Quadrant-Electrometer. 297 
on induction-plate last, because the charging of the in- 
duction-plate slightly diminishes the charge of the jar, and 
considerably displaces the zero-reading by giving an induc- 
tive charge to the quadrant A. It is also necessary to begin 
with the charge of the jar minutely too high, so that after sepa- 
rating the induction-plate from the interior of the jar, the latter 
shall have exactly the correct charge as indicated by the gauge. 
The deflection thus obtained was precisely 2984, repeated in 
many experiments. The double deflection given by seventy 
Daniell cells was 43°6 scale-divisions. By comparison with 
two Clark’s cells, the value of which I know, the potential 
of the seventy Daniells was found to be 74°2 volts; hence the 
potential of the jar is 1016 volts, when charged to the poten- 
tial indicated by the gauge. 
The constant » of the instrument was next determined by 
the formula 
g=(A—B)(C-AS* ) 
Four modes of connecting are available for this :-— 
mw—O—(4-2 yolts, b=: 
B=C=74:2 volts, A=0 ; 
a—C—(0, b= 74:2 volte ; 
B=C=0, A=74:2 volts. 
In each ease the deflection was 253°5 if the charge on the 
needle was positive in relation to the quadrant with which it 
was not connected; and was 247 when the needle was nega- 
tive. This at first appeared anomalous; but the explanation 
is very simple. The needle is aluminium, the quadrants are 
either brass or brass-gilded, I am not sure which. There is 
therefore a contact-difference of potential between the needle 
2 
and the quadrants; call it z. Thus, instead of o=—, we have 
= +rA(F +0) 
and 
d=N(—A) (- t)3 
this gives 
6? 
fo ON A 
The result was verified by using fourteen cells instead of 
seventy: the deflections were 10:0 and 88, which gives the 
same value toz. It is worth noting that the same cause 
=0°482 volt. 
