Development, design and construction of electrical condensers

Development, design and construction of electrical condensers

Journal of The Franklin Institute D e v o t e d to S c i e n c e a n d t h e M e c h a n i c Arts Vol. 248 SEPTEMBER, 1949 No. 3 DEVELOPMENT, DESI...

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Journal of

The Franklin Institute D e v o t e d to S c i e n c e a n d t h e M e c h a n i c Arts Vol. 248

SEPTEMBER, 1949

No. 3

DEVELOPMENT, DESIGN AND CONSTRUCTION OF ELECTRICAL CONDENSERS * BY WILLIAM

DUBILIER t

A n y electrical a p p a r a t u s , h o w e v e r c o m p l e x , is c o m p o s e d , e s s e n t i a l l y , of one or m o r e d e v i c e s e m p l o y i n g inductance, resistance, or capacity. T h e y are the b u i l d i n g b l o c k s of e l e c t r i c a l e n g i n e e r i n g . T h e u s e of i n d u c t a n c e s p e r m i t t e d the d e v e l o p m e n t of t r a n s f o r m e r s , electromagnets, electric motors, generators and similar appliances. T h e u s e of r e s i s t o r s m a d e p o s s i b l e the d e v e l o p m e n t of electric l i g h t s , e l e c t r i c h e a t e r s , c o n t r o l s , a n d the l i k e . But m o d e r n electrical e n g i n e e r i n g w o u l d h a v e been i m p o s s i b l e w i t h out the u s e of c o n d e n s e r s - - m i l l i o n s of c o n d e n s e r s , of m a n y t y p e s a n d s i z e s - - s o m e of t h e m n o t m u c h l a r g e r than a m a t c h h e a d , o t h e r s a s big a s a r o o m . T H E C O M I N G OF T H E M O D E R N C O N D E N S E R

T h e c o n d e n s e r is a n old i n v e n t i o n b u t a r e c e n t d e v e l o p m e n t . T h e f i r s t electrical c o n d e n s e r , the L e y d e n jar, was i n v e n t e d in 1746 by D e e n V a n K l e i s t . I t was r e f e r r e d t o by B e n j a m i n F r a n k l i n as a n " a c c u m u l a t o r " of electrical e n e r g y . L a t e r , it was u s e d e x t e n s i v e l y in medical and wireless telegraph equipment. T h e Leyden jar remained p r a c t i c a l l y u n c h a n g e d for more than 200 y e a r s , r e t a i n i n g e s s e n t i a l l y the same shape, d e s i g n and construction. F o r t y y e a r s ago, in 1910, we a l r e a d y h a d a s i z a b l e electrical ind u s t r y - a l l of it b a s e d on electrical d e v i c e s e m p l o y i n g p r i m a r i l y only i n d u c t a n c e a n d resistance. T h e only p r a c t i c a l p o w e r c o n d e n s e r then a v a i l a b l e was the g l a s s L e y d e n jar, or its e q u i v a l e n t , the g l a s s p l a t e s . * Based on a p a p e r presented before the Seminar of Electrical Engineering, Princeton U n i v e r s i t y , P r i n c e t o n , N. J., February 28, 1949. 1 Founder and Technical Director, Cornell-Dubilier Electric Co., New York, N. Y. (Note---The Franklin Institute is not responsible for the statements and opinions advanced by contributors in the JOURNAL.) ~93

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[J. I~. I.

The telephone and telegraph industry, operating with only minute energies, had a primitive low-voltage, low-power paper condenser. For higher voltages and higher powers we used glass plates, or the equally inefficient, cumbersome, fragile Leyden jars. The standard, fragile glass jar was then used in the many thousands of high-tensi0n electro-medical machines, in thousands of wireless telegraph installations. R a d i o communications with ships at sea, and between the ships of all the navies of the world, all depended on the Leyden jars. Here is an interesting footnote to history. Up to the end of 1911, practically all of the Leyden jars were made in Germany. All commercial and military r a d i o installations used German-made Leyden jars. The German government subsidized the industry and discouraged foreign development. The British Navy realized that its entire communications system was dependent on a foreign power. When I was visiting England in 1911, the government invited me to assist them in finding a substitute for the Leyden jar. Thus the modern power capacitor was born. T~

R O C K Y R O A D OF P R O G ~ S S

We began with crude tools. We were lone workers, shaping the individual bricks of knowledge. Today, the edifice of knowledge is a towering vastness: new bricks are being added daily, raising it ever higher--and I doubt that a single h u m a n mind could encompass all the knowledge it contains. We did not know n e a r l y as much a b o u t electrons and a t o m i c structure as we do today; wave mechanics were n e a r l y two decades in the future. In the beginning, our progress was slow. We gained new knowledge experimentally, by trial and error. This gave us experience and developed our imagination. Experience and imagination make an ideal marriage. The rocky road of progress is hard. There are many obstacles to be overcome, and the inertia of public opinion is not the least of these. If radio broadcasting had been developed before the telephone, and then someone had invented a method of making broadcasting secret, by simply guiding the waves along a thin metal wire, he would have received public acclaim. A little more than a century ago, when trains were first developed, a speed of twelve miles an hour was considered the limit of h u m a n endurance. The medical profession, newspapers and others, were quick to point out t h e i r terrible menace and danger to humanity. They predicted that a speed of fifteen miles an hour would cause nosebleeds, deafness and even death. I recall that in my own boyhood, automobiles were prohibited in many sections: they frightened horses, and a speed of twenty-five

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miles an hour--"without tracks," as an old ordinance put it--was "dangerous." There were local laws to keep automobiles off the roads; and there was broken glass to cut our tires, and stretched wire to cut our necks. The man who first drove an automobile through Central Park in New York was arrested for disorderly conduct. Long ago, when I was a lad, a bearded German professor named Roentgen discovered some mysterious X-rays which made it possible to photograph bones in the living body. The now-defunct P a l l M a l l Gazette, of London, was horrified. It protested against what it called

FIG. 1.

Representative group of small electrical condensers showing variety of shapes and sizes.

the "revolting indecency of making pictures of our insides," and hoped something would be done to "thwart the shameless experimenters who were beginning to supply the discovery." This newspaper stated that it would be possible to see through the clothes of people as they walked in the streets. We, too, had similar obstacles to overcome. The condenser appears to be the simplest of all electrical devices--a pair of conducting sheets separated by insulating layers. Actually, the condenser is probably the most complex of the electrical elements.

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WILLIAM DUBILIER

[J. F. I.

It has more hidden problems than any other electrical device, and is more difficult to produce. Today, we take our condensers for granted, and expect them to do t h e i r job with precision. We can do so only because m u c h ingenuity and many inventions resulting from patient research went into t h e i r design and construction. The eventual adoption of the new compact and efficient power capacitors permitted the development of many branches of the electrical industry, with its high-frequency equipment and vacuum tube applications. Powerful broadcasting and communications stations, and other high-frequency equipment, would have been impractical because of the prohibitive size, cost and inefficiency of the glass jar. THE MICA CAPACITOR

The first development that broke the German Leyden-jar monopoly was our mica condenser. Up to 1910, all technical publications and text-books were in agreement that mica was, theoretically, a more efficient and suitable material than glass. Yet there was no practical mica condenser, one that could withstand for long periods of time high voltages or high power without deterioration and breakdown. The phenomena of corona, hysteresis, eddy currents, ionization, and above all mechanical losses in capacitors, were either misinterpreted or unknown. In those days whenever higher voltages were involved, it was the customary practice to make the insulating material thicker and larger. I found that corona, or brush discharge, was particularly destructive, and that it invariably started at potentials of around 1000 volts. To overcome this, I did not " m a k e it thicker and larger." We made and tested hundreds of condensers, employing materials of various thicknesses, applying to them different potentials. These condensers were then carefully dissected and examined. Tests showed that, at five t i m e s the thickness of the insulating material, the corona began at less than two t i m e s the previous corona potential. I therefore concluded that the voltage across any two adjacent electrodes must always be less than 1000, irrespective of the total voltage applied to a condenser. This was the origin of the corona-free condenser. A f t e r the corona was eliminated, some condensers still became hot and broke down. We found that in some cases certain spots in the armatures became discolored because of concentrated heat. I recalled my school days and the teacher's demonstration of the "talking book" telephone. The "talking book" was a loosely arranged condenser made by placing light metal foils between the pages of a book. When connected to a microphone and a battery, the book "talked." I connected the book across a 60-cycle supply, and it gave

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D E V E L O P M E N T OF ~ L E C T R I C A L CONDENSERS

FtG. 2.

FICA. 3.

An automatic machine for making mica condensers.

Finished condensers being ejected from the machine shown in Fig. 2.

i97

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WILLIAM DUBILIER

[J. F. I.

a t r e m e n d o u s h u m . I r e c a l l e d t h a t I t r i e d t o stop it by p r e s s i n g the b o o k with m y h a n d , a n d soon f o u n d t h a t the b o o k b e c a m e h o t . I t was this s c h o o l l e s s o n t h a t h e l p e d m e t o e l i m i n a t e the h o t s p o t s . I r e a s o n e d t h a t , w h a t e v e r the f r e q u e n c y , be i t a h u n d r e d t h o u s a n d o r half a m i l l i o n c y c l e s i n s t e a d of s i x t y or a few h u n d r e d , a m i n u t e m o v e -

FIG. 4.

The author, with a large mica condenser rated at 2250 KVA. for operation at 15,000 volts, 150 amperes.

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ment of the electrodes resulted in h e a v y power losses. Hence, it was necessary to obtain an intimate contact between the foil electrodes and the mica sheets. I found that the surface of mica sheets, although it appears perfectly smooth to the eye, is in reality full of microscopic hills and dales. Therefore, a special type of soft m e t a l foil was developed, one that would flow and imbed itself into the fine crevices of the mica sheets, ensuring an intimate contact throughout the condenser. This was the second m a j o r improvement which made the mica condenser practical. With h e a v y currents, such as are encountered in high-frequency equipment, X2R loss b e c a m e a serious factor. To reduce this loss to a minimum, changes were made in the shapes, sizes, and assembly. The u n i t s were evacuated and impregnated with certain compounds at various temperatures. We were able to reduce the size of the mica condenser to less than 10 per cent of the equivalent Leyden jar, and the losses by more than 90 per c e n t . Ours being a new development, we were subjected to most rigid specifications. Although the size (and therefore the radiating surface) was considerably reduced, and consequently heat generation due to losses was also reduced, instead of allowing us a temperature rise equivalent to that allowed in Leyden jars, we were limited to 10° C., in places where the equivalent Leyden jar b e c a m e so hot that many times the-glass melted, and cooling fans were therefore necessary. Not only did we reduce the overall dimensions of the capacitor, but we also eliminated a g r e a t deal of such associate equipment. The importance of this can be realized when we recall that in a radio installation on b o a r d a battleship, Leyden jars usually occupied more than 50 per cent of the equipment space. For a while, the government distrusted our new and revolutionary condenser, as lives and s h i p s depended on its continued safe operation. To meet the exacting government specifications, we produced u n i t s where the heat radiation was equal to the heat generation, and thermal s t a b i l i t y was reached within less than 10° C. T o d a y , it is a source of some satisfaction to us that every radio broadcasting station, every radio transmitting station on land, at sea or in the air, and o t h e r high-frequency electrical equipment use mica condensers as originated by us f o r t y years ago. PAPER CONDENSERS

P a p e r condensers employ impregnated paper as the dielectric. P a p e r condensers look q u ite simple, but their appearance is deceiving. They are extremely difficult to engineer and produce. Consider the paper that serves as the dielectric. Many of our capacitors contain enough paper to cover the walls of a room. A

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[J. F. I.

single microscopic defect, a mere pinpoint, will cause the entire condenser to fail. In a m o d e m condenser factory, the materials controls which a p p l y to the p a p e r begin with the manufacturer of the paper itself. He must carefully control the chemical content of the water used for washing the pulp from which the paper will be made. He must control, within close limits, the ash content, the acidity, the alkalinity, the porosity, and the residual moisture in the finished product. M i n u t e metal particles from paper-making machines find t h e i r way into the paper, impairing its dielectric efficiency. P a p e r thickness is exceedingly important, as the cost and the bulk of condensers increase as the square of the thickness.

FIG. 5.

W i n d i n g p a p e r condensers.

The design of metal electrodes, and the composition of the metal foil, are equally important. U n d e r certain operating conditions, metal particles detach themselves from the foil and penetrate the paper, resulting in disintegration and breakdown of the condenser. Aluminum foil has been found to minimize such ionic effects. P a p e r condensers are impregnated with various insulating materials, such as waxes, oils, chlorinated diphenyl preparations, and others. These must be constantly tested before using, and must be free from contamination. Blending some of the impregnants results in improve-

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2OI

m e n t s ;-other b l e n d s p r o d u c e poor r e s u l t s . C o n d e n s e r s are i m p r e g n a t e d u n d e r v a c u u m , a n d the p r o d u c t i o n c y c l e s o m e t i m e s t a k e s more t h a n 150 hr. T h e v a c u u m is a s low a s 100 m i c r o n s , a n d the t e m p e r a t u r e s are a b o u t 250 ° F. T h e m a n u f a c t u r e of p a p e r c o n d e n s e r s is a controlled p r e c i s i o n o p e r a tion from s t a r t t o finish. M a t e r i a l s m u s t be k e p t free from c o n t a m i n a t i o n . W e m u s t even g u a r d o u r s e l v e s a g a i n s t the e f f e c t of p e r s p i r a t i o n of the a s s e m b l y w o r k e r s . T h e f i b r o u s p a p e r a n d the i m p r e g n a n t s a l i k e h a v e a g r e a t affinity for m o i s t u r e . U n l e s s t h o r o u g h l y s e a l e d , t h e y can a b s o r b a g r e a t d e a l of m o i s t u r e from the a t m o s p h e r e . On a n o r d i n a r y h u m i d day, in a few h o u r s the u n i t s m a y b e c o m e u n f i t for use. T o g u a r d a g a i n s t m o i s ture a b s o r p t i o n , p a p e r c o n d e n s e r s are a s s e m b l e d a n d i m p r e g n a t e d u n d e r c a r e f u l l y c o n t r o l l e d c o n d i t i o n s , a n d then h e r m e t i c a l l y s e a l e d . T h e seal itself m u s t w i t h s t a n d the i n t e r n a l e x p a n s i o n a n d c o n t r a c t i o n over a wide t e m p e r a t u r e r a n g e . T h e r e are m a n y o t h e r , u n s e e n d a n g e r s . A s l i g h t l y d e f e c t i v e cond e n s e r m a y o p e r a t e satisfactorily for a l e n g t h of t i m e , b u t u n d e r p o w e r s t r a i n s , i n t e r n a l h e a t is g e n e r a t e d due t o the v a r i o u s l o s s e s . U n l e s s p r o p e r p r e c a u t i o n s are t a k e n , f a t t y a c i d s m a y be f o r m e d i n s i d e the u n i t . In m a n y i n s t a n c e s , i n h i b i t o r s m u s t be a d d e d t o the i m p r e g n a n t s . T h e c o m p l e t e d u n i t is t e s t e d a t a p p r o x i m a t e l y t w i c e its n o r m a l o p e r a t i n g v o l t a g e , a n d its t e r m i n a l s a t five t i m e s . It is then p a s s e d t h r o u g h long h e a t e d o v e n s for d e t e c t i o n of m i n u t e f l a w s . F i n a l l y , the u n i t is g i v e n a n a c c e l e r a t e d h e a t test u n d e r p o w e r o v e r l o a d . T h e s e are only s o m e of the p r o b l e m s in p a p e r c o n d e n s e r m a n u f a c t u r e . M a n y o t h e r , u n f o r e s e e n p r o b l e m s a r i s e , w h i c h m u s t be s o l v e d . H e r e is a n i n t e r e s t i n g p u z z l e t h a t g a v e us m a n y a s l e e p l e s s n i g h t . W e m a d e a l a r g e n u m b e r of p a p e r c o n d e n s e r s , r a t e d a t 600 v o l t s , for a n e w h i g h - v o l t a g e a n o d e s u p p l y s o u r c e o p e r a t i n g over long c o n t i n u o u s p e r i o d s . B e f o r e s h i p m e n t , o u r c o n d e n s e r s w e r e all t e s t e d a t 6000 v o l t s . T h e b r e a k d o w n t e s t s s h o w e d m o r e t h a n 10,000 v o l t s . T h e c o n d e n s e r s w e r e i m p r e g n a t e d with a refined paraffin wax, a s we h a d p r a c t i c e d succ e s s f u l l y for m a n y y e a r s , a n d u n d e r n o r m a l c o n d i t i o n s w o u l d h a v e l a s t e d a l m o s t indefinitely. A few m o n t h s l a t e r , m a n y of t h e s e c o n d e n s e r s f a i l e d in s e r v i c e a n d w e r e r e t u r n e d t o us. W e w e r e m y s t i f i e d . I n v e s t i g a t i o n s h o w e d t h a t d a r k s t r e a k s a n d s p o t s h a d d e v e l o p e d on the p a p e r i n s i d e the c o n d e n s e r s . I t took m u c h t i m e a n d w o r k t o a n a l y z e a n d s o l v e this r i d d l e . W e d i s c o v e r e d t h a t w h e n the c o n d e n s e r u n i t s w e r e r e m o v e d from the i m p r e g n a t i n g t a n k s , the o u t s i d e of the u n i t s c o o l e d r a p i d l y a n d f o r m e d a h a r d s o l i d shell. B u t the c e n t e r r e t a i n e d its h e a t for a m u c h l o n g e r p e r i o d , k e e p i n g the w a x soft a n d f l u i d . A s the c e n t e r f i n a l l y c o o l e d and c o n t r a c t e d , m i n u t e s p a c e s w e r e f o r m e d internally, s e a l e d b y the s o l i d o u t e r c r u s t . G a s e s filled t h e s e s p a c e s , a n d the g a s e s g l o w e d

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a t p o t e n t i a l s of 200 v o l t s or l e s s . Our c o n d e n s e r s , in effect, c o n t a i n e d m i n i a t u r e glow l a m p s . W h e n s u b j e c t e d t o long c o n t i n u e d p e r i o d s of o p e r a t i o n , e n o u g h h e a t was g e n e r a t e d t o d e s t r o y the i n s u l a t i o n , c a u s i n g b r e a k d o w n s . In add i t i o n , the u n e v e n c o n t r a c t i o n of the w a x also i m p o s e d s e v e r e m e c h a n i c a l s t r a i n s a n d d i s t o r t i o n a l o n g the i n s u l a t i n g l a y e r s . T h u s , c o n d e n s e r s m a d e t o o p e r a t e r e l i a b l y a t t h o u s a n d s of v o l t s m a y b r e a k down a t a p o t e n t i a l of a few h u n d r e d v o l t s , u n l e s s p r e c a u t i o n s are t a k e n t o prevent ionic discharges within t h e u n i t . T h i s was a s e r i o u s p r o b l e m , a n d one t h a t we h a d t o o v e r c o m e a t o n c e . F i n a l l y , one of m y a s s i s t a n t s s u g g e s t e d c o o l i n g the c o n d e n s e r s u n d e r oil. T h i s a l l o w e d the e n t i r e i m p r e g n a t e d m a s s t o cool w i t h o u t the h a r d s h e l l . T h i s c o o l i n g p r o c e s s i n c r e a s e d the life of the c o n d e n s e r t e n f o l d , a n d its r a t e d safe o p e r a t i n g v o l t a g e from t h r e e t o five t i m e s . T h i s s i m p l e m a n u f a c t u r i n g i m p r o v e m e n t p r o v e d t o be one of the m o s t i m p o r t a n t a n d r e v o l u t i o n a r y d e v e l o p m e n t s in w a x - i m p r e g n a t e d c o n d e n s e r s , a n d now is u n i v e r s a l l y u s e d in the m a n u f a c t u r e of h i g h - v o l t a g e c a p a c i t o r s . ELECTROLYTIC

CONDENSERS

Up t o 1925, one of the b a s i c p r e c a u t i o n s t a u g h t t o e v e r y c o n d e n s e r w o r k e r was t o be a l w a y s on g u a r d a g a i n s t a n y risk of c o n t a m i n a t i o n of c o n d e n s e r m a t e r i a l s b y o u t s i d e c h e m i c a l s - - a l k a l i s or a c i d s . W e k n e w , from o u r own sad experiences, t h a t a s i n g l e d r o p of w a t e r or acid in a g a l l o n of i m p r e g n a t i n g oil o r w a x w o u l d m a k e the e n t i r e b a t c h of m a t e r i a l u n f i t for use. W e w e r e so t h o r o u g h l y t r a i n e d in s t r i c t p r e c a u t i o n s a g a i n s t c h e m i c a l c o n t a m i n a t i o n of o u r c o n d e n s e r m a t e r i a l s , t h a t the v e r y i d e a of m a k i n g a chemical c o n d e n s e r s e e m e d a t f i r s t f r i g h t e n i n g . A l t h o u g h we k n e w t h a t m e t a l l i c o x i d e s w e r e good i n s u l a t o r s , we a l s o k n e w t h a t s t r o n g a c i d s a r e n e e d e d for the f o r m a t i o n of s u c h o x i d e s on a l u m i n u m f o i l - - a n d s t r o n g a c i d s c o u l d n e v e r be t o l e r a t e d in or n e a r a c o n d e n s e r p l a n t . T h e s e w e r e the m e n t a l r o a d b l o c k s t h a t the n e w electrolytic cond e n s e r h a d t o o v e r c o m e . Yes, we w e r e slow a n d c a u t i o u s in a c c e p t i n g the electrolytic c o n d e n s e r . B u t t o d a y , i t h a s f o u n d its r i g h t f u l p l a c e in i n d u s t r y , e s p e c i a l l y for s t a r t i n g s m a l l a-c. m o t o r s , a n d in filtering circuits. A l t h o u g h t h e electrolytic c o n d e n s e r s e e m s t o the eye t o be m u c h more c o m p l e x , the p r o b l e m s of its m a n u f a c t u r e are n o w h e r e n e a r a s g r e a t a s t h o s e of m i c a a n d p a p e r c o n d e n s e r s . CERAMIC CONDENSERS

D u r i n g the F i r s t W o r l d W a r , the G e r m a n s p u r c h a s e d , t h r o u g h H o l l a n d , a s m a l l n u m b e r of o u r t r a n s m i t t i n g c o n d e n s e r s , a n d c o p i e d t h e m , J a p a n e s e f a s h i o n . T h o s e w e r e s h i p p e d by us in 1916, a n d I k n e w t h a t t h e y w e r e d e s t i n e d for G e r m a n y . A f t e r t h e W a r , in 1920,

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D E V E L O P M E N T OF E L E C T R I C A L CONDENSERS

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I visited Berlin. The Telefunken Company, which was then the largest radio organization in the world, showed me thousands of units made exactly.like ours. Most of them failed a f t e r a few weeks' service. I knew they would fail--another proof that experience is necessary for Success.

Ceramic condensers originated in England, a b o u t a quarter of a century ago. However, they received t h e i r highest development in Germany, a f t e r the First World War again with the aid of financial subsidies from the German government. At the end of the First World War, the new mica condensers were used extensively, in transmitters and o t h e r electrical equipment. Germany had no mica supply of her own. The best mica came mostly from England and India. Therefore, Germany heavily subsidized and encouraged the development of ceramics to replace mica. The resultant ceramics were much stronger and had lower losses than the old wet-process porcelains. Units were made of many materials, such as magnesium, and l a t e r the titanates. T h e i r production required higher firing temperatures and closer manufacturing tolerances and controls. The development of low-loss ceramic insulators, particularly those with high-capacitance possibilities, soon affected the electric power industry. Ceramic condensers were gradually improved until the overall efficiency of the best German-made condensers during the last War was about equal to mica condensers of similar dimensions. Since then, American manufacturers concentrated on the development of titanates, mostly for use in low-voltage applications. Titanate compounds have the advantage of possessing a large capacity, anti occupy but a small space. We have made some ceramic condensers with a K as high as 500,000. The losses, however, were more than 30 per cent. (The K of mica is a b o u t 7; that of paper, around 4; of paraffin, around 2.) Improvement in ceramic condensers has been rapid. At present, large quantities of ceramic condensers are being made, with a K of between 3000 and 5000, with losses as low as 2 per cent. Recent developments in low-loss ceramics show most encouraging results. W H A T O F THE

FUTURE?

The future progress in electricity and radio is inseparable from progress in condenser design. Inductances and resistances have never presented serious design or manufacturing problems: they are not likely to present them in the future. But the third of the building blocks of electrical engineering-the condenser--remains a limiting factor in applied electrlcal and radio engineering.

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WILLIAM DUBILIER

[J. F. L

F o r t y y e a r s ago, o u r h o r i z o n s l i m i t e d by the old L e y d e n jar, we c o u l d n o t f o r e s e e the v a r i e t y , the c o m p l e x i t y , the u t i l i t y of m o d e r n electrical a p p a r a t u s . N e w a n d b e t t e r c o n d e n s e r s w e r e a m a j o r f a c t o r in m a k i n g the m o d e r n electrical a n d r a d i o i n d u s t r y p o s s i b l e . T o d a y , f o r t y y e a r s l a t e r , o u r eyes c a n see f a r t h e r , a n d o u r v i s i o n is clearer. W i t h o u r m i n d ' s eye we can see the m a g n i f i c e n t v i s t a s w h i c h the d e v e l o p m e n t of n e w e r a n d b e t t e r c o n d e n s e r s m a y b r i n g a b o u t c o m p a r a t i v e l y few y e a r s from now. T h e r e a l w a y s is s o m e t h i n g n e w e r a n d b e t t e r w a i t i n g t o be d e v e l o p e d . F o r t y y e a r s ago, t h e n e w c o m p a c t m i c a c a p a c i t o r r e p l a c e d the L e y d e n jar. T o d a y , its s u p r e m a c y is a l r e a d y t h r e a t e n e d by the n e w e r t y p e s , s u c h a s for i n s t a n c e the c e r a m i c c a p a c i t o r , a n d the y e t - n e w e r v a c u u m t u b e t y p e . A n d y e t , s u c h is the excellence of the m o d e r n m i c a cond e n s e r t h a t it will n e v e r be e l i m i n a t e d or s u p p l a n t e d by o t h e r s ; r a t h e r , the n e w e r c a p a c i t o r s will open n e w f i e l d s , will m a k e p o s s i b l e n e w app a r a t u s a s y e t in the d r e a m s t a g e . I t is a t h a n k l e s s t a s k t o p r o p h e s y ; but, g i v e n c a p a c i t o r s s m a l l e r a n d m o r e efficient t h a n the b e s t we h a v e t o d a y , s u c h o b v i o u s n e w d e v i c e s a s v e s t p o c k e t t e l e p h o n e t r a n s m i t t e r s , c a p a b l e of r e a c h i n g a r e l a y s t a t i o n a few m i l e s a w a y , w o u l d be a p r a c t i c a l possibility. T h o s e s a m e c o n d e n sers w o u l d also p e r m i t us n o t m e r e l y t o b o u n c e a few r a d a r p u l s e s a g a i n s t the s u r f a c e of the m o o n , a s was d o n e some two y e a r s ago, b u t t o m a i n tain c o m m u n i c a t i o n s w i t h the s p a c e s h i p s w h i c h will i n e v i t a b l y v e n t u r e f o r t h in a few y e a r s , r a n g i n g f a r t h e r a f i e l d t h a n the m o o n , i n t o the s o l a r system, perhaps beyond.