118 Mr. H. H. Poole on the Dielectric Constant and 



value of the charge on the condenser. This correction might 

 have been neglected without serious error, as it was always 

 very small. 



The galvanometers H and I were similar and of the 

 D'Arsonval type, their resistances being about 800 ohms 

 each. As the throws to be observed were large the scales, 

 which were 50 cm. long divided in mm., were set up so 

 that the luminous images were at the ends of the scales when 

 the galvanometers were short-circuited. The scales were 

 set so that perpendiculars to them at their middle points 

 bisected the mirrors. A careful standardization was neces- 

 sary to test the relation between throw and quantity for 

 each galvanometer. This was carried out with a standard 

 -J microfarad condenser charged to various pressures up to 

 about 10 volts, measured on a Weston voltmeter, and dis- 

 charged through the galvanometer, the key D being used. 

 It was also necessary to know the capacity of the condenser 

 E (fig. 1), or, what comes to the same thing, to find what 

 throw will be produced in the galvanometer H by charging 

 E to some known potential. By combining a Tucker hygro- 

 metric battery giving about 600 volts with a set of small 

 dry cells of the flash-lamp type and with the continuous 

 current supply of the laboratory, pressures up to about 

 1000 volts could be obtained. These pressures were measured 

 in sections by means of a Kelvin multicellular voltmeter 

 reading up to 300 volts. Knowing the variation of sensi- 

 tiveness of the galvanometer H with the throw, a scale of 

 voltage was constructed. The characteristics of the two 

 galvanometers are shown in the following table. Here 

 is the throw, V the factor by which the throw of H must be 

 multiplied in order to find the voltage of the source, and Q 

 the factor by which the throw of I must be multiplied in 

 order to find the charge on the mica condenser in micro- 

 coulombs. 



0. 



V. 



Q- 



50 



1221 



6-71 XlO" 3 



100 



12-29 



6-78 



150 



1237 



6-84 



200 



1244 



6-90 



250 



12-50 



6-96 



300 and upwards. 



12-56 



6-99 



The figures in the V column refer to the air-condenser 



