1880.] Strained Material, Leyden Jars, and Voltameters. 413 



certain time, by removing tbe battery and connecting togetber the 

 platinnm plates of tbe voltameter through a resistance, and finally the 

 platinum plates be insulated, that tbe difference of potentials between 

 tbe plates gradually rises, presenting phenomena exactly like the 

 soaking out of the residual charge in a Leyden jar that has been in- 

 sulated after being discharged for a short time. 



The unsatisfactory nature of the ordinary experiments with con- 

 densers has led us to try if we could find the actual constants in 

 Professor Clerk Maxwell's differential equation — 



d"v . d' l ~ l v . . , . , ,du , . ,d n - l u n , 



+ »— — + .... +mv=a'u + h'—+ .... +m'—— . . (1>, 

 dt" dt 11 ' 1 dt dt >l ~ l 



where v is the difference of potentials of tbe two surfaces of the 

 condenser, and u the current flowing into or from the condenser at 

 the time t, for it is quite evident that if his theory be correct these 

 constants completely determine the residual charge phenomena of the 

 substance. The difficulty in obtaining these constants has involved 

 us in a very large amount of labour, and we have only yet reduced a 

 small number of tbe curves we have experimentally obtained during 

 the last five years ; in addition we have found that many of our ex- 

 periments will have to be repeated with somewhat different conditions 

 for the investigation to be regarded as complete. But some of the 

 results already obtained from this reduction are of interest. Thus, in 

 one set of observations, we have tried to determine tbe constants from 

 curves of the soaking out of the residual charge of a Leyden jar dis- 

 charged for a very short time, having been previously kept charged 

 for so long a time in a room of nearly constant temperature, that no 

 loss was observed during two or three days, that is, the jar may be 

 regarded as possessing almost infinite insulation. 



The Leyden jar employed for these experiments was that of a 

 Thomson's quadrant electrometer, and the measure of its charge was 

 determined from the deflection produced in the needle of the electro- 

 meter itself when the electrodes of a Latimer Clark's constant cell 

 attached to the electrometer quadrants were reversed. In reality 

 three such cells in succession were used in order to detect any irregu- 

 larity in the cells themselves. The Leyden jar was first rather highly 

 charged, and then left with the replenisher untouched for some days 

 until the loss of charge from day to day became imperceptible. The 

 readings obtained with each of the three Clark's cells were then accu- 

 rately taken, when one of them being left on the electrometer elec- 

 trodes the Leyden jar was discharged for a very short time and then 

 insulated ; tbe soaking out of the residual charge was now measured 

 by taking time-readings of tbe increase of deflection produced by the 

 constant cell. To allow for alteration of zero of the electrometer 

 arising from alterations of its charge frequent reversals of the cell 



