of the Electrical Conductivity of Mica. 199 



by bringing an earthed pin-point near the machine ; but it 

 was found that the best position for the pin was near the 

 isolated collecting comb of the machine instead of the comb 

 connected to the condensers. 



The mica condenser was placed in an oven heated by gas. 

 Although no self-regulating device was employed, the oven 

 could be fairly rapidly brought to a nearly constant tem- 

 perature, and the variation during an experiment never 

 exceeded 1° C. On the other hand, it was hard to be certain 

 how accurately the thermometer, which was close above the 

 condenser, indicated the temperature of the latter. Raising 

 the thermometer 7 cm., which brought its bulb near the top 

 of the oven, caused a rise in its temperature o£ about 2 0, 5 C. 

 This result was obtained at several different temperatures 

 between 100° C. and 200° C. Different thermometers were 

 used for temperatures above and below 100° C. The low 

 temperature one was standardized at air temperature and 

 also in steam, ana found correct ; the high-temperature 

 thermometer was also standardized against a platinum thermo- 

 meter, and the necessary corrections, which nowhere exceeded 

 1° C, were applied to its readings. 



The galvanometers H and I were standardized by means 

 of a set of standard condensers. It was found that by 

 suitably adjusting the scales the deflexion could be made 

 proportional to the quantity except near the ends of the 

 scales, where small corrections, amounting to a few scale- 

 divisions, had to be applied to the readings. The voltage 

 applied to the condenser is proportional to the throw of H, 

 the constant of proportionality being found, as before, with 

 the aid of a hygrometric battery giving a P.D. of about 

 600 volts, which could be measured in two parts by means of 

 a Kelvin multicellular voltmeter. The ratio of the throws 

 of I and H, when multiplied by a suitable coefficient, gives 

 the apparent value of the dielectric constant. The deflexion 

 of G, which measures the leakage current, was found to be 

 proportional to the current over the range employed. 



Experimental Results. 



The results of a typical set of readings are shown below. 

 Here X is the potential gradient in megavolts per cm., k the 

 apparent value of the dielectric constant, and C the con- 

 duction current in microamperes per sq. cm. 



