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VIII. On the Dielectric Constant and Electrical Conductivity 

 of Mica in Intense Fields. By H. H. Poole*. 



Introduction. 



IN view of the possible analogy between electric polariza- 

 tion and magnetization, it seems not unreasonable to 

 suppose that some variation in the Dielectric Constant of a 

 dielectric might be observed as the potential gradient 

 approaches the sparking value. In the following paper an 

 account is given of some experiments on mica (Muscovite), 

 which was chosen on account of the ease of obtaining it in 

 the form of uniform sheets of very small thickness. The 

 method finally adopted consisted in charging a small mica 

 condenser to a known voltage by means of a Wimshurst 

 machine, and then discharging it through a suspended-coil 

 ballistic galvanometer, suitable precautions being taken to 

 avoid piercing the insulation of the latter. As a satisfactory 

 electrostatic voltmeter for reading the pressures involved 

 was not available, another condenser of nearly the same 

 capacity and charged to the same pressure was simultaneously 

 discharged through a second similar galvanometer. In 

 many of the experiments a small leyden-jar was used for 

 this purpose. As the potential gradient in the comparatively 

 thick glass of the jar was only a very small fraction of that 

 in the mica, it was assumed that the capacity of the jar 

 remained constant. To remove any uncertainty on this 

 point, an air-condenser was finally constructed and used in 

 the latest experiments without, however, any appreciable 

 effect on the results obtained. It was found that the ratio 

 of the throws of the two galvanometers was almost inde- 

 pendent of the time of charge if this was not less than 

 20 seconds : this condition was always fulfilled. 



It soon became evident that, at high pressures, a very 

 noticeable leak occurred through the mica, so the arrange- 

 ment was modified so as to include a third galvanometer to 

 measure this leakage current, the possibility of surface leak 

 being eliminated by means of an earthed guard-ring. 



Apparatus. 



The connexions are shown diagrammatically in fig. 1. A 

 Wimshurst machine A is used to keep the system charged 

 to an approximately constant potential, which is roughly 

 measured by the electrostatic voltmeter B. A large leyden- 

 jar C makes the pressure regulation easier by adding to the 



* Communicated by the Author. 



