1872.] Louis Schwendler — On Differential Galvanometers. 
Fig. 1. 
Now, as far as the 
magnetic effect of the 
two coils is concerned, 
we may substitute for the 
parallel circuit, Fig. 1, 
the simple circuit, Fig. 2, 
if we only reverse the 
magnetic action of one 
of the two coils, (say the 
right one). 
145 
(Fig. 2). 
And in order to have, in 
this case, for the same 
electromotive force E the 
same current C flowing 
through the coils as be- 
fore, (see Fig. 1), we 
must necessarily intro- 
duce a resistance x 
hence — 
E 
c = 
E 
E 
2y + x 
9 + w 
therefore w — g -f- CD . 
But to obtain the maximum magnetic effect in any single circuit (Fig. 2), 
it is necessary that the resistance of the coil should be equal to the total 
external resistance* and therefore in this case (Fig. 2) 
x =2g (ID 
Eliminating x from equation I and II we have 
w m 
(I) 
To oltain the most delicate reading with a differential galvanometer , the 
two coils of wh ich have equal magnetic momenta, and also equal resistances, 
* This law holds good, — as can easily be shown, — for any number of coils con- 
nected into a single circuit, no matter if the magnetio effects of these coils have the 
same or opposite Bigu with respect to a given magnetic point. 
