[ 109 ] 
IV. 0)1 the Capacit)j cutd Residual Charge of Dielectrics as Affected hy Tempera- 
ture and Time. 
By J, Hopkinson, F.R.S., and E. Wilson. 
Received December 15, 1896,—Read January 28, 1897. 
Before describing the experiments^^ forming the principal subject of this communi¬ 
cation, and their results, it may be convenient to shortly state the laws of residual 
charge. 
Let Xi be the potential at any time t of a condenser, e.y., a glass flask, let Vt 
be the time integral of current through the flask up to time f, or, in other words, 
let yt be the electric displacement, including therein electric displacement due to 
ordinary conduction. If the j^otential be applied for a short time let the dis¬ 
placement at time t, after time m has elapsed from the application of force 
be (w) ; this assumes that the effects produced are proportional to the 
forces producing them; that is, that we may add the effects of simultaneously- 
applied electromotive forces. Generalise this to the extent of assuming that we may 
r'” 
add the effects of successively-applied electromotive forces, then i/j = Xi_^xlj {(o) do). 
'' * 0 
This is nothing else than a slight generalisation of Ohm’s Law, and of the law that 
the charge of a condenser is proportional to its potential. Experiments were tried 
some years ago for the purpose of supporting this law of superposition as regards 
capacity. It was shown that the electrostatic capacity of light flint glass remained 
constant up to 5,000 volts per millimetre (‘Phil. Trans.,’ 1881, Part IL, p. 365). 
The consequences of deviation from proportionality were considered (‘ Proc. Hoy. Soc.,’ 
1886, vol. 41, p. 453), and it was shown that, if the law held, the capacity as 
determined by the method of attractions was equal to that determined by the method 
of condensers; this is known to be the case with one or two doubtful exceptions 
{ibid., p. 458). Rough experiments have been made to show that residual charge is 
proportional to potential ; they indicate that it is (‘Phil. Trans.,’ vol. 167, Part IL). 
Xi_^ijj (co) dfi) includes in itself ordinary conduction, residual 
0 
* These experimeuts were commenced in the summer of 1894, and we have to thank Messrs. 
C. J. Evans and R. E. Shawcross for valuable assistance rendered during the period of their 
Demonstratorship in the Siemens Laboratory, King’s College, London. 
12.4.97 
