Electrical Conductivity of Mica in Intense Fields, 119 



finally used ; with the leyden-jar the figures were about 

 3 per cent, smaller. 



The galvanometer G was of the Ayrton & Mather sus- 

 pended-coil type, the coil being enclosed in a silver tube, so 

 that the galvanometer was dead-beat even on open circuit. 

 Its resistance was about 296 ohms, the constant was found to 

 be 8*0 x 10 -3 microampere per scale-division, the deflexion 

 being very nearly proportional to the current over the range 

 employed. 



The Wimshurst machine, having vulcanite plates and 

 insulation, generally excited itself without any special warm- 

 ing or other precautions, but it was impossible to sav which 

 way it would build up, so that the condensers were some- 

 times charged the reverse way accidentally. For determi- 

 nations at voltages exceeding 2000 it was found best to run 

 the machine at a nearly constant speed by means of a motor, 

 and regulate the voltage by the pin-point method. This 

 consists in fixing an earthed pin-point near the plate of the 

 machine. The best position seemed to be just in advance of 

 the collecting-comb connected to the condensers. By moving 

 this pin in and out by means of a rack and pinion, the 

 voltage could be adjusted over a wide range, and kept 

 approximately constant for a considerable time. At voltages 

 less than 4000 the electrostatic voltmeter was used for 

 approximately indicating the pressure before discharge, 

 above that the galvanometer G served the same purpose. 

 The actual value of the pressure was obtained from the throw 

 of H on discharge. The electrostatic voltmeter was of the 

 vertical needle type, reading to the nearest 100 volts. As 

 the divisions were very close together towards the upper 

 part of the range, it could only be read rather roughly. 



Experimental Results. 



The first definite results were obtained with a mica sheet 

 about 0*017 mm. thick. The value obtained for the di- 

 electric constant k at voltages not exceeding 1000 was about 9. 

 At higher voltages the value of k steadily increased. It was 

 subsequently found that this increase was due to a brush 

 discharge, as already mentioned. The conduction current 

 through the mica was inappreciable for potential gradients 

 less than 0*5 megavolt per cm. At higher gradients it 

 increased rapidly, as shown by the following figures, which, 

 with the exception of the last, are means taken from the 

 curve obtained by plotting the results. The last figure is 

 the highest obtained in this series. 



