GOG PROF. J. J. THOMSON AND MR. G. F. C. SEARLE ON THE RATIO OF THE 
needle remains undeflected, the quantity of electricity which passes through the 
galvanometer in unit time must be zero. But this quantity is r<Z + z^, so that 
nZ + Zj = 0 . 
Substituting this relation in equation (G), we get 
|i_£_1 
1 _7 [ (7 + /3) (/j + 7 + «)j 
n bj 3 ^ _7 ^ 
(5 + « + 7 ) /3 
( 7 ) 
From this equation, if we know the resistances and the speed, we can calculate the 
capacity. 
In order to apply the correction for the diflerence of potential between the guard 
ring and the inner cylinder, we require {A} — {C]7[Al — {B}, now 
{A} - {0} ^ 5 . {A} - {!)} 
{A} - {R} « (A) - {B} 
a + j3 
+ 
7) 
Where n was equal to G4 the approximate values of the resistances were 
h = 200,000 ohm.s. 
a = 20,000 ,, 
^ = 500,000 „ 
y = 3,000 ,, 
substituting the values 
(Aj -{C} 
{A}-{B}- 
We shall now go on to discuss the details of the method whose theory we havejust 
given. 
The Commutator. 
The general view of this is shown in figs. (4), (5), (G). The framework is 
strongly made of cast iron, and somewhat resembles tlie headstock of a lathe, it is 
provided with two hardened steel centres, capable of adjustment, between which runs 
the axle of the commutator. This was made of tool steel but not hardened. The 
centres were very good, and the apparatus ran with very little friction and wear. 
