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Constant shunt method for the measurement of large continuous currents
476
Electrical
Section.
[J. ~'. I.,
C O N S T A N T S H U N T M E T H O D FOR THE M E A S U R E M E N T oF LARGE C O N T I N U O U S C U R R E N T S . BY CLAYTONW. PIKE. [Read at lhe stated meeting of the Section, held ~Warch z, zggz. ]
T h e constant, V i e n n a or Munich, s h u n t m e t h o d , as it is v a r i o u s l y termed, is s o m e t h i n g w h i c h is b y no m e a n s n o v e l and is, indeed, well known to some of the section, h a v i n g b e e n used b y t h e m at the Electric E x h i b i t i o n of the Institute, in I884. B u t I b e l i e v e t h a t the m e t h o d is not so well known, or at least not so m u c h in use as it should be, and I hope, b y calling a t t e n t i o n to some of its a d v a n t a g e s .and by p o i n t i n g o u t and s h o w i n g h o w to avoid certain errors incident to it, one or two of w h i c h are n o t g e n e r a l l y known, to do s o m e t h i n g t o w a r d b r i n g i n g it into more general use. F o r the m e a s u r e m e n t of c o n t i n u o u s currents we have the v a r i o u s forms of ammeter, the S i e m e n s d y n a m o m e t e r , the T h o m s o n platform g a l v a n o m e t e r s , and the T h o m s o n balances, the t a n g e n t sine and cosine g a l v a n o m e t e r s . W i t h the o b j e c t i o n s to the a m m e t e r s all are d o u b t l e s s familiar. T h e y are i n a c c u r a c y of g r a d u a t e d scale, errors due to residual m a g n e t i s m or to loss of p e r m a n e n t m a g n e t i s m , and lack of wide range. T h e T h o m s o n p l a t f o r m galvan o m e t e r does not m e a s u r e large currents w i t h o u t the use of the p e r m a n e n t m a g n e t w h i c h is o b j e c t i o n a b l e if considerable a c c u r a c y is desired, and it r e q u i r e s a k n o w l e d g e of t h e horizontal c o m p o n e n t of the earth's m a g n e t i s m H. T h e T h o m s o n b a l a n c e is an i n s t r u m e n t of m u c h g r e a t e r v a l u e than the above, b e i n g i n d e p e n d e n t of H, h a v i n g g r e a t e r r a n g e and g r e a t e r precision, b u t b e i n g even more costly. T h e t a n g e n t g a l v a n o m e t e r requires the v a l u e of H, and has only a limited range, w h i c h the sine and cosine, t h o u g h b e t t e r in this last respect, are t e d i o u s to use on a c c o u n t of time require d in s e t t i n g the coils. T h e g r e a t a d v a n t a g e of the s h u n t m e t h o d over all t h e s e is that, b y the aid of fairly simple and cheap a p p a r a t u s , c u r r e n t s v a r y i n g from one to I,ooo a m p e r e s or m o r e m a y
June, t89z,]
Electrical Section.
477
be m e a s u r e d with q u i c k n e s s and a v e r y g r e a t degree of a c c u r a c y over the w h o l e range. T h e s h u n t m e t h o d is really a comparison method, and d e p e n d s u p o n Ohm's l a w that the potential difference b e t w e e n two ends of a resistance is proportional to the c u r r e n t flowing t h r o u g h t h e resistance. L e t A / ~ be a c o n d u c t o r w h o s e t e m p e r a t u r e is k e p t constant, its resistance will therefore be constant, b u t need n o t be known. Pass t h e c u r r e n t C, which we desire to m e a s u r e t h r o u g h A B, t h e r e will then be a p o t e n t i a l difference P D b e t w e e n A and/3. L e t G b e a mirror g a l v a n o m e t e r with telescope and scale, then the deflection d i s proportional to the current t h r o u g h it, w h i c h is proportional to P D, as long as resistance .of galv a n o m e t e r and leads is constant, di ~ P D~ or d~ a C1. Pass a n y o t h e r current G t h r o u g h A B then d~ a C~ and
4 If the resistance of the g a l v a n o m e t e r and leads be n o t c o n s t a n t , t h e algebraieal result is less simple and, m o r e o v e r if the deflections b e c o m e v e r y large, t h e y are n o t strictly proportional to the c u r r e n t t h r o u g h the g a l v a n o m e t e r which we will call C~-. H e n c e the comparison of large with small c u r r e n t s will be s o m e w h a t inaccurate. T h i s leads us to a little different m e t h o d . Place a resistance box R in the g a l v a n o m e t e r circuit. P a s s c u r r e n t 6; t h r o u g h A/~, c a u s i n g a potential difference Y Dj, a n d s e n d i n g t h r o u g h G a current P/3~ C1 g - R1 + g w h e r e g is the g a l v a n o m e t e r resistance. Then
,ti a C , g a P D___~_~ R1 + g or
G a 4 (R, + g) in the s a m e w a y
G a 4 (& + g)
478
Electrical Section.
[J. F. I.,
then
c, _ d, (R, + g) G 4 (x, + g) or
cl - e~ (xl + g ) c~ 4 (x~ + g) Now, if when C~ were flowing we had accurately meas~lred
it by a tangent galvanometer or by the electrolytic cell, the quantity
G
/2 (R2 + g) would be known. Call it K the constant of the shunt. Then we have the general formula for the shunt that the current whose value we desire is equal to
d (R, + g) K Now, by using the resistance box, and by adjusting it to
June. I892. ]
Electrical Section..
479
give a b o u t the s a m e d e f e c t i o n s on t h e g a l v a n o m e t e r in different m e a s u r e m e n t s we h a v e secured two i m p o r t a n t advantages. (i) W e h a v e done a w a y w i t h the error of a s s u m i n g t h a t the c u r r e n t s t h r o u g h the g a l v a n o m e t e r was strictly proportional to its deflections. (2) T h e resistances R b e i n g large in v a l u e with reference to g a l v a n o m e t e r and leads, and b e i n g of platinoid or some metal with v e r y low t e m p e r a t u r e coefficient, a n y c h a n g e in t e m p e r a t u r e of g a l v a n o m e t e r and leads will p r o d u c e little effect on the result. Like all other m e t h o d s this one is liable to certain errors, some of which I will m e n t i o n and briefly s u g g e s t their remedies. T h e w a y in which t h e y affect the result will also be stated. (i) Reading of Deflection.--Suppose y o u could read to w i t h i n "ol cm. on the scale. If the deflection were I o c m . the error w o u l d be "OI --
Io
i
part in i,ooo.
T h e error is e v i d e n t l y d i m i n i s h e d as we increase t h e deflection. (2) Error Due to Tem~Oerature Change of Galvanometer and Leads.--If t h e s e are of copper their resistance w o u l d c h a n g e four-tenths per cent. p e r d e g r e e Centigrade. T h i s correction could be applied, b u t if the resistance in R is large with respect to g, this correction will be negligible; (2 °) then conflicts with (I°), and we m u s t compromise. S u p p o s e R: : 20 g -~ 2, and t e m p e r a t u r e rise = :0 ° C. then the true value of R 1 -j- ff ~--- 20 -q- 2 -[ 2 >< 4 X I0~ I00
22"08
and the error in n e g l e c t i n g the '08-
"08 22
t in 275
(3) Error Due to Temperature C/tange o f Bow.--This is "02 per cent. per d e g r e e C e n t i g r a d e for platinoid coils, and m a y be
480
[J. F. 1.,
. Electrical Section.
corrected for, or neglected, according to the desired precision of our results. (4) Error Due to Determining Constant R ' . - - T h e e o n s t a n t is best d e t e r m i n e d in m o s t cases by electrolytic cells, sos; three in series, If one has a t a n g e n t g a l v a n o m e t e r of proper range, I-5 amperes, and knows H to the desired a c c u r a c y this m a y be used. A T h o m s o n deci-amp~re balance would b e v e r y conv e n i e n t for s u b s e q u e n t d e t e r m i n a t i o n s of the constant, if its r e a d i n g s were k n o w n to be correct w i t h i n the desired degree of accuracy. A n error of 'i per cent in K c a u s e s an equal error in the s u b s e q u e n t m e a s u r e m e n t s . (5) Error Z)ue ~o a Rise in Temperature o f tile S h u n t . - - T h i s m a y be due to two causes: a rise in the t e m p e r a t u r e of the room, or the h e a t i n g effect of the current. Both m a y be obviated by p l a c i n g the s h u n t in liqtiid artificially cooled
II II
--7
A
B F I G . I.
b y cold w a t e r coils, and k e e p i n g c o n s t a n t the r e a d i n g of a t h e r m o m e t e r placed in the liquid close to the s h u n t . Suppose the rise in t e m p e r a t u r e were 5 ° C. If the s h u n , were of G e r m a n silver the c h a n g e in its resistance would be "o4 per cent. X 5 = '2 per cent. or two parts in i,ooo, and the results of the m e a s u r e m e n t would be in error by tfiis a m o u n t . If of platinoid, the error w o u l d be b u t half as much. (6) Error l)ue to Change o f Value o f H . - - T h i s w o u l d be possible if the g a l v a n o m e t e r h a d the e a r t h ' s field for a r e s t i n g force. If g r e a t accuracy be desired, obtain the value of the c o n s t a n t frequently, a n d see if it c h a n g e s . As I have before said a T h o m s o n balance w o u l d be m o s t desirable for this, a n d m a n y laboratories can afford one T h o m s o n balance. Also if we h a d a g a l v a n o m e t e r whose d a m p i n g could be removed, we could d e t e r m i n e w h e t h e r H h a d c h a n g e d b y its t i m e of vibration. T h e sources of
June. 1892.]
Electrical Section.
48 t
error so far m e n t i o n e d are well u n d e r s t o o d and looked out for, b u t the following are, I believe, n o t so well known. A t least t h e a m o u n t of the effect is not so g e n e r a l l y appreciated, and t h e y have, so far as I know, received e x h a u s t i v e s t u d y only a.t the h a n d s of Mr. W, L. Puffer, of the M a s s a c h u s e t t s I n s t i t u t e of T e c h n o l o g y . H i s results are to be f o u n d in the Proceedings a f tlze American Academy o f Arts and Sciences, J a n u a r y , 1888. (7) Error Due to Tlwrma-electric Effects.--We are obliged to m a k e the resistance of our s h u n t v e r y l o w to p r e v e n t u n d u e heating, hence the potential differences set up at its termi• rials ar.e v e r y low. In this case a n y t h e r m a l E. M. P.'s, which m i g h t be caused b y differences of t e m p e r a t u r e at the j u n c t i o n s of t h e different metals, w o u l d be liable to affect the readings of t h e g a l v a n o m e t e r . This trouble m a y be o b v i a t e d b y so m a k i n g t h e s h u n t as to p u t the j u n c t i o n s u n d e r the surface of liquid w h o s e t e m p e r a t u r e !s m a i n t a i n e d c o n s t a n t a n d m a y be d e t e c t e d b y r u n n i n g the g a l v a n o m e t e r leads to a m e r c u r y cup c o m m u t a t o r and t e s t i n g with reversals. (8) Error Due to Electro-chemical A etion.--The shun t is placed in liquid and u n d e r the action of the p o t e n t i a l differences set up at its terminals t h e r e is electrolytic decomposition, (if the liquid be one which can be d e c o m p o s e d as water). T h i s will cause the end h a v i n g t h e h i g h e r potential to be ~tarkened, o w i n g to oxidization of the G e r m a n silver b y the oxygen set free, while t h e o t h e r end will b e c o m e w h i t e r or cleaner in appearance, o w i n g to the r e d u c t i o n of the film of okide on its surface b y electrolytic h y d r o g e n . T h e s e actions will e v i d e n t l y set up E. M. F.'s, and if the s h u n t resis.tance is v e r y low, will cause c o n s i d e r a b l e error i n the result. This source of error could be a v o i d e d b y placing the s h u n t in s o m e l i q u i d i n c a p a b l e of d e c o m p o s i t i o n w i t h low E. M. F.'s. Chemically pure w a t e r w o u l d do, b u t for the t r o u b l e of o b t a i n i n g and k e e p i n g it so. P e t r o l e u m oils w o u l d act Slightly on the b a r e piece of m e t a l c o m p o s i n g the shunt, t h u s p r o d u c i n g a b a t t e r y effect. W e m u s t either i m m e r s e the b a r in some liquid incapable of acting on it or cover the s h u n t v e r y carefully w i t h some s u b s t a n c e n o t acted upon VoL. CXXXIII. 3t
482
Elcctrica! SccNou.
[J. F. l.,
by the liquid in w h i c h it is placed. E v e r y t h i n g considered it seems to me t h a t very carefully covering t h e s h u n t with several coats of shellac a n d a f t e r w a r d s w i t h copal varnish, a n d i m m e r s i n g i t i n distilled wa£er would be the m o s t satisfactory m e t h o d . I)esz~o'u and Construction of Api~aratus.-- Suppose we desire to m e a s u r e c u r r e n t s of from one to I,OOO ampSres. This would require a set of five t a n g e n t g a l v a n o m e t e r s of different sizes, the three l a r g e s t of w h i c h would be impracticable. (The d i a m e t e r of the l a r g e s t w o u l d be a b o u t seventy-five feet.) W i t h the sine or cosine g a l v a n o m e t e r s or Siemens d y n a m o m e t e r we could do b e t t e r in this respect, b u t they are still out of the question. W i t h the T h o m s o n p l a t f o r m or balance we s h o u l d need three different i n s t r u m e n t s , w h i c h are e x c e e d i n g l y costly.
.
.
.
.
G
A
8 FIG. 2.
Moreover, none of these could be used n e a r d y n a m o s and this m e a n s t h a t we m u s t lead the whole of the c u r r e n t we wish to m e a s u r e p e r h a p s a considerable distance, t h u s e n t a i l i n g expense for wire and p e r h a p s leakage. Suppose we m a k e one s h u n t of resistance "ooi ~o. If of G e r m a n silver 9 inches wide, T ~ inch thick, two strips 24 inches long joined in parallel will be n e a r l y the r e q u i r e d dimensions. T h e object of m a k i n g t h e m so t h i n is to offer large surface to the w a t e r so t h a t t h e y m a y quickly dissipate the h e a t p r o d u c e d by the current. T h e rate of p r o d u c t i o n of h e a t w i t h the m a x i m u m c u r r e n t I,OOO, amperes, will be •24 C~ R = "24 X 1,ooo 5 X "0oi = 24o calories or e n o u g h per second to h e a t 240 g r a m m e s of w a t e r i o C. if none of the h e a t be dissipated. W e m u s t now provide a c o n t a i n i n g vessel large e n o u g h to hold so m u c h w a t e r t h a t it will not be
June. x892.]
t:•l~'ctrical Section.
483
u n d u l y raised in t e m p e r a t u r e , and large e n o u g h to a d m i t of placing in it coils of pipe which shall carry cooling w a t e r if this be f o u n d necessary with the l a r g e r currents. • T h e two German-silver strips should be joined t h u s b y massive cast copper blocks, to w h i c h t h e y are carefully soldered. To m a k e the j u n c t i o n d o u b l y s e c u r e , thin strips of copper 9 x i x ¼ inch are screwed a n d soldered on over t h e G e r m a n silver, as shown. T h e vertical p r o j e c t i n g copper bars D and F are for c o n n e c t i n g the s h u n t into the circuit where we wish to m e a s u r e the current. T h e a r r a n g e m e n t so far described would collapse, owing" to the t h i n n e s s of the G e r m a n silver. So it is necessary to hold the copper blocks a p a r t at the top a n d b o t t o m by some a r r a n g e m e n t w h i c h shall be i n s u l a t e d from the s h u n t . T h i s can be done b y s c r e w i n g on to b o t h top and b o t t o m of the copper blocks~ a piece of board z4 x 4 x I inch, as shown in drawing. T h e board on top should be pierced w i t h a large n u m b e r of holes, as shown, in order to facilitate the circulation of water• T h e ends of the boards t o u c h i n g the blocks should be soaked t h o r o u g h l y in m e l t e d paraffrri arid coated w i t h shellac. T h e Screw holes in t h e b l o c k s should be filled w i t h paraffin, the screw h e a d s c o u n t e r s u n k in the wood, as shown, and melted paraffin p o u r e d over them. T h e whole a r r a n g e m e n t should t h e n be coated w i t h shellac and copal v a r n i s h thorolto~hly. T h e c o n t a i n i n g vessel m a y be of wood, paper or galvanized iron, a b o u t 3 2 x I 4 x I3 inches. Such a vessel will contain, besides the shunt, a b o u t 85,ooo g r a m m e s of water. Now, the n u m b e r of h e a t units, small calories, developed in one second b y i,ooo a m p e r e s flowing t h r o u g h "ooi ~o is in one second as before stated, such as would h e a t 24ogTammes of w a t e r 15 C. H e n c e i t w o u l d take to h e a t 85,ooogrammes ~7"b-I °, a t i m e -83000 seconds, or a b o u t six minutes, if there were no radiation. But it is probable t h a t this h e a t p r o d u c e d will not be i n s t a n t l y c o m m u n i c a t e d to the entire mass of water, hence, for t h e large c u r r e n t m e a s u r e m e n t s it will be advisable to lead coils c a r r y i n g cold w a t e r close to the German-silver strips, b o t h inside a n d outside, if we desire g r e a t a c c u r a c y in our m e a s u r e m e n t s .
484
Electrwal
[J. F. I.,
Section.
Galvanometer.--Any f o r m of s e n s i t i v e r e f l e c t i n g galvan o m e t e r w h i c h is d e a d h e a t and w o u n d w i t h coils of low r e s i s t a n c e m a y be used. If t h e i n s t r u m e n t has t h e earth's field alone for a r e s t o r i n g force, as is g e n e r a l l y t h e case, ( n e g l e c t i n g torsion), it w o u l d b e c o n v e n i e n t to h a v e the
o
o
o¢,~ .-
9J" °°
N,,
1
5"__1 0 " II
i
I~ t
6 a
'
~1
0
I I
I I
OC~
,~
I I
!
J I ¢;
~---- 6 -~" ~
11
FIG. 3. d a m p i n g c o p p e r m a s s easily r e m o v a b l e . T h e f o l l o w i n g will i l l u s t r a t e t h e d e s i g n : S u p p o s e t h e scale of telescope is d i v i d e d i n t o millimetres, a n d we can r e a d w i t h an e r r o r of "I ram. b y eye e s t i m a t i o n , t h e n w i t h a deflection Io cm. t h e e r r o r in r e a d i n g w o u l d be = I in i,ooo. If o u r scale w e r e d i s t a n t f r o m m i r r o r of t e l e s c o p e b y I" 5 m e t r es , t h e deflection of t h e m i r r o r for a r e a d i n g IO cm. w oul d be a b o u t 2 ° for i1~°~= t a n . 2 a = 2 t a n .
June, I892.J
Electrical
Section.
485
a a p p r o x i m a t e l y , t a n . a - ~ loOo - ~ "o33 a n d a t h e deflection of m i r r o r = I° 54 ~. W h e n I,ooo a m p e r e s are flowing, t h e p o t e n t i a l di fference a c t i n g to s end a c u r r e n t t h r o u g h t h e g a l v a n o m e t e r is IOO × "ooi or one volt. S u p p o s e we p u t in t h e gal vanom e t e r c i r c u i t Io, ooo ohms, t h e m a x i m u m of an o r d i n a r y box, th en t h e c u r r e n t t h r o u g h t h e g a l v a n o m e t e r will be "oooi amp&es. W i t h sufficient a c c ur acy, we m a y a p p l y t h e t a n g e n t g a l v a n o m e t e r f or m ul a , C = IO 11 ~- t a n . a 2
~lg
from w h i c h we h a v e
/ iott
~, t a n .
2 r, C
¢z
io × "2 >( ~- X .o33 2 × 3"1416 × "oooI
A s s u m e a m e a n r a d i u s / ' = 2"5 era., t h e n n t h e n u m b e r of t u r n s = 266 = 4I'78 m e t r e s or a b o u t I36 f e e t of wire. If we t a k e a c o p p e r wire w hos e d i a m e t e r b a r e is "o36 i nches t h e r e s i s t a n c e will be a b o u t i"i a, a n d t h e w i re will lie in a g r o o v e I × ~ inch, whose m e a n r a d i u s is 2"5 era., or a b o u t one inch. W h e n one a m p e r e goes t h r o u g h t h e s h u n t t he p o t e n t i a l d if f er en c e will be i X "ooi = I volt. T h e n to gi ve a b o u t IO cm. deflection we should i n s e r t s u ch a r e s i s t a n c e t h a t t h e g a l v a n o m e t e r c u r r e n t w o u l d be "oooi, and this r e s i s t a n c e w o u l d be a b o u t n i n e ohms. T h e n s h o u l d t h e t e m p e r a t u r e of t h e g a l v a n o m e t e r coil rise 5 o C., t h e r e s u l t i n g e r r o r w o u l d be b u t tw o p a r t s in 1,ooo f r o m this c a u s e alone. A g a l v a n o m e t e r of t h e D ' A r s o n v a l p a t t e r n m i g h t be used a n d wo u l d h a v e t h e a d v a n t a g e of b e i n g i n d e p e n d e n t of a n y c h a n g e s in 11, a nd of b e i n g v e r y d e a d heat. It s h o u l d be d e s i g n e d w i t h as low r e s i s t a n c e as is c o n s i s t e n t w i t h t h e c o n d i t i o n t h a t a c u r r e n t "oooi a m p e r e s shall p r o d u c e a deflection of 2 ° . T w o c o m m u t a t o r s are necessary. T h e l a r g e r recei ves t h e six coarse wire s o l d e r e d i nt o the c o p p e r Iugs D F of the s h u n t , s h o w n in F~g. 3. T h e s e wires s h o u l d be No. oooo ]3. a n d S. t h r e e in parallel, and t h e c o m m u t a t o r of t h e m e r c u r y cup
486
Electrical Sectton.
[ J. F. I.,
variety, so arranged t h a t the s h u n t can be entirely disconn e c t e d from the d y n a m o circuit. Into each of t h e h c l e s at f-f and K, Fig. 3, s h o u l d be soldered, carefully, a No. i8 B. and S. copper wire t h o r o u g h l y insulated, and the j o i n t c o v e r e d carefully with shellac and copal. T h e s e are led o u t to the small m e r c u r y c o m m u t a t o r for t h e g a l v a n o m e t e r Circuit a r r a n g e d like the o t h e r so t h a t t h e g a l v a n o m e t e r can be isolated from s h u n t and m a i n circuit. T h e n a n y t h e r m o electric or electro chemical effects can b e d e t e c t e d and studied. F o r instance, disconnect g a l v a n o m e t e r entirely from c o m m u t a t o r and note reading. Connect it to c o m m u t a t o r w i t h s h u n t uncon-
!
eq
J
I
'
~.
.... 5
L
..J
"1~
J.
Fro. 4neeted, and there s h o u l d be, of course, no deflection even b y r e v e r s i n g the c o m m u t a t o r . Connect s h u n t (still with no c u r r e n t in i t ) t o g a l v a n o m e t e r c o m m u t a t o r , and see if there is any deflection, r e v e r s i n g this t i m e the connection from s h u n t to g a l v a n o m e t e r . Finally, run c u r r e n t t h r o u g h shunt, a n d s e e w h e t h e r c h a n g i n g the c o m m u t a t o r s p r o d u c e s a n y c h a n g e in deflection, b e i n g sure t h a t this c u r r e n t is m a i n t a i n e d constant. D i s c o n n e c t s h u n t from d y n a m o circuit, and see w h e t h e r there is a n y appreciable deflection. T h e time occupied for these tests is v e r y short, if the comm u t a t o r s are properly arranged, and if the t e s t s s h o w no appreciable thermo-electric or chemical effects, we m a y feel
June, 1892 I
487
Electrical Section.
sure that o u r a p p a r a t u s is s a t i s f a c t o r y in this respect. Finally, provide a sensitive t h e r m o m e t e r w h o s e range is from a b o u t 60 ° F. to 80 ° F., or 15 ° ~. to 27 ° C., and a r r a n g e so t h a t it will be placed close to one of the German-silvEr strips of the shunt. R e a d w h e n no c u r r e n t is flowing, and then w h e n the m a x i m u m c u r r e n t flows. T h e n if these readings differ 5 ° C., or 9 ° F., the error w o u l d b e two per cent., as previously stated. If g r e a t e r a c c u r a c y be required send cold w a t e r t h r o u g h the coils, and read the t h e r m o m e t e r , then b y s e n d i n g the w a t e r t h r o u g h fast enough, the h e a t i n g effect of the c u r r e n t m a y be nullified to t h e desired extent. T h e sketch below s h o w s the general a p p e a r a n c e of the c o m p l e t e d shunt, t o g e t h e r with t h e c o m m u t a t o r s . R e c e n t researches of Prof. A y r t o n on t h e D ' A r s o n v a l g a l v a n o m e t e r indicate t h a t such a t y p e w o u n d with plati-
A
~
FiG. 5.
noid with phosphor-bronze strip suspensions w o u l d be most desirable, and, therefore, the following a r r a n g e m e n t comm e n d s itself, more especially w h e r e the field a r o u n d the g a l v a n o m e t e r is s u b j e c t to fluctuations. U s e for the s h u n t bar a s h e e t of platinoid, to w h i c h the wires carrying the main current s h o u l d be soldered. P r e f e r a b l y there should be several such wires soldered on the end edge of the sheet and d i s t r i b u t e d at equal distances along the edge so as to secure uniform d i s t r i b u t i o n of c u r r e n t in the strip. Inside at the points A and B t h e leads (of platinoid) to t h e galvanom e t e r c o m m u t a t o r should be fastened. If then a D'Arsonval, m a d e as a b o v e and h a v i n g its telescope and scale rigidly a t t a c h e d to itself, be provided, it m a y b e u s e d w h e r e the other a r r a n g e m e n t w o u l d be o u t of the question, namely, near the d y n a m o room. Moreover, b y m a k i n g e v e r y t h i n g of platinoid, we h a v e d o n e a w a y w i t h t e m p e r a t u r e correc-
488
Electrical
Section.
[ J. F. I.,
tions, provided the p l a t i n u m strip be of such d i m e n s i o n s t h a t the m a x i m u m c u r r e n t d o e s n ' t h e a t it appreciably. T h e strip t h e n m a y be kept in air, t h e only d a n g e r b e i n g a g r e a t e r liability to thermo-electric effects d u e to differences in t e m p e r a t u r e at t h e junctions, and this can be tested b y commutation. U s i n g the D'Arsonval, we can g e t a g r e a t e r deflection for a g i v e n p o t e n t i a l difference at its terminals, therefore, the resistance of the strip m a y be m a d e lower and less e n e r g y be u s e d up in the m e t h o d of m e a s u r e m e n t , and in some cases this is an i m p o r t a n t actvantage. On t h e o t h e r hand, since we m e a s u r e with smaller p o t e n t i a l differences, a n y thermo-electric effects will p r o d u c e a g r e a t e r error in o u r results. CEREBRAL
RADIATION.
BY PROF. EDWIN J. HOUSTON.
[Read before the Electrical SecNon, March s, 1392. ] GENTLEMEN: I h a v e t h o u g h t it possible t h a t it m i g h t interest y o u to consider s o m e r a t h e r wild speculations in w h i c h I h a v e i n d u l g e d for a n u m b e r of y e a r s past, b u t w h i c h I h a v e h e r e t o f o r e refrained from publishing. In t h e s e speculations, to w h i c h I Was first led b y a s u g g e s t i o n from a friend, I h a v e e n d e a v o r e d to correlate, to some extent, the p h e n o m e n a of t h o u g h t with g r o s s e r physical p h e n o m e n a . A l t h o u g h t h e s u g g e s t i o n s I h a v e to offer as a basis for a h y p o t h e s i s of t h e m e c h a n i s m of cerebration, are confessedly incomplete, and, p e r h a p s improbable, y e t I h a v e c o n c l u d e d to place t h e m on record as of possible interest to t h e scientific world. I am, of course, a w a r e of the fact t h a t the psychical operations of the brain are b y no m e a n s u n d e r s t o o d . It is • g e n e r a l l y believed, however, t h a t t h e seat of psychical a c t i v i t y is the cerebrum. T h e m a n n e r in which the brain acts to produce, record, and r e p r o d u c e t h o u g h t is u n k n o w n , and will p r o b a b l y remain u n k n o w n .
Electrical
Section.
[J. ~'. I.,
C O N S T A N T S H U N T M E T H O D FOR THE M E A S U R E M E N T oF LARGE C O N T I N U O U S C U R R E N T S . BY CLAYTONW. PIKE. [Read at lhe stated meeting of the Section, held ~Warch z, zggz. ]
T h e constant, V i e n n a or Munich, s h u n t m e t h o d , as it is v a r i o u s l y termed, is s o m e t h i n g w h i c h is b y no m e a n s n o v e l and is, indeed, well known to some of the section, h a v i n g b e e n used b y t h e m at the Electric E x h i b i t i o n of the Institute, in I884. B u t I b e l i e v e t h a t the m e t h o d is not so well known, or at least not so m u c h in use as it should be, and I hope, b y calling a t t e n t i o n to some of its a d v a n t a g e s .and by p o i n t i n g o u t and s h o w i n g h o w to avoid certain errors incident to it, one or two of w h i c h are n o t g e n e r a l l y known, to do s o m e t h i n g t o w a r d b r i n g i n g it into more general use. F o r the m e a s u r e m e n t of c o n t i n u o u s currents we have the v a r i o u s forms of ammeter, the S i e m e n s d y n a m o m e t e r , the T h o m s o n platform g a l v a n o m e t e r s , and the T h o m s o n balances, the t a n g e n t sine and cosine g a l v a n o m e t e r s . W i t h the o b j e c t i o n s to the a m m e t e r s all are d o u b t l e s s familiar. T h e y are i n a c c u r a c y of g r a d u a t e d scale, errors due to residual m a g n e t i s m or to loss of p e r m a n e n t m a g n e t i s m , and lack of wide range. T h e T h o m s o n p l a t f o r m galvan o m e t e r does not m e a s u r e large currents w i t h o u t the use of the p e r m a n e n t m a g n e t w h i c h is o b j e c t i o n a b l e if considerable a c c u r a c y is desired, and it r e q u i r e s a k n o w l e d g e of t h e horizontal c o m p o n e n t of the earth's m a g n e t i s m H. T h e T h o m s o n b a l a n c e is an i n s t r u m e n t of m u c h g r e a t e r v a l u e than the above, b e i n g i n d e p e n d e n t of H, h a v i n g g r e a t e r r a n g e and g r e a t e r precision, b u t b e i n g even more costly. T h e t a n g e n t g a l v a n o m e t e r requires the v a l u e of H, and has only a limited range, w h i c h the sine and cosine, t h o u g h b e t t e r in this last respect, are t e d i o u s to use on a c c o u n t of time require d in s e t t i n g the coils. T h e g r e a t a d v a n t a g e of the s h u n t m e t h o d over all t h e s e is that, b y the aid of fairly simple and cheap a p p a r a t u s , c u r r e n t s v a r y i n g from one to I,ooo a m p e r e s or m o r e m a y
June, t89z,]
Electrical Section.
477
be m e a s u r e d with q u i c k n e s s and a v e r y g r e a t degree of a c c u r a c y over the w h o l e range. T h e s h u n t m e t h o d is really a comparison method, and d e p e n d s u p o n Ohm's l a w that the potential difference b e t w e e n two ends of a resistance is proportional to the c u r r e n t flowing t h r o u g h t h e resistance. L e t A / ~ be a c o n d u c t o r w h o s e t e m p e r a t u r e is k e p t constant, its resistance will therefore be constant, b u t need n o t be known. Pass t h e c u r r e n t C, which we desire to m e a s u r e t h r o u g h A B, t h e r e will then be a p o t e n t i a l difference P D b e t w e e n A and/3. L e t G b e a mirror g a l v a n o m e t e r with telescope and scale, then the deflection d i s proportional to the current t h r o u g h it, w h i c h is proportional to P D, as long as resistance .of galv a n o m e t e r and leads is constant, di ~ P D~ or d~ a C1. Pass a n y o t h e r current G t h r o u g h A B then d~ a C~ and
4 If the resistance of the g a l v a n o m e t e r and leads be n o t c o n s t a n t , t h e algebraieal result is less simple and, m o r e o v e r if the deflections b e c o m e v e r y large, t h e y are n o t strictly proportional to the c u r r e n t t h r o u g h the g a l v a n o m e t e r which we will call C~-. H e n c e the comparison of large with small c u r r e n t s will be s o m e w h a t inaccurate. T h i s leads us to a little different m e t h o d . Place a resistance box R in the g a l v a n o m e t e r circuit. P a s s c u r r e n t 6; t h r o u g h A/~, c a u s i n g a potential difference Y Dj, a n d s e n d i n g t h r o u g h G a current P/3~ C1 g - R1 + g w h e r e g is the g a l v a n o m e t e r resistance. Then
,ti a C , g a P D___~_~ R1 + g or
G a 4 (R, + g) in the s a m e w a y
G a 4 (& + g)
478
Electrical Section.
[J. F. I.,
then
c, _ d, (R, + g) G 4 (x, + g) or
cl - e~ (xl + g ) c~ 4 (x~ + g) Now, if when C~ were flowing we had accurately meas~lred
it by a tangent galvanometer or by the electrolytic cell, the quantity
G
/2 (R2 + g) would be known. Call it K the constant of the shunt. Then we have the general formula for the shunt that the current whose value we desire is equal to
d (R, + g) K Now, by using the resistance box, and by adjusting it to
June. I892. ]
Electrical Section..
479
give a b o u t the s a m e d e f e c t i o n s on t h e g a l v a n o m e t e r in different m e a s u r e m e n t s we h a v e secured two i m p o r t a n t advantages. (i) W e h a v e done a w a y w i t h the error of a s s u m i n g t h a t the c u r r e n t s t h r o u g h the g a l v a n o m e t e r was strictly proportional to its deflections. (2) T h e resistances R b e i n g large in v a l u e with reference to g a l v a n o m e t e r and leads, and b e i n g of platinoid or some metal with v e r y low t e m p e r a t u r e coefficient, a n y c h a n g e in t e m p e r a t u r e of g a l v a n o m e t e r and leads will p r o d u c e little effect on the result. Like all other m e t h o d s this one is liable to certain errors, some of which I will m e n t i o n and briefly s u g g e s t their remedies. T h e w a y in which t h e y affect the result will also be stated. (i) Reading of Deflection.--Suppose y o u could read to w i t h i n "ol cm. on the scale. If the deflection were I o c m . the error w o u l d be "OI --
Io
i
part in i,ooo.
T h e error is e v i d e n t l y d i m i n i s h e d as we increase t h e deflection. (2) Error Due to Tem~Oerature Change of Galvanometer and Leads.--If t h e s e are of copper their resistance w o u l d c h a n g e four-tenths per cent. p e r d e g r e e Centigrade. T h i s correction could be applied, b u t if the resistance in R is large with respect to g, this correction will be negligible; (2 °) then conflicts with (I°), and we m u s t compromise. S u p p o s e R: : 20 g -~ 2, and t e m p e r a t u r e rise = :0 ° C. then the true value of R 1 -j- ff ~--- 20 -q- 2 -[ 2 >< 4 X I0~ I00
22"08
and the error in n e g l e c t i n g the '08-
"08 22
t in 275
(3) Error Due to Temperature C/tange o f Bow.--This is "02 per cent. per d e g r e e C e n t i g r a d e for platinoid coils, and m a y be
480
[J. F. 1.,
. Electrical Section.
corrected for, or neglected, according to the desired precision of our results. (4) Error Due to Determining Constant R ' . - - T h e e o n s t a n t is best d e t e r m i n e d in m o s t cases by electrolytic cells, sos; three in series, If one has a t a n g e n t g a l v a n o m e t e r of proper range, I-5 amperes, and knows H to the desired a c c u r a c y this m a y be used. A T h o m s o n deci-amp~re balance would b e v e r y conv e n i e n t for s u b s e q u e n t d e t e r m i n a t i o n s of the constant, if its r e a d i n g s were k n o w n to be correct w i t h i n the desired degree of accuracy. A n error of 'i per cent in K c a u s e s an equal error in the s u b s e q u e n t m e a s u r e m e n t s . (5) Error Z)ue ~o a Rise in Temperature o f tile S h u n t . - - T h i s m a y be due to two causes: a rise in the t e m p e r a t u r e of the room, or the h e a t i n g effect of the current. Both m a y be obviated by p l a c i n g the s h u n t in liqtiid artificially cooled
II II
--7
A
B F I G . I.
b y cold w a t e r coils, and k e e p i n g c o n s t a n t the r e a d i n g of a t h e r m o m e t e r placed in the liquid close to the s h u n t . Suppose the rise in t e m p e r a t u r e were 5 ° C. If the s h u n , were of G e r m a n silver the c h a n g e in its resistance would be "o4 per cent. X 5 = '2 per cent. or two parts in i,ooo, and the results of the m e a s u r e m e n t would be in error by tfiis a m o u n t . If of platinoid, the error w o u l d be b u t half as much. (6) Error l)ue to Change o f Value o f H . - - T h i s w o u l d be possible if the g a l v a n o m e t e r h a d the e a r t h ' s field for a r e s t i n g force. If g r e a t accuracy be desired, obtain the value of the c o n s t a n t frequently, a n d see if it c h a n g e s . As I have before said a T h o m s o n balance w o u l d be m o s t desirable for this, a n d m a n y laboratories can afford one T h o m s o n balance. Also if we h a d a g a l v a n o m e t e r whose d a m p i n g could be removed, we could d e t e r m i n e w h e t h e r H h a d c h a n g e d b y its t i m e of vibration. T h e sources of
June. 1892.]
Electrical Section.
48 t
error so far m e n t i o n e d are well u n d e r s t o o d and looked out for, b u t the following are, I believe, n o t so well known. A t least t h e a m o u n t of the effect is not so g e n e r a l l y appreciated, and t h e y have, so far as I know, received e x h a u s t i v e s t u d y only a.t the h a n d s of Mr. W, L. Puffer, of the M a s s a c h u s e t t s I n s t i t u t e of T e c h n o l o g y . H i s results are to be f o u n d in the Proceedings a f tlze American Academy o f Arts and Sciences, J a n u a r y , 1888. (7) Error Due to Tlwrma-electric Effects.--We are obliged to m a k e the resistance of our s h u n t v e r y l o w to p r e v e n t u n d u e heating, hence the potential differences set up at its termi• rials ar.e v e r y low. In this case a n y t h e r m a l E. M. P.'s, which m i g h t be caused b y differences of t e m p e r a t u r e at the j u n c t i o n s of t h e different metals, w o u l d be liable to affect the readings of t h e g a l v a n o m e t e r . This trouble m a y be o b v i a t e d b y so m a k i n g t h e s h u n t as to p u t the j u n c t i o n s u n d e r the surface of liquid w h o s e t e m p e r a t u r e !s m a i n t a i n e d c o n s t a n t a n d m a y be d e t e c t e d b y r u n n i n g the g a l v a n o m e t e r leads to a m e r c u r y cup c o m m u t a t o r and t e s t i n g with reversals. (8) Error Due to Electro-chemical A etion.--The shun t is placed in liquid and u n d e r the action of the p o t e n t i a l differences set up at its terminals t h e r e is electrolytic decomposition, (if the liquid be one which can be d e c o m p o s e d as water). T h i s will cause the end h a v i n g t h e h i g h e r potential to be ~tarkened, o w i n g to oxidization of the G e r m a n silver b y the oxygen set free, while t h e o t h e r end will b e c o m e w h i t e r or cleaner in appearance, o w i n g to the r e d u c t i o n of the film of okide on its surface b y electrolytic h y d r o g e n . T h e s e actions will e v i d e n t l y set up E. M. F.'s, and if the s h u n t resis.tance is v e r y low, will cause c o n s i d e r a b l e error i n the result. This source of error could be a v o i d e d b y placing the s h u n t in s o m e l i q u i d i n c a p a b l e of d e c o m p o s i t i o n w i t h low E. M. F.'s. Chemically pure w a t e r w o u l d do, b u t for the t r o u b l e of o b t a i n i n g and k e e p i n g it so. P e t r o l e u m oils w o u l d act Slightly on the b a r e piece of m e t a l c o m p o s i n g the shunt, t h u s p r o d u c i n g a b a t t e r y effect. W e m u s t either i m m e r s e the b a r in some liquid incapable of acting on it or cover the s h u n t v e r y carefully w i t h some s u b s t a n c e n o t acted upon VoL. CXXXIII. 3t
482
Elcctrica! SccNou.
[J. F. l.,
by the liquid in w h i c h it is placed. E v e r y t h i n g considered it seems to me t h a t very carefully covering t h e s h u n t with several coats of shellac a n d a f t e r w a r d s w i t h copal varnish, a n d i m m e r s i n g i t i n distilled wa£er would be the m o s t satisfactory m e t h o d . I)esz~o'u and Construction of Api~aratus.-- Suppose we desire to m e a s u r e c u r r e n t s of from one to I,OOO ampSres. This would require a set of five t a n g e n t g a l v a n o m e t e r s of different sizes, the three l a r g e s t of w h i c h would be impracticable. (The d i a m e t e r of the l a r g e s t w o u l d be a b o u t seventy-five feet.) W i t h the sine or cosine g a l v a n o m e t e r s or Siemens d y n a m o m e t e r we could do b e t t e r in this respect, b u t they are still out of the question. W i t h the T h o m s o n p l a t f o r m or balance we s h o u l d need three different i n s t r u m e n t s , w h i c h are e x c e e d i n g l y costly.
.
.
.
.
G
A
8 FIG. 2.
Moreover, none of these could be used n e a r d y n a m o s and this m e a n s t h a t we m u s t lead the whole of the c u r r e n t we wish to m e a s u r e p e r h a p s a considerable distance, t h u s e n t a i l i n g expense for wire and p e r h a p s leakage. Suppose we m a k e one s h u n t of resistance "ooi ~o. If of G e r m a n silver 9 inches wide, T ~ inch thick, two strips 24 inches long joined in parallel will be n e a r l y the r e q u i r e d dimensions. T h e object of m a k i n g t h e m so t h i n is to offer large surface to the w a t e r so t h a t t h e y m a y quickly dissipate the h e a t p r o d u c e d by the current. T h e rate of p r o d u c t i o n of h e a t w i t h the m a x i m u m c u r r e n t I,OOO, amperes, will be •24 C~ R = "24 X 1,ooo 5 X "0oi = 24o calories or e n o u g h per second to h e a t 240 g r a m m e s of w a t e r i o C. if none of the h e a t be dissipated. W e m u s t now provide a c o n t a i n i n g vessel large e n o u g h to hold so m u c h w a t e r t h a t it will not be
June. x892.]
t:•l~'ctrical Section.
483
u n d u l y raised in t e m p e r a t u r e , and large e n o u g h to a d m i t of placing in it coils of pipe which shall carry cooling w a t e r if this be f o u n d necessary with the l a r g e r currents. • T h e two German-silver strips should be joined t h u s b y massive cast copper blocks, to w h i c h t h e y are carefully soldered. To m a k e the j u n c t i o n d o u b l y s e c u r e , thin strips of copper 9 x i x ¼ inch are screwed a n d soldered on over t h e G e r m a n silver, as shown. T h e vertical p r o j e c t i n g copper bars D and F are for c o n n e c t i n g the s h u n t into the circuit where we wish to m e a s u r e the current. T h e a r r a n g e m e n t so far described would collapse, owing" to the t h i n n e s s of the G e r m a n silver. So it is necessary to hold the copper blocks a p a r t at the top a n d b o t t o m by some a r r a n g e m e n t w h i c h shall be i n s u l a t e d from the s h u n t . T h i s can be done b y s c r e w i n g on to b o t h top and b o t t o m of the copper blocks~ a piece of board z4 x 4 x I inch, as shown in drawing. T h e board on top should be pierced w i t h a large n u m b e r of holes, as shown, in order to facilitate the circulation of water• T h e ends of the boards t o u c h i n g the blocks should be soaked t h o r o u g h l y in m e l t e d paraffrri arid coated w i t h shellac. T h e Screw holes in t h e b l o c k s should be filled w i t h paraffin, the screw h e a d s c o u n t e r s u n k in the wood, as shown, and melted paraffin p o u r e d over them. T h e whole a r r a n g e m e n t should t h e n be coated w i t h shellac and copal v a r n i s h thorolto~hly. T h e c o n t a i n i n g vessel m a y be of wood, paper or galvanized iron, a b o u t 3 2 x I 4 x I3 inches. Such a vessel will contain, besides the shunt, a b o u t 85,ooo g r a m m e s of water. Now, the n u m b e r of h e a t units, small calories, developed in one second b y i,ooo a m p e r e s flowing t h r o u g h "ooi ~o is in one second as before stated, such as would h e a t 24ogTammes of w a t e r 15 C. H e n c e i t w o u l d take to h e a t 85,ooogrammes ~7"b-I °, a t i m e -83000 seconds, or a b o u t six minutes, if there were no radiation. But it is probable t h a t this h e a t p r o d u c e d will not be i n s t a n t l y c o m m u n i c a t e d to the entire mass of water, hence, for t h e large c u r r e n t m e a s u r e m e n t s it will be advisable to lead coils c a r r y i n g cold w a t e r close to the German-silver strips, b o t h inside a n d outside, if we desire g r e a t a c c u r a c y in our m e a s u r e m e n t s .
484
Electrwal
[J. F. I.,
Section.
Galvanometer.--Any f o r m of s e n s i t i v e r e f l e c t i n g galvan o m e t e r w h i c h is d e a d h e a t and w o u n d w i t h coils of low r e s i s t a n c e m a y be used. If t h e i n s t r u m e n t has t h e earth's field alone for a r e s t o r i n g force, as is g e n e r a l l y t h e case, ( n e g l e c t i n g torsion), it w o u l d b e c o n v e n i e n t to h a v e the
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11
FIG. 3. d a m p i n g c o p p e r m a s s easily r e m o v a b l e . T h e f o l l o w i n g will i l l u s t r a t e t h e d e s i g n : S u p p o s e t h e scale of telescope is d i v i d e d i n t o millimetres, a n d we can r e a d w i t h an e r r o r of "I ram. b y eye e s t i m a t i o n , t h e n w i t h a deflection Io cm. t h e e r r o r in r e a d i n g w o u l d be = I in i,ooo. If o u r scale w e r e d i s t a n t f r o m m i r r o r of t e l e s c o p e b y I" 5 m e t r es , t h e deflection of t h e m i r r o r for a r e a d i n g IO cm. w oul d be a b o u t 2 ° for i1~°~= t a n . 2 a = 2 t a n .
June, I892.J
Electrical
Section.
485
a a p p r o x i m a t e l y , t a n . a - ~ loOo - ~ "o33 a n d a t h e deflection of m i r r o r = I° 54 ~. W h e n I,ooo a m p e r e s are flowing, t h e p o t e n t i a l di fference a c t i n g to s end a c u r r e n t t h r o u g h t h e g a l v a n o m e t e r is IOO × "ooi or one volt. S u p p o s e we p u t in t h e gal vanom e t e r c i r c u i t Io, ooo ohms, t h e m a x i m u m of an o r d i n a r y box, th en t h e c u r r e n t t h r o u g h t h e g a l v a n o m e t e r will be "oooi amp&es. W i t h sufficient a c c ur acy, we m a y a p p l y t h e t a n g e n t g a l v a n o m e t e r f or m ul a , C = IO 11 ~- t a n . a 2
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io × "2 >( ~- X .o33 2 × 3"1416 × "oooI
A s s u m e a m e a n r a d i u s / ' = 2"5 era., t h e n n t h e n u m b e r of t u r n s = 266 = 4I'78 m e t r e s or a b o u t I36 f e e t of wire. If we t a k e a c o p p e r wire w hos e d i a m e t e r b a r e is "o36 i nches t h e r e s i s t a n c e will be a b o u t i"i a, a n d t h e w i re will lie in a g r o o v e I × ~ inch, whose m e a n r a d i u s is 2"5 era., or a b o u t one inch. W h e n one a m p e r e goes t h r o u g h t h e s h u n t t he p o t e n t i a l d if f er en c e will be i X "ooi = I volt. T h e n to gi ve a b o u t IO cm. deflection we should i n s e r t s u ch a r e s i s t a n c e t h a t t h e g a l v a n o m e t e r c u r r e n t w o u l d be "oooi, and this r e s i s t a n c e w o u l d be a b o u t n i n e ohms. T h e n s h o u l d t h e t e m p e r a t u r e of t h e g a l v a n o m e t e r coil rise 5 o C., t h e r e s u l t i n g e r r o r w o u l d be b u t tw o p a r t s in 1,ooo f r o m this c a u s e alone. A g a l v a n o m e t e r of t h e D ' A r s o n v a l p a t t e r n m i g h t be used a n d wo u l d h a v e t h e a d v a n t a g e of b e i n g i n d e p e n d e n t of a n y c h a n g e s in 11, a nd of b e i n g v e r y d e a d heat. It s h o u l d be d e s i g n e d w i t h as low r e s i s t a n c e as is c o n s i s t e n t w i t h t h e c o n d i t i o n t h a t a c u r r e n t "oooi a m p e r e s shall p r o d u c e a deflection of 2 ° . T w o c o m m u t a t o r s are necessary. T h e l a r g e r recei ves t h e six coarse wire s o l d e r e d i nt o the c o p p e r Iugs D F of the s h u n t , s h o w n in F~g. 3. T h e s e wires s h o u l d be No. oooo ]3. a n d S. t h r e e in parallel, and t h e c o m m u t a t o r of t h e m e r c u r y cup
486
Electrical Sectton.
[ J. F. I.,
variety, so arranged t h a t the s h u n t can be entirely disconn e c t e d from the d y n a m o circuit. Into each of t h e h c l e s at f-f and K, Fig. 3, s h o u l d be soldered, carefully, a No. i8 B. and S. copper wire t h o r o u g h l y insulated, and the j o i n t c o v e r e d carefully with shellac and copal. T h e s e are led o u t to the small m e r c u r y c o m m u t a t o r for t h e g a l v a n o m e t e r Circuit a r r a n g e d like the o t h e r so t h a t t h e g a l v a n o m e t e r can be isolated from s h u n t and m a i n circuit. T h e n a n y t h e r m o electric or electro chemical effects can b e d e t e c t e d and studied. F o r instance, disconnect g a l v a n o m e t e r entirely from c o m m u t a t o r and note reading. Connect it to c o m m u t a t o r w i t h s h u n t uncon-
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Fro. 4neeted, and there s h o u l d be, of course, no deflection even b y r e v e r s i n g the c o m m u t a t o r . Connect s h u n t (still with no c u r r e n t in i t ) t o g a l v a n o m e t e r c o m m u t a t o r , and see if there is any deflection, r e v e r s i n g this t i m e the connection from s h u n t to g a l v a n o m e t e r . Finally, run c u r r e n t t h r o u g h shunt, a n d s e e w h e t h e r c h a n g i n g the c o m m u t a t o r s p r o d u c e s a n y c h a n g e in deflection, b e i n g sure t h a t this c u r r e n t is m a i n t a i n e d constant. D i s c o n n e c t s h u n t from d y n a m o circuit, and see w h e t h e r there is a n y appreciable deflection. T h e time occupied for these tests is v e r y short, if the comm u t a t o r s are properly arranged, and if the t e s t s s h o w no appreciable thermo-electric or chemical effects, we m a y feel
June, 1892 I
487
Electrical Section.
sure that o u r a p p a r a t u s is s a t i s f a c t o r y in this respect. Finally, provide a sensitive t h e r m o m e t e r w h o s e range is from a b o u t 60 ° F. to 80 ° F., or 15 ° ~. to 27 ° C., and a r r a n g e so t h a t it will be placed close to one of the German-silvEr strips of the shunt. R e a d w h e n no c u r r e n t is flowing, and then w h e n the m a x i m u m c u r r e n t flows. T h e n if these readings differ 5 ° C., or 9 ° F., the error w o u l d b e two per cent., as previously stated. If g r e a t e r a c c u r a c y be required send cold w a t e r t h r o u g h the coils, and read the t h e r m o m e t e r , then b y s e n d i n g the w a t e r t h r o u g h fast enough, the h e a t i n g effect of the c u r r e n t m a y be nullified to t h e desired extent. T h e sketch below s h o w s the general a p p e a r a n c e of the c o m p l e t e d shunt, t o g e t h e r with t h e c o m m u t a t o r s . R e c e n t researches of Prof. A y r t o n on t h e D ' A r s o n v a l g a l v a n o m e t e r indicate t h a t such a t y p e w o u n d with plati-
A
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FiG. 5.
noid with phosphor-bronze strip suspensions w o u l d be most desirable, and, therefore, the following a r r a n g e m e n t comm e n d s itself, more especially w h e r e the field a r o u n d the g a l v a n o m e t e r is s u b j e c t to fluctuations. U s e for the s h u n t bar a s h e e t of platinoid, to w h i c h the wires carrying the main current s h o u l d be soldered. P r e f e r a b l y there should be several such wires soldered on the end edge of the sheet and d i s t r i b u t e d at equal distances along the edge so as to secure uniform d i s t r i b u t i o n of c u r r e n t in the strip. Inside at the points A and B t h e leads (of platinoid) to t h e galvanom e t e r c o m m u t a t o r should be fastened. If then a D'Arsonval, m a d e as a b o v e and h a v i n g its telescope and scale rigidly a t t a c h e d to itself, be provided, it m a y b e u s e d w h e r e the other a r r a n g e m e n t w o u l d be o u t of the question, namely, near the d y n a m o room. Moreover, b y m a k i n g e v e r y t h i n g of platinoid, we h a v e d o n e a w a y w i t h t e m p e r a t u r e correc-
488
Electrical
Section.
[ J. F. I.,
tions, provided the p l a t i n u m strip be of such d i m e n s i o n s t h a t the m a x i m u m c u r r e n t d o e s n ' t h e a t it appreciably. T h e strip t h e n m a y be kept in air, t h e only d a n g e r b e i n g a g r e a t e r liability to thermo-electric effects d u e to differences in t e m p e r a t u r e at t h e junctions, and this can be tested b y commutation. U s i n g the D'Arsonval, we can g e t a g r e a t e r deflection for a g i v e n p o t e n t i a l difference at its terminals, therefore, the resistance of the strip m a y be m a d e lower and less e n e r g y be u s e d up in the m e t h o d of m e a s u r e m e n t , and in some cases this is an i m p o r t a n t actvantage. On t h e o t h e r hand, since we m e a s u r e with smaller p o t e n t i a l differences, a n y thermo-electric effects will p r o d u c e a g r e a t e r error in o u r results. CEREBRAL
RADIATION.
BY PROF. EDWIN J. HOUSTON.
[Read before the Electrical SecNon, March s, 1392. ] GENTLEMEN: I h a v e t h o u g h t it possible t h a t it m i g h t interest y o u to consider s o m e r a t h e r wild speculations in w h i c h I h a v e i n d u l g e d for a n u m b e r of y e a r s past, b u t w h i c h I h a v e h e r e t o f o r e refrained from publishing. In t h e s e speculations, to w h i c h I Was first led b y a s u g g e s t i o n from a friend, I h a v e e n d e a v o r e d to correlate, to some extent, the p h e n o m e n a of t h o u g h t with g r o s s e r physical p h e n o m e n a . A l t h o u g h t h e s u g g e s t i o n s I h a v e to offer as a basis for a h y p o t h e s i s of t h e m e c h a n i s m of cerebration, are confessedly incomplete, and, p e r h a p s improbable, y e t I h a v e c o n c l u d e d to place t h e m on record as of possible interest to t h e scientific world. I am, of course, a w a r e of the fact t h a t the psychical operations of the brain are b y no m e a n s u n d e r s t o o d . It is • g e n e r a l l y believed, however, t h a t t h e seat of psychical a c t i v i t y is the cerebrum. T h e m a n n e r in which the brain acts to produce, record, and r e p r o d u c e t h o u g h t is u n k n o w n , and will p r o b a b l y remain u n k n o w n .