[ 141 ] 



XIX. Proceedings of Learned Societies, 



ROYAL SOCIETY. 



[Continued from p. 73.] 



January 18, 1877. — Dr. J. Dalton Hooker, O.B., President, in the 



Chair. 



^HE following paper was read :— 



-*- " Kesidual Charge of the Leyden Jar. — II. Dielectric Proper- 

 ties of various Grlasses." By J. Hopkinson, M.A., D.Sc. 



I. The two following propositions are included under the law 

 that the effects of simultaneous electromotive forces are super- 

 posable. 



(a) If two jars be made of the same glass but of different thick- 

 nesses, if they be charged to the same potential for equal times, 

 discharged for equal times, and then insulated, the residual charge 

 will after equal times have the same potential in each. 



(6) Eesidual charge is proportional to exciting charge. 



These propositions are verified experimentally within the limits 

 of errors of observation. 



II. Electric displacement through a dielectric may be supposed 

 to depend not only on the electromotive force at the instant, but 

 also in part on the electromotive forces at all previous times. 

 If we assume that the effect of the electromotive force at any 

 previous time decreases according to some law as the time elapsed 

 increases, and that these effects are superposable, we may write 



where ctj, is the electromotive force at time ^, and yt is the surface 

 integral of electric displacement divided by the instantaneous capa- 

 city of the jar. 



If ^ (w) is determined for all values of w, the properties of the 

 glass as regards conduction and residual charge are completely ex- 

 pressed. 



>//■ ( GO ) is equal to the reciprocal of the specific resistance of 

 the material multiplied by 47r and divided by the specific induc- 

 tive capacity. During insulation yt is constant ; hence 





,\l/{(i)) dio. 



This is the fundamental equation of the following experiments. 

 Two methods of finding values of ^ (w) present themselves. 

 1st. Let X. be constant = X for a time T : insulate for time t. 





,\p{(ij)dio; 



^|=-X^(. + T)- %.f„c(.. 



