of Edinlurgli, Session 1883-84 
417 
small pin (h') in the 
brass top passed through 
the smaller hole (li) in 
the tongue of the plate, 
so that when the disc 
and top were screwed 
together by a screw 
passing through (s), the 
disc was prevented from 
rotating when brushed. 
The end of the wire {lo) 
(fig. 3), connecting the 
disc with the electrode 
of the electrometer, was 
screwed in between the 
head of the screw and 
the disc. 
To obtain quantitative 
results, it is necessary 
to be able to give the 
disc a constant rub. I 
found that a small 
camel’s-hair brush forms 
a very convenient rub- 
ber. The hair is a 
snfhciently good insu- 
lator, not very difficult 
to discharge after a whisk 
across the metal, and it 
does not scratch the 
metal — a most import- 
ant consideration. I 
always gave the disc a 
single whisk — never a 
plurality — for one read- 
ing. I drew the brush 
across the middle zone 
{z) (fig. 3), exerting 'as 
