Or ELECTROSTATIC WITH ELECTROMAGNETIC EORCE. 
649 
by increasing considerably the resistance of the great battery-circuit, the current could 
be made more uniform. 
When a sufficient number of experiments on equilibrium had been made, a current 
was made to pass through the secondary coil of the galvanometer, and was then divided 
between a shunt of 31 units B. A. and the primary coil of the galvanometer with a 
resistance S' added. S' was then varied till the needle was in equilibrium. In this 
way the magnetic effects of the two coils were compared. 
The resistance of the galvanometer and of all the coils were tested by Mr. Hockin, 
who also made all the observations with the galvanometer and its adjusting shunts. 
To determine v from these experiments, we have first, since the attraction is equal to 
the repulsion, 
E 2 a 2 . ,2A 
SP= w b! 
( 6 ) 
If x is the current of the great battery passing through the great resistance R, and if x 1 
of this passes through the galvanometer whose resistance is G, and x— x' through the 
shunt S to earth, then 
E=Rr+G^ (7) 
and 
Ga/=S0r-O (8) 
Also if (j ! is the magnetic effect of the principal coil of the galvanometer, and g 2 that of 
the secondary coil, then when the needle is in equilibrium 
g^-g.y. (9) 
In the comparison of the coils of the galvanometer, if x x and y x are the currents 
through each, we have 
Fi=Mi ( 10 ) 
But y x is divided into two parts, of which x x passes through the galvanometer G and 
the shunt S', and the other, y x —x!, passes through the shunt of 31 Ohms. Hence 
^(G+S')=(y- ffJSl. 
( 11 ) 
From these equations we obtain as the value of v. 
v ~4V^ii T> \/ £5 ( R S G +E+G ) 
31 
G + S' + 31 
( 12 ) 
an equation containing only known quantities on the right-hand side. Of these, n and 
n 1 are the numbers of windings on the two coils, a is the mean of the radii of the sus- 
pended disk and the aperture, b is the distance between the fixed disk and the suspended 
2A 
disk, -g- is found from a x and a 2 , the mean radii of the coils, and V their mean distance 
by equation (3). 
It is the great resistance, G that of the galvanometer, S that of the shunt in the prin- 
cipal experiment, and S' that of the additional resistance in the comparison of galvano- 
meter-coils. 
