﻿Electricity on Insulators and Metals. 263 



suspended by a string over a deep metal jar into which the 

 specimen could be lowered. The jar was placed inside, and 

 insulated by quartz supports from, a larger earth-connected 

 jar, which served to increase the capacity and to shield the 

 inner jar from outside influences. The inner jar was connected 

 by a fine wire passing through sulphur plugs in the outer, to 

 the upper plate of a parallel plate condenser, and also to one 

 of the terminals of a quadrant-electrometer, the lower plate 

 of the condenser and the other terminal of the electrometer 

 being permanently earthed. The deflexions of the needle 

 were observed by a telescope and illuminated scale. The zero 

 of the scale was obtained by earthing the terminal of the 

 electrometer by means of a suitable mercury-cup key operated 

 by a string. The whole measuring system was enclosed in a 

 wooden box with a window for the electrometer, the inside of 

 the box being covered with tinfoil and earthed. The box 

 served toshield the whole apparatus from external influences 

 and by means of drying agents inside it kept the apparatus 

 perfectly dry. The needle of the electrometer was suspended 

 by a platinum wire "01 mm. in diameter, and throughout 

 the experiments was maintained at a potential of 100 volts 

 by a small battery of Weston cells. 



In measuring the charge on a specimen after a rub, the 

 zero of the electrometer was first observed, the earth con- 

 nexion removed, and the specimen lowered into the jar and 

 the deflexion noted. 



The insulation of the apparatus was frequently tested, and 

 the holder cleaned with a warm cloth, so that the specimen 

 should lose no appreciable charge during its removal from 

 the lever to the jar. Provided the holder were clean and 

 dry, no appreciable change in deflexion was noticed, even 

 when the charged specimen was replaced in the socket and 

 then again removed and lowered into the jar. Good insula- 

 tion of the specimen was thus secured by the use of amber 

 as a connecting piece in the holder, and the upper piece of 

 ebonite could be freely handled when being placed into or 

 withdrawn from the socket without any leakage of charge 

 from the specimen. 



Before each rub the specimen, holder, and rubber were 

 completely discharged by exposure to radiations from radium. 



Preliminary experiments showed that it was necessary to 

 reduce considerably the sensitiveness of the electrometer 

 owing to the large values of the charges produced on some 

 of the specimens. This was done by shortening the sus- 

 pending wire, so that the needle could still be maintained 

 throughout the experiments at a potential much higher tlnn 



