Electrification of Dielectrics by Mechanical Means. 233 



that in many ways the characteristic properties of dielectrics 

 can be illustrated by experiments with a model of this kind, 

 which will, it is hoped, afford valuable help in illustrating 

 the behaviour of dielectrics. 



The question may be asked, In what respect the action of 

 this model differs from that of a simple twisted wire as an 

 illustration of dieleetria effects? The answer is that expe- 

 riment has shown that although the current flowing out of a 

 charged condenser is a simple power-function of the time, 

 yet the exponent varies with the time of charging. 



In the case of the simple twisted wire, whether the wire 

 is twisted quickly or slowly, the twist per unit of length of 

 the wire is approximately the same. 



In the case of the model here described, it is evident that 

 the effect on the compression makes itself felt, first on the top 

 piston ; and hence that the amount of total compression depends 

 not only upon the compressing force, but upon the time during 

 which this acts, and that this compression is propagated gra- 

 dually down the chain of pistons. In this respect the model 

 imitates closely the electrical behaviour of dielectrics such as 

 indiarubber when subjected for various times to different 

 electromotive forces. 



In the above model the velocity of return of the upper 

 piston to its zero position is very approximately represented 

 in terms of the time t reckoned from the instant of release 

 by an equation of the form 



w here A is a constant, and B is a function of the time T during 

 which the operation of compression lasted. 



In the case of the charge and discharge of a condenser, an 

 equation of the above form has been shown by one of us to 

 connect the value of the discharge-current and the times of 

 discharge and charge. 



XVIII. Note on the Electrification of Dielectrics hy Mechanical 

 Means. By A. W. Ashton, B.Sc* 



npiIE apparatus used consisted of two brass plates P x and P 2 , 

 J- the upper one, P ]? being 6 inches diameter and '25 inch 

 thick. These two plates were connected to a quadrant electro- 

 meter, E. The electrometer gave a deflexion of 4*72 cms. for 

 one volt. The sheet of rubber was laid on P 2 , and F x was laid 

 gently on top of the sheet. The first sheet tested was pure Para 

 rubber, which had been rolled to a thickness of "012 inch, and 



Communicated by the Physical Society : read May 31, 1901. 



