Temperature Variation of the Conductivity of Mica. 195 



the approximate value of p t , 10*38 being the value correct 

 to two places of decimals. 



Tables of the first ten roots* of J n {x) have been calcu- 

 lated to six figures for the following values of n : n = 0, 

 1, 2, 8, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, 40, 50, 75, 100, 200, 

 300, 400, 500, 750, 1000. Bourget's tables give the first nine 

 roots t of J (V) to J 5 (#) to four or five figures, in a number 

 of cases incorrectly. 



XXIV. On the Temperature Variation of the Electrical 

 Conductivity of Mica. By H. H. Poole J. 



Introduction. 



A DESCRIPTION was given in a previous paper (Phil. 

 Mag. July 1916) of some experiments on the dielectric 

 constant and electrical conductivity of mica in intense fields. 

 The method adopted consisted in measuring the charge and 

 leakage current of a small mica condenser of known small 

 thickness when charged to a known voltage, the latter being 

 found by measuring the charge on an air-condenser or a thick- 

 walled leyden-jar. No certain variation of the dielectric 

 constant with field was observed, but the conduction current 

 was found to increase very rapidly, according well with the 

 formula C = aXe 6X , where C is the current density, X the 

 potential gradient, a and b constants. It was suggested that 

 the occurrence of the exponential term might be connected 

 with a distribution of electronic velocities in accordance with 

 Maxwell's law. On this assumption b would vary inversely 

 as the mean electronic energy, and hence, presumably, as the 

 absolute temperature. It accordingly seemed to be desirable 

 to repeat the observations at different temperatures. 



Experimental Modifications. 



For a description of the experimental details reference 

 must be made to the previous paper. The condenser described 

 there was suitable for only quite a moderate range of tempe- 

 rature owing to the materials employed. Some experiments 

 were made with it which indicated that a increased very 



* Report of the British Association, Math. Tables Committee, ] 917. 

 f Anncdes de VEcole Normale, iii. (1866) ; Lord Rayleigh, ' Theory of 

 Sound,' vol. i. Table B. p. 330. 

 \ Communicated by the Author. 



