in W i e d e m a n n ' s A n n a l e n led the former to f u r t h e r pursue his researches, and to examine the action at the moist edges of t h e silver electrode u n d e r a p o w e r f u l microscope. H i s observations thereon and m y own i n v e s t i g a t i o n s in 1899 coincide in all essentials, and clearly d e m o n s t r a t e that the p h e n o m e n o n is one of electrolysis, due to the combined action of the Hertzinn and local currents. T h e y d e m o n s t r a t e , however, s e v eral features n e w to the generally accepted idea of electrOlytic action; and open up lines of speculation and research of u n u s u a l interest to the physicist, and rife with suggestion. T h e p h e n o m e n o n n o t e d a b o v e exist to a more prominent degree with tinfoil electrodes in place of silver, and these I shall now a t t e m p t to describe.
Oct.. 19o4.]
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W i t h the telephone to the ear and the eye at the microscope the action thus d o u b l y observed affords in fact one of the most fascinating, most b e a u t i f u l " p a s t i m e s (as I m a y well term it) ever g r a n t e d to the i n v e s t i g a t o r in these fields. W h e n the local E.M.F. is first applied to the gap, m i n u t e s t metallic particles, all b u t invisible even with a i,ooo-power lens, are seen torn off from the anode, u n d e r the stress of the electric forces, a p p a r e n t l y mechanical in action; and these dust-like particles, floating in the fluid, m o v e across to the cathode, s o m e rapidly, some slowly, b y s t r a n g e and g r o t e s q u e p a t h w a y s , or directly to their goal. T i n y ferry-boats, each laden with its little electric charge, and u n l o a d i n g its invisible cargo at the opposite electrode, retrace their j o u r n e y i n g s , or, c a u g h t b y a cohesive force, build up little bridges, trees, with branches of q u a i n t and crystalline patterns. D u r i n g this f o r m a t i v e period (lasting p e r h a p s for half a minute) the ear hears an irregular boiling sound, and the average deflection of the g a l v a n o m e t e r indicates a g r a d u a l decrease of resistance, until one or more of these tin trees or tentacles has been b u i l t c o m p l e t e l y across t h e gap. T h e n silence ensues until the current across this b r i d g e is s u d d e n l y increased as b y the H e r t z i a n oscillations from an electric spark m a d e in the neighborhood, or even from a source of so low f r e q u e n c y as the ordinary 6o-cycle alternating current. I n s t a n t l y all is c o m m o t i o n and c h a n g e a m o n g the tentacles, and especially w h e r e these join the cathode. T i n y b u b b l e s of h y d r o g e n gas appear, and enlarging s u d d e n l y break or b u r s t apart the bridges, while the click in the telephone indicates the r u p t u r e of the curreflt's path. Vet t h e y are persevering, these little p o n t o o n ferrymen, and instantly re-form, locking hands and h a s t e n i n g from their s u d d e n r o u t back to build n e w paths and chains. So the process continues, the local current re-establishing, the electric oscillations b r e a k i n g up, its h i g h w a y s of passage, with furious b u b b l i n g s and a g i t a t i o n s - - a v e r i t a b l e t e m p e s t in a microscopic tea-pot.
248
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[J. F. I,
T h e h y d r o g e n gas, having, of course, twice the v o l u m e of the oxygen, is m o s t in evidence, and therefore the r u p t u r e of the tentacle occurs chiefly at its cathodic terminal. A n d w h e r e s e g r e g a t e d b r a n c h e s of the tin trees are broken off, the b u b b l e s of gas are generally noticed at the cathodes. Moreover, the o x y g e n to a large e x t e n t enters into chemical c o m b i n a t i o n w i t h the tin, and after the slit has b e e n used for some time a greyish deposit of s t a n n o u s oxide m a y be scraped from the anode. A peculiar f e a t u r e of this p h e n o m e n o n is the rapid disappearance of the gas b u b b l e s . T h e h y d r o g e n , and w h a t oxygen does not c o m b i n e with the tin, s e e m s to be a b s o r b e d b y the liquid ; or the two, b e i n g nascent, m a y r e c o m b i n e to form water. One fact m u s t be b o r n e in mind, t h a t the fine tentacles (whose diameter, b y the way, is of t h e order of some hund r e d - t h o u s a n d t h s of an inch) do not come into actual metallic c o n t a c t with t h e anode terminal. A film of electrolyte of almost m o l e c u l a r thickness m u s t exist b e t w e e n the two, c o n d u c t i n g n o r m a l l y b y electrolytic ionization and conduction, y e t easily d e c o m p o s e d and t r a n s f o r m e d b y a sudden increase of c u r r e n t into an i n s u l a t i n g g a s e o u s film, the expansion of w h i c h still f u r t h e r increases the resistance of the gap. T h e character of the electrolytic action w h e n soft metals, such as tin, silver, lead, are used as anode, and w h e n t h e distance to the cathode is decreased to the order of T~0-inch or less, is quite surprising. F o r example, the electro-motive forces n e e d e d for electro-deposition from anode to c a t h o d e are e x t r e m e l y small, n o w h e r e a p p r o a c h i n g the critical E.M.F. of polarization of the electrolyte. Moreover, distilled water, so-called chemically-pure glycerine, oils, etc., contain e n o u g h of i m p u r i t i e s (such as acid traces) to enable t h e m to act here like our ordinary electrolytic solution. I h a v e in ten m i n u t e s plated a firm deposit of tin on a gold cathode, u s i n g distilled w a t e r only, and a potential difference not e x c e e d i n g ~ of a volt. F9r the s a m e reasons it is possible to d e c o m p o s e the w a t e r films interposed b e t w e e n b r a n c h e s of these tin trees
Oct., 19o4.]
Electrolytic Receivers zn Wireless Telegraphy.
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by a v e r y m i n u t e difference of potential there-between. T h e result is t h a t this electrolytic reslvonder does not neceasarily possess a "critical potential," as is c o m m o n to a coherer ; and it does not, therefore, cease to respond to electric impulses the potential of w h i c h is less t h a n the E.M.F. of polarization of the electrolyte, as c o m m o n l y understood. This receiver is in fact a current.operated device, as d i s t i n g u i s h e d from potentially-operated forms such as the coher~r, as I h a v e proved in a v a r i e t y of ways. R e t u r n i n g to the silver-mirror slit e x p e r i m e n t of Neugschwender, m y investigations soon showed me t h a t the device was a l t o g e t h e r unreliable as a practical w a v e - r e s p o n d i n g detector. In the slit form above described t h e t e a r i n g off of metal from the anode and the b u i l d i n g up of c o n d u c t i n g bridges and tentacles across the gap was found to continue u n t i l t h e whole gap b e c a m e clogged with an a g g l o m e r a t i o n of crystalline formations, affording so m a n y short-circuiting paths in parallel t h a t v e r y soon the auto-cohering could be obtained only in response to powerful sparks. Again, an a c c u m u l a t i o n of h y d r o g e n gas m a y obtain, completely p r e v e n t i n g the r e f o r m i n g of a w a t e r film between tentacles and electrodes or b e t w e e n sections of the tentacles. T h e r e results t h e r e u p o n the r e g u l a r coherer a r r a n g e m e n t , and at the first electric impulse p e r m a n e n t welding of the chain ensues, so that the device becomes inoperative until cleaned out or readjusted. S y s t e m a t i c and p a i n s t a k i n g e x p e r i m e n t s e x t e n d i n g over two years were required before the electrolytic " r e s p o n d e r , " as I have t e r m e d it, a s s u m e d a commercial form. As described in U. S. P a t e n t 716,oo% a viscous supporting m e d i u m , such as glycerine, h e a v y oil, or one mixed with an absorbent such as lycopodium powder, m a y be inserted between the electrodes to p r e v e n t the a g g l o m e r a t i o n of conducting paths, to confine the action to a few only and to prevent the p e r m a n e n t formation of gas a m o n g the tentacles. In addition a depolarizer was added to chemically dispose of the h y d r o g e n bubbles. This last factor has proved of greatest value, r e n d e r i n g the responder of e x t r e m e sensitiveness and reliability.
250
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kJ F . t .
Other electrolytes, such as dilute acids, alkalies, alcohols, a m m o n i a , etc., were early tried, b u t I found generally t h a t such lead to a too violent d i s i n t e g r a t i o n of the m e t a l s ill the responder. It has f r e q u e n t l y been asserted t h a t the Marconi coherer when connected directly b e t w e e n the foot of an u p r i g h t aerial conductor, of a b o u t 6o m e t e r s height, and the earth, failed to respond to impulses from a typical I2-inch induction-coil t r a n s m i t t e r set at distances exceeding t~renty miles. For longer distances it was necessary to insert a " j i g g e r " or transformer, by m e a n s of which the potential difference across t h e t e r m i n a l s of the coherer could be stepped up to t h a t required by the so-called " c r i t i c a l potent i a l " of the coherer. A carbon m i c r o p h o n e auto-coherer is a current-operated device. It can, therefore, be i n s e r t e d w i t h o u t a step.up t r a n s f o r m e r directly at the foot of the aerial wire, and shows no such failure to respond for the distances above mentioned. If h i g h - f r e q u e n c y electric oscillations from a d i s t a n t source be impressed upon a vertical conductor g r o u n d e d at its lower end, curr~nt oscillations will be i n d u c e d in the surface of the conductor, t r a v e r s i n g it from top to b o t t o m . A t the upper e x t r e m i t y the electro-static c o m p o n e n t s of the oscillations will be reflected w i t h o u t c h a n g e of sign, and at the base reflected w i t h c h a n g e of sign. T h e exact converse holds for the e l e c t r o . m a g n e t i c or c u r r e n t c o m p o n e n t s of the oscillation. T h e result is a loop of electro-static force at the upper end of the wire, and a c u r r e n t loop at its base. A current-operated device, therefore, will r6spond best w h e n inserted in the base of the aerial at a point where the coherer is at its g r e a t e s t d i s a d v a n t a g e . T h e e x t r e m e distances f r o m which messages have been received on the electrolytic responder of the types j u s t described, as compared to records with the coherer u n d e r similar conditions, afford a practical proof t h a t it is a current-operated" device, in a d d i t i o n to the consideration of t h e electrolytic principles involved in its action. As an i n t e r e s t i n g i n s t a n c e of the work done by the re-
Oct., i9o4.]
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sponder u n d e r such conditions, I m a y cite the following a e r o g r a m received at the Coney Island S t a t i o n of the de F o r e s t Company, March I4, I9o3, and addressed to P r e s i d e n t Schurman, of Cornell University. T h e m e s s a g e was sent b y Charles De Garnio, professor of p e d a g o g y in that institution, from on board the S. S. " Coamo," b o u n d to',Porto Rico, u s i n g a to-inch coil and an a n t e n n a of only 9 ° feet, the distance b e i n g t io miles: " G r e e t i n g s from Old Ocean, who no longer s u n d e r s friends. C o u r t e s y de F o r e s t wireless telegraph system." Signed. I call y o u r attention n o w to a r e m a r k a b l e fact which I have discovered in connection with the electrolytic action of H e r t z i a n waves. T h e sensitive cell, as a b o v e described, s h o w s n o t h i n g of a c o u n t e r electro-motive force of polarization, a l t h o u g h the electrodes m a y be of a g r e a t v a r i e t y of shape and materials. T h e locally applied electro-motive force m a y r a n g e from a few millivolts to 3 volts or more, b u t always with a p r o p o r t i o n a t e increase of indicated current-flow. If, however, the electrodes be slowly s e p a r a t e d from one another, with one or more of the m i n u t e tentacles clinging thereto, a c o u n t e r electro-motive force of polarization m a y be o b s e r v e d w h e n the distance b e t w e e n the n e a r e s t electrodes exceeds a certain small limit. This c o u n t e r E.M.F. exists w h e t h e r the electrodes be of like or unlike material or shape. Now, upon the first application of the local b a t t e r y to the terminals of this cell, a t e m p o r a r y flow of c u r r e n t is observed, the resistance of the cell b e i n g at first slight, especially if a dilute alkali or acid form the electrolyte. Immediately thereafter, however, the current flow falls almost to n o t h i n g ; the c o u n t e r E.M.F. of the cell asserts itself. This, as is well k n o w n to the science of electro-chemistry, is due to the formation of a layer of gas i n s u l a t i n g the faces of the electrodes. This requires that, unless some depolarizing m e a n s b e added, the local applied electro-motive force m u s t be raised a b o v e the opposing electro-motive force of the cell; that is to say, a b o v e the critical v o l t a g e required for the d e c o m p o s i t i o n of the electrolyte.
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T h i s c o u n t e r E.M.F. of the cell exists w h e t h e r the fine tentacles be a t t a c h e d to the anode or cathode. N o w , w h e n the cell is placed in the p a t h of the h i g h f r e q u e n c y electrical oscillations t h e effect is f o u n d to be a temporary, more or less complete, a n n u l m e n t of the c o u n t e r electro-motive force of polarization. T h i s effect is scarcely noticeable w h e n the tentacles are m a d e the c a t h o d e ; b u t when m a d e the anode, so t h a t o x y g e n is t h e gas s u r r o u n d i n g and i n s u l a t i n g the fine tentacles, the effect o f the Hertzialx oscillation is to decrease to a m a r k e d degree the a p p a r e n t resistance of the cell. T h e sensitiveness of the electrolytic cell to this second action of the H e r t z i a n oscillations is n e a r l y inversely pro. portional to the exposed area of the anode. It can be obtained even for such coarse anodes as are f u r n i s h e d by a i m i l l i m e t e r p l a t i n u m wire ; b u t the d i a m e t e r of these is I00 times t h a t of the fine tentacles or trees previously described. L a s t year a G e r m a n experimenter, Schloemilch, r e p o r t e d the i n d e p e n d e n t discovery of this same effect in w h a t h e called " p o l a r i z a t i o n cells." A n o d e s h a v i n g d i a m e t e r s sufficiently small to give g r e a t s e n s i t i v e n e s s to the r e s p o n d e r are now o b t a i n e d by m e c h a n i c a l means, u s i n g the W a l l a s ton wire h a v i n g gold or p l a t i n u m core a n d a silver s h e a t h ; wherewith, after d r a w i n g the whole to a fineness of 2 or 3 millimeters, the silver s h e a t h is dissolved in c o n c e n t r a t e d acid in the m a n n e r long k n o w n to the arts. T h e order of d i a m e t e r t h u s o b t a i n e d is c o m p a r a b l e to t h a t of the metallic trees built up by electrolytic d e p o s i t ; b u t the wire a n o d e s are more stable and b e t t e r suited to the work in this second type of the electrolytic responder. To r e c a p i t u l a t e these points : Electrolytic effects of the H e r t z i a n oscillations are available in a practical d e t e c t o r w h e n e v e r e x t r e m e l y fine t e n t a c l e s or wires are used for one of the electrodes: (i) W h e n such tentacle is b r o u g h t into close p r o x i m i t y of the other e.lectrode, preferably the anode. T h e n o r m a l resistance of the cell t h e n becomes very small, g e n e r a l l y a few h u n d r e d ohms. T h e cell offers no c o u n t e r E M.F. of polarization. A m i n u t e film of liquid intervenes, conduct-
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i n g by ionization. T h e passage of the I-Iertzian oscillation will now decompose this film, insulatfng the fine electrode, and c a u s i n g a t e m p o r a r y and m a r k e d decrease in the local c u r r e n t flow. T h i s is a current-operated device, and is ext r e m e l y sensitive w h e n inserted as a potential node, directly at the base of the receiving antenna. (2) W h e n the anodic tentacle is w i t h d r a w n from p r o x i m i t y w i t h the o t h e r electrode, of w h a t e v e r shape or material. T h e n o r m a l a p p a r e n t resistance of the cell now becomes v e r y great, 3o,ooo or 5o,ooo ohms, due to the c o u n t e r E.M.F. of polarization. A m i n u t e film of o x y g e n gas normally envelopes the s u b m e r g e d surface of the fine cathode tentacle, practically i n s u l a t i n g it. T h e passage of the Hertzian oscillation will now cause a t e m p o r a r y dissipation of this gas, a p p a r e n t l y into the liquid, causing a m a r k e d increase in the local curr e n t flow. T h i s .is a potential-operated device, and is best located w h e r e can exist potential loops of opposite sign at its two electrodes. One investigator, p r o m i n e n t for his inventions in the wireless field, m a i n t a i n s t h a t the p h e n o m e n o n observed in this second form of t h e electrolytic responder is a heat effect. I h a v e been unable, however, to find a single fact warranting such a view. First of all, the fact that the device is a valve-effect, not indifferent to t h e direction of applied local potential, b u t practically i n o p e r a t i v e if the tentacle be m a d e negative, shows t h a t the results obtained are not chiefly C2R effects, as would be required were the action a h e a t p h e n o m e n o n due to t h e a m o u n t of c u r r e n t flowing. Second, t h a t it is a potential-operated and not a current-operated device, possessing a capacity, and normally insulating the fine anode. Third, t h a t its action is sensibly unaffected by e x t r e m e s of t e m p e r a t u r e , h i g h or low. M a n y other considerations; based upon theoretical calculations as well as practical experiment, upon which I c a n n o t dwell at this time, point to the correctness of t h e explanation of the action involved, based entirely upon electrolytic p h e n o m e n a . T h e electrolytic responders which I have described seem
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to possess in the h i g h e s t degree the qualities necessary to place the art of a e r o g r a p h y on a basis to c o m p e t e with existing telegraphic service, b y land as well as by sea. T h e y are extraordinarily sensitive, r e g u l a r in response, strictly quantitative, a u t o m a t i c in action, h a v i n g a p p r o x i m a t e l y constant normal factors as r e g a r d s resistance and capacity, allowing thus a speed of word-transmission limited only b y the ability of operators to send and receive. By such m e a n s the problems of electrical tuning, or syntony, are e n o r m o u s l y simplified ; for heretofore the erratic n a t u r e of the coherer has rendered it impossible to closely r e g u l a t e the c o n s t a n t s of the t u n e d electrical circuits in which t h e y are placed; t h u s m a k i n g syntonization b y their e m p l o y at best crude and uncertain. In r e g u l a r i t y of action the r e s p o n d e r is strictly comparable with the R u t h e r f o r d m a g n e t i c d e t e e t o r as d e v e l o p e d b y Wilson, Shoemaker, Marconi and Ewing, while as regards relative sensitiveness there is to-day no comparison b e t w e e n that of the m a g n e t i c d e t e c t o r and the electrolytic receiver. Moreover, the device is practically indestructible, while its syntonizing qualities enable me to so c u t out foreign signals and electric d i s t u r b a n c e s as to render wireless communication by this means practically i m m u n e from interruption. In corroboration of these s t a t e m e n t s allow me to call attention to the service w h i c h the A m e r i c a n De F o r e s t Company has now.'established b e t w e e n Cleveland and Buffalo. T h e distance here is I8o miles, extending, as the two stations are located, almost entirely overland. D u r i n g the recent severe weather, when this land was frozen to a depth of several feet, and Lake Erie a m a s s of ice e x t e n d i n g from the shore for miles (conditions which afford the g r e a t e s t difficulties for aerographic transmission), the daily e x c h a n g e of m e s s a g e s at high rates of s p e e d has been continuous. T h e loudness and clearness of the signals are most surprising, originating, one m i g h t suppose, from some station less than 30 miles distant. T u n i n g here is so close that the addition or s u b t r a c t i o n of 6 to 8 inches of wire in the syntonizing coils cuts o u t the signals a l t o g e t h e r and enables the operator to dispose entirely of a t m o s p h e r i c d i s t u r b a n c e s
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arising from severe l i g h t n i n g storms in the close neighborhood of the stations. D u r i n g a recent test to Lockport, 36 miles overland from the Buffalo station, m e s s a g e s were received on two small wires e x t e n d i n g to the roof of a b u i l d i n g b u t 85 feet a b o v e the earth, with the u t m o s t clearness and accuracy. Again, the recent excellent record which the U n i t e d States Signal Corps has m a d e b e t w e e n F o r t S c h u y l e r and Port W r i g h t , 98 miles apart, concerning which so m u c h has recently a p p e a r e d in the daily press, was m a d e with receivers, t u n i n g devices and oscillator ~ets p u r c h a s e d from the de F o r e s t Companies. A s a result of this work, following upon the failure of several coherer s y s t e m s to cover the distance, the a p p a r a t u s is to be installed next s u m m e r in A l a s k a b e t w e e n N o m e and St. Michaels, over a r a n g e which the Signal Corps has s o u g h t in vain for several years to cover.*
T h e exact n a t u r e of the electro.magnetic w a v e s b y which wireless signals are t r a n s m i t t e d has afforded field for a g r e a t a m o u n t of speculation and t h e o r y a m o n g scientists and laymen. Considerable interest has a t t a c h e d to the discussion of this subject, which has recently been r u n n i n g in the c o l u m n s of the Electrical World and Engineer. U n f o r t u n a t e l y the classic work in exploration of the field of force and the n a t u r e of p r o p a g a t i o n from the dumb-bell oscillator, b y which Professor H e r t z immortalized his name, is not entirely applicable to the case; b u t the a n a l o g y is very close. W h e n the lower half of the H e r t z oscillator is s u b s t i t u t e d for a connection of t h a t side of the spark-gap to a c o n d u c t i n g or s e m i - c o n d u c t i n g m e d i u m of u n l i m i t e d extent (such as sea w a t e r or the earth's surface), the laws of electrical i m a g e s do not strictly hold. T h i s was, however, the view taken b y A b r a h a m in his m a t h e m a t i c a l i n v e s t i g a t i o n of the s u b j e c t ; b y Blondel, at least until recently, and others. Mr. J. E. Taylor, engineer of the British General P o s t * Since this paper was written these Alaskan stations have been opened. Cat:t. W i l d m a n reports the system as giving complete satisfaction.
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Office, and a zealous pioneer in wireless work, was, I believe, first to apply in a clear and iucid m a n n e r the t h e o r y and d i a g r a m s of H e r t z to the ease of the g r o u n d e d oscillator. T a y l o r pointed o u t that we had in practiee the sliding wave, following in general the shape worked o u t b y H e r t z ; b u t with the feet of the lines of force w h i c h form the polarized wave-front gliding over the surface of the earth or sea, following the c o n t o u r of hill and dale; and scaling such obstacles as do not a b s o r b portions of the w a v e e n e r g y and east t h e r e b y m o r e or less s e r i o u s shadows. T h e Hertzian fielcl of force does not call for approxim a t e l y spherical radiation of energy, as s e e m s to be taken for g r a n t e d b y s o m e ; and m y o w n e x p e r i e n c e goes to s h o w that the falling off in the s t r e n g t h of received e n e r g y does not follow a n y t h i n g like t h e law of the inverse s q u a r e s of the distance, as this a s s u m p t i o n w o u l d require. T h e electrolytic r e s p o n d e r is q u i t e exactly a q u a n t i t a t i v e instrument, and with it I find, at distances far from the course of oscillations, a decrease in s t r e n g t h of r e c e i v e d signals a p p r o x i m a t e l y proportional to the first p o w e r of the distance. One fact is certaiu, t h a t the p r o p a g a t i o n is not by hemispherical waves. One searches in vain the writings of H e r t z for a n y t h i n g s u g g e s t i n g such a t h e o r y there. H e r t z points out t h a t even with the free polarized w a v e in space the force d i m i n i s h e s c o n t i n u o u s l y w i t h the inverse distance; at first rapidly, as the inverse cube, b u t later slowly, as the inverse of the distance itself. So at g r e a t distances will the effect of oscillation be marked only in the equatorial plane ; that is, in our case, in the region nearest the earth's surface. On the other hand, the electric force in the direction of the axis of the oscillator, or vertically a b o v e the r a d i a t i n g antenna, falls off v e r y rapidly, at first as the cube, later as the square of the distance. T h u s , at g r e a t distances, an appreciable effect from the H e r t z i a n w a v e s sliding over the earth's surface need n o t be expected at distances far a b o v e the surface. This does n o t require an h y p o t h e s i s of the reflecting p o w e r of a highlyc o n d u c t i n g s t r a t u m of tariffed air, at an elevation of several
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miles, as has been i n g e n i o u s l y s u g g e s t e d to account for the d e p a r t u r e of the p h e n o m e n a from the law of the invet'se squares. P o s s i b l y such strata m a y influence the radiation outwardly into space to some extent. But an exact k n o w l e d g e of the actual field of force, b o t h on the earth's surface and in the e t h e r s u r r o u n d i n g the t r a n s m i t t i n g station, can only be had b y a t h o r o u g h exploration with d e t e c t i n g oscillators, and u n d e r conditions such as obtain in practical, long-distance w o r k i n g ; an exploration such as H e r t z c o n d u c t e d in the l a b o r a t o r y at Karlsruhe, in that m a s t e r l y w a y which so characterized his work. Such work m u s t o b v i o u s l y be u n d e r t a k e n in a captive balloon or air-ship, and will be a t t e n d e d with o b v i o u s and costly difficulties. A b e g i n n i n g was m a d e several years ago by Captain Ferrie, of the F r e n c h Artillery, b u t has never been pursued. W i t h the u n e q u a l l e d facilities offered this s u m m e r at the World's Exposition at St. Louis, b o t h as regards a transm i t t i n g station of unusual power, or others of lesser range, and as regards vehicles for aerial navigation, it is to be earnestly h o p e d that more light m a y then be thrown upon this subject, so full of academic and practical interest. T h e results of y e a r s of p a t i e n t experiment, careful research, a c c o m p a n i e d often with d i s h e a r t e n i n g failures, on the part of the investigators in the wireless field, f r e q u e n t l y fail to meet the full expectations of the public. T h e difficulties to be overcome can of necessity not be realized b y those u n a c q u a i n t e d with the exact details of development. Such, alas, has been the h i s t o r y of all invention, all progress. It is g r a t i f y i n g to know that the f u n d a m e n t a l obstacles at least are in so large a m e a s u r e overcome, that develop. ment in this, the field possessing perhaps the most popular interest e x h i b i t e d in any age t o w a r d s any invention, any discovery, has y e t been steady, a l t h o u g h at m a n y times slow and faltering. T h e difficulties r e m a i n i n g are manifold and great. T h e development of the past is an e a r n e s t of the g r e a t e r things VOL. CLVIII. No. 946. ~7
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which are so surely in store for this n e w and h u m a n i t a r i a n art. DISCUSSION.
PROF. GEO. A. FIOADLE¥ :--I u n d e r s t a n d t h a t the Hertzian waves decrease t h e resistance of t h e coherer in the Marconi s y s t e m to such an e x t e n t t h a t it is possible to use a local circuit a n d Morse i n s t r u m e n t as a receiver. In this system, since the H e r t z i a n waves increase the resistance of the electrolyte, such an i n s t r u m e n t c a n n o t be employed. W h a t is to be used, a telephone receiver? DR. DE FOREST : - - I n r e a d i n g we always e m p l o y the telephone w i t h the electrolytic receiver. T h i s c o m b i n a t i o n is the s i m p l e s t a n d m o s t sensitive possible and requires no a t t e n t i o n or a d j u s t m e n t from the operator. T h e result is a speed and accuracy of r e a d i n g impossible by r e l a y ; and the trained ear of the o p e r a t o r enables h i m to i n s t i n c t i v e l y discard false or i n t e r f e r i n g signals, a t m o s p h e r i c or otherwise, and to a t t e n d exclusively to the sigr~als h a v i n g the distinctive sound of his t r a n s m i t t e r spark. W e can thus, w i t h two telephones in series, and two operators, read two distinct messages w i t h o u t a n y o t h e r t u n i n g . A relay can, however, be used with the electrolytic receiver. On account of its sensitiveness and r e g u l a r i t y of normal, I use this m e r e l y to ring a calling signal. But the records w h i c h m y s y s t e m has to its credit, often in competition w i t h tape-register systems, justify, I think, this policy of s i m p l i f y i n g the a p p a r a t u s to the g r e a t e s t possible extent. MR. E. A. SCOTT:--In consideration of the fact t h a t the earth, as a mass, is a c o n d u c t o r w i t h o u t resistance, would it not be practicable to m a k e a g r o u n d connection w i t h the two s t a t i o n s of sufficiently low resistance to use the earth as a c u r r e n t c h u t e as the w a t e r is now used ? DR. DE FORESw:--While the c o n d u c t i v i t y of the earth as a mass m a y be considered as infinite, y e t only t h a t of the surface l y i n g in the zone of influence from a n y t r a n s m i t t e r station can e n t e r into the question. T h i s c o n d u c t i v i t y of course varies w i t h the locality, and the q u a l i t y and dampness of the soil i n t e r v e n i n g b e t w e e a the two stations. If this chanced to be especially b e t t e r t h a n the conductivity=
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of the s u r r o u n d i n g c o u n t r y we would find the c o n d u c t i n g strip a c t i n g in a m e a s u r e as a " w a v e - c h u t e , " t e a d i n f f a g r e a t e r portion of the wave e n e r g y in t h a t direction t h a n was p r o p a g a t e d in o t h e r directions of the compass. Such a condition would be offered in an arid c o u n t r y where the c o n d u c t i n g surface was confined l a r g e l y to the n e i g h b o r h o o d of a river or its valley, l y i n g in the line between the two a e r o g r a p h stations. A s y s t e m of railroad tracks would act in this m a n n e r , and if b o t h t r a n s m i t t e r and receiver i n s t r u m e n t s were connected to these tracks t r a n s m i s s i o n along the line would be especially good. MR. CARL HERING : - - W o u l d not the presence of large quantities of telegraph wires, or large m e t a l masses, interfere with t r a n s m i s s i o n of y o u r messages ? W o u l d not the s u b m a r i n e cables play a part in transatlantic wireless ? DR. DE FOREST : - - A mas:; of horizontally e x t e n d i n g telegraph wires would have little effect on the transmission of " wireless " signals if the wires e x t e n d e d at r i g h t angles to the direction of propagation. T h e lines of static displacement in the wave are vertical, and hence it is only the vertical components in the telegraph wires which would cause a reflection or absorption of the wave energy. In the case, however, of large vertical masses of metal, such as elevators, g a s r e s e r v o i r s , steel-framed buildings, etc., a v e r y considerable shadow effect of the waves is observed. T h i s is in accordance with the e x p e r i m e n t s of H e r t z and Lodge on the polarized properties of the H e r t z i a n waves. On the o t h e r hand, if the line of telegraph wires e x t e n d i n t h e direction of propagation, t h e y afford a c o n d u c t i n g path for the waves, and we find the propagation a c t u a l l y aided, with a t e n d e n c y to confine or concentrate the e n e r g y propagated along t h a t line. The same t h i n g is noticed in the laboratory, where a simple wire l y i n g along the floor, and not connected e i t h e r to t r a n s m i t t e r or receiver, will g r e a t l y aid the t r a n s m i s s i o n , if it lie in the proper directions.
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[J. F. I.,
I h a v e noticed the same t h i n g to a m a r k e d degree at one of m y stations, where a single t e l e p h o n e wire l e a d i n g into s t a t i o n s e x t e n d e d for several miles in a s t r a i g h t line t o w a r d s the f r a n s m i t t i n g station. It was possible to o b t a i n the m e s s a g e s by c o n n e c t i n g the receiver b e t w e e n this wire and e a r t h f u l l y as well as from the I8o feet a n t e n n a , a l t h o u g h t h e d i s t a n c e to the trans. m i t t e r was some 15 miles. If this wire were e x t e n d e d at r i g h t angles to the line of propagation, we would expect no such action from it. DR. E. GOLDSMITH asked t h e lecturer w h e t h e r the earth or the water was not necessary for the t r a n s m i s s i o n of aerial m e s s a g e s ; a n d also w h e t h e r the h i g h poles built into the g r o u n d did not assist to help the t r a n s m i s s i o n ; how h i g h h a v e the poles to be in order to send m e s s a g e s across the A t l a n t i c Ocean ? It s e e m e d to h i m t h a t iron ships flqating on the w a t e r helped in a m e a s u r e the t r a n s m i s s i o n of the waves.
DR. DE FOREST : - - T h e c o n d u c t i v i t y of the e a r t h or w a t e r surfaces over which the sliding waves are p r o p a g a t e d e n t e r s very g r e a t l y into the p h e n o m e n a . T h e r e f o r e is it t h a t we are enabled to operate over g r e a t e r distances by sea t h a n b y l a n d w i t h a given q u a n t i t y of t r a n s m i t t e d energy, o t h e r conditions b e i n g the same. Moreover, o v e r l a n d the vertical wave-fronts are more or less cut into by obstacles which t h e y there m e e t - - f o r e s t s , precipitous hill-sides, b u i l d i n g s , m e t a l structures, etc. E v e r y such obstacle absorbs or refleets a g r e a t e r or less a m o u n t of the e n e r g y of the waves, d e p e n d i n g on the dimensions, c o n d u c t i v i t y and shape of the obstruction. It is necessary to e x t e n d the vertical t r a n s m i t t i n g a n d receiving wires to a c e r t a i n h e i g h t , r o u g h l y d e p e n d i n g u p o n the distances to be covered. T h e c o m m o n l y - a c c e p t e d law is t h a t w i t h a p a r i t y of o t h e r conditions, the distances a t t a i n a b ] e are p r o p o r t i o n a t e to the s q u a r e of t h e h e i g h t of t h e aerial conductors. But to-day a g r e a t n u m b e r of o t h e r conditions and a r r a n g e m e n t s m o d i f y this rule. Signals have been sent across the A t l a n t i c u s i n g ant e n n a 225 feet in the vertical, a l t h o u g h the l e n g t h of t h e s e
Oct., 19o4.]
Notes and Comments.
26I
wires is m u c h greater. T h a t h e i g h t is a b o u t the limit to be o b t a i n e d in practice. Obviously it is not necessary t h a t the tops of the two masts, or towers, should lie in a line not cut by the curvature of the earth, for reasons already explained. T h e distances a t t a i n a b l e are n e a r l y proportionate to the a m o u n t of e n e r g y t r a n s m i t t e d , a l t h o u g h the l e n g t h of wave used plays a considerable part. R o u g h l y speaking, the g r e a t e r the e n e r g y the longer the period of o s c i l l a t i o n of the r a d i a t o r circuit, and the g r e a t e r m u s t be the l e n g t h and the capacity of the a n t e n n a to be in resonance with such periods. No b e t t e r g r o u n d i n g conditions can be afforded t h a n those f o u n d on an iron-hull vessel floating in sea w a t e r . H e n c e the long distances which are often a t t a i n e d u n d e r such circumstances.
E L E C T R I C A L T R A N S M I S S I O N OF P I C T U R E S AND SCRIPT. A correspondent of ( L o n d o n ) N a t u r e refers as follows to a method and apparatus for this purpose lately described by Dr. Arthur Korn: The problem of distant electrical vision is one to which much speculation and e x p e r i m e n t i n g have been devoted. Before this problem can be attempted with any hope of success, however, the preliminary one of the electrical transmission of photographs over a distance has to be solved. This problem, it may be stated at once, has been mastered, and it is now possible to transmit photographs in this manner, and successful results have been obtained over telegraph and telephone lines 800 kilometers long. It does not need much consideration to see how important such a process would be for journalistic and police work if it could be industrially exploited, and it were possible simply to band a sketch or photograph in at the telegraph office and send the same as one now sends an ordinary telegram. The evening papers would be able then to publish photographs taken at the seat of war in Korea on t h e same day. Unfortunately, with the apparatus at present to be had, the time taken to transmit a half-plate photograph is half an hour. The cost of the use of a telegraph line of any length for half an hour would be, it is needless to point out, prohibitive. The lessening of the required time of transmission is, however, simply a matter of further development, and 11o good reason can be seen why in a few years' time the l:rocess should not be an adjunct to every existing telegraph line. The m e t h o d shortly consists of the following : A ray of light is made to pass systematically all over the transparent film to be transmitted. After passing t h r o u g h the film it impinges upon a selenium cell the resistance of which varies p r o p o r t i o , a t e l y to the amount of light which passes lhrough the pho-