124 
MESSRS. J. HOPKIN'SOR' AND E. WILSON ON THE 
unreliable, when a knowledge of their actual resistance at the time of contact 
is taken to be the same as measured in the ordinary way on a Wheatstone bridge. 
Time of Contact .—The connections were altered from those in fig. 5 to those 
in fig. 6. Eight dry cells having low internal resistance were used for charging. 
In fig. 6 let K be the capacity of the condenser, equal to microfarad. Let h be 
the capacity of the quadrant electrometer at rest in zero position, equal to 
'000015 microfarad. Let L be the insulation resistance of K, and r the resistance 
through which the condensers are charged. Let E be the E.M.F. of the battery, 
V be the E.M.F. of condenser, and t the time of contact in seconds. 
Then 
(K+2')V + |= 7(E-V) 
Y = f \ ^ _) • 
R + /■ V 10- K 4 kj 
To determine E. Let E = oc , K = 0, ?■ = 0 ; the deflection of the electrometer 
needle from zero after the pendulum has struck gives E in scale divisions. 
Fi^e. 
To determine b Let K be known and great as conq)ared with k. Let K = cc , 
and let r be such that the steady deflection from zero, Y, after })endulum has struck, 
is about equal to half E. 
V = E (1 - e-K,), 
which gives 
t ? K log ^ * 
