ELECTROMAGNETIC UNIT OF ELECTRICITY TO THE ELECTROSTATIC UNIT. 603 
The difference of potential between the guard ring and the cjdinder depends upon 
the speed of the commutator, so that the correction on this account is made on the 
electromagnetic measure of the capacity. 
The Electromagnetic Measure of the Capacity. 
The arrangement employed in this measurement is represented in fig. 3, 
ABCD is a Wheatstone’s bridge with the galvanometer at G, and the l)attery 
between B and C. The arm AB is broken at II and S, which are two poles of a 
commutator, which alternately come into contact with a spring P, connected with the 
middle part of the inner cylinder of the condenser. The outer cylinder is connected 
to S. The points C and B are connected respectively with L and M, the two poles 
of a commutator, which alternately come into contact with a spring Q, attached, 
to the guard ring of the condenser. The system is arranged so that when the 
commutators are working the order of events is as follows :— 
I. 
P on S. 
Q on M. 
Condenser discharged. 
Guard ring discharged. 
II. 
P on Pt. 
Q on M. 
Condenser begins to charge. 
III. 
P on P. 
Q on L. 
Condenser completely charged to potential (A)-(B). 
Guard ring charged to potential (C)-(B^. 
IV. 
P on S. 
Q on L. 
Condenser begins discharging. 
V. 
P on S. 
Q on M. 
Condenser discharged. 
Guard ring discharged. 
4 H 2 
