504 Mr. A. W. Ashton on the 



used consisted of 13 sheets of mica varying from '0009 in. to 

 •001 in. in thickness. 



5. The experiments were carried out in the Pender Elec- 

 trical Engineering Laboratory under the direction of Dr. J. A. 

 Fleming. The measurements of current were made by means 

 of a sensitive Kelvin Astatic Galvanometer of 6057 ohms 

 resistance provided with a shunt-box. The E.M.F. of the 

 battery of small Lithanode cells was measured during the 

 earlier experiments by a Weston voltmeter, but as the current 

 taken by this instrument was sufficient to cause the E.M.F. 

 of the cells to fall during an experiment, a Kelvin electro- 

 static voltmeter was used instead. The battery, condenser, 

 and galvanometer were connected up to a three-way key in 

 such a way that, v\ ith the key in its normal position, the con- 

 denser was charged through the galvanometer, and when the 

 key was depressed the plates of the condenser were connected 

 directly to the galvanometer. The galvanometer was stan- 

 dardized by means of a Crompton potentiometer and standard 

 resistances. In this way the current corresponding to various 

 points on the scale over the whole range used in any parti- 

 cular experiment was determined by reference to the Clarke 

 cell. The u okonite " cable was immersed in a tank of water 

 3 in. x 3 in. X 2J in. deep. The two ends of the copper 

 conductor were brought out to a block of paraffin-wax having 

 four mercury cups. To two of the latter the ends of the 

 copper were joined; the other two mercury cups being con- 

 nected to a Wheatstone-bridge in order that the resistance of 

 the conductor might be measured during an experiment. 

 The volume of water was so great that the variation in tem- 

 perature was never more than 0°*2 G. during the day, while 

 the resistance of the copper conductor never varied more 

 than 0*2 per cent, in a day. The temperature of the dielec- 

 tric was taken as a mean between that of the water and that 

 of the copper conductor. To prevent leakage over the ends 

 of the cable, the braid was removed to a length of 2 feet and 

 the surface of the indiarubber kept clean. A Price's guard- 

 ring was also used to test for end leakage. In the mica and 

 paraffined-paper experiments no special precautions were 

 taken to ensure constancy of temperature during an experi- 

 ment, or to measure the actual temperature of the dielectric, 

 as the experiments were only intended to ascertain whether 

 equations of similar form to those found for the cable 

 would satisfy the curves of charge and discharge for these 

 dielectrics. 



6. In the experiments to determine the variation of re- 

 sistance with pressure, a battery of cells just sufficient to give 



