218 Proceedings of the Asiatic Society. [ Serr. 
The reasoning being in both cases virtually the same, I will 
therefore only deal with the first case—the galvanometer wound 
with two coils. 
Let @ be the deflection produced on any particular galvanometer 
by a current C, then for small deflections 
WO a> 
where 7 is the number of convolutions, and a a constant depending 
on the form of the galvanometer and the size and power of the 
magnetic needle. Let this current C be produced by a battery 
having an electromotive force Z, and an internal resistance # then 
E 
RkR+G | 
G being the resistance of the galvanometer 
C= by Ohm’s law 
wi 
ES Aaa Racecar a tah, veces eorecece ae 
or if we consider z and @ constant 
E varies as (2+ G@)d......... fe 2k AC 
Now if, G be very large compared with # then 
F varies as G X& d..... ...very nearly ; 
that is to say the electromotive forces of batteries are directly 
proportional to the deflections they produce on a galvanometer 
having a very large resistance compared with the resistance of 
the batteries. This method will therefore answer with batteries of 
small internal resistance. But in the case of batteries consisting 
of a large number of cells joined in series (such batteries, in fact, 
as are necessarily in use in the Indian Telegraph on account of the 
great length of the lines) the internal battery resistance is itself 
large. Consequently, with such batteries it is impossible to use any 
cheap galvanometer, that is, a galvanometer not containing a large 
amount of wire, which will fulfil the condition that the galvano- 
meter resistance shall be large compared with the battery resistance. 
The electromotive forces, therefore, of such batteries can only be 
very roughly compared by using the so-called ‘intensity’ coil, 
unless the internal resistances of the batteries be in some way 
previously ascertained. 
The internal resistance, if high, could be found in the following 
way: 
