710 
MR. J. J. THOMSON - ON THE HUMBER OF ELECTROSTATIC 
But r/X is the quantity of electricity that flows through the galvanometer whilst the 
condenser is being charged. If the condenser is charged and discharged n times in a 
second, the quantity of electricity which flows through the galvanometer in one second 
is nrjX, and if this is to balance the steady current, wo must have 
n--\-iv= 0 
A 
or 
..Q {(a + c + <7)(a + b + d) — a~}a 
{(a + b + cl) (a + c)— a(a -f cl) } { (a + cl) (a + c+g) — a(a + c) } 
or 
o 3 
(a + c +g)(cc + b + d) 
UUTT7ULIAU 
+ b + d)) \ d(a + c+g) 
Now in the actual experiment the resistances a, b, c, cl, g had about the following 
values :— 
a— 1,200 B.A. units. 
b= 2,500 
c=100,100 
cl— 900,000 
9= 1B000 
So that in this case the formula nC=a/cd is correct to within 0’1 per cent., and it 
is the one we shall use to calculate the electromagnetic measure of the capacity of the 
condenser. 
With these values of the resistances we find that X is greater than 5000, thus the 
time constant of the system is very small compared with the time during which the 
plates of the condenser are connected together, so that the condenser is completely 
discharged each time. 
The electrostatic measure of the capacity must be calculated from the geometrical 
constants of the condenser. It was necessary to use a guard ring in order to simplify 
the calculation, and to avoid the influence of the irregular distribution of electricity 
near the edges of the condenser, but as a condenser with a guard ring could not be 
worked by the commutator, the capacity of the guard ring condenser had to be 
compared experimentally with that of a condenser without a guard ring which could 
be worked by the commutator. 
The investigation thus divides itself naturally into three parts :— 
First, the theoretical calculation of the electrostatic capacity of the guard ring 
condenser. For this purpose it was necessary to determine the geometrical constants 
of the guard ring condenser. 
nC = 
a\ 1 
cell 1 
c(ci 
