New Self-Registerung Electrometer. 299 
the indications due to a change in charge is always neces- 
sary and afforded by 1. 
If now a wire from the water dropper is connected to the 
hook wire or avr electrode, the pendulum will be acted on 
by the electric force of the air where the water drops, and 
if the jar is positive, will be attracted towards this elec- 
trode when the air is negative, and repelled when it is 
positive, in proportion to the electric potential. And as the 
pendulum is so made that the deflections are seldom large, 
its angular deviation may be taken as proportional to the 
_ force. 
I have arranged this Hlectrometer so as to. be continu- 
ously self-registering by photography, on the same principle 
as is adopted at Kew for the magnetographs, a full descrip- 
tion of which arrangement is given in the British Associa- 
tion reports for 1859. 
This apparatus may be therefore styled “The Pendulum 
Magnetograph.” The method of procedure adopted is 
this: The electrometer being adjusted and charged, and the 
water cistern (which is made to contain twenty-four lfours 
supply) filled and dropping, the revolving cylinder covered 
with sensitive paper, and the clock going; at about 9 am., 
the earth electrode is connected, but the other disconnected, 
it 1s left thus for five or six minutes, and the scale reading 
obtained and entered as the earth reading ; at the end of 
this time the water dropper is connected to the earth elec- 
trode, it is left so till 9 am. next day, when the cistern is 
disconnected and filled, the light shifted so as to get the 
second day’s curve and the earth reading, and left on five or 
six minutes, then the cistern is connected. At 9 am. next 
day the same process is gone through, with the addition of 
removing the sensitive sheet from the cylinder and putting 
on a fresh one. 
The sheets when photographicively finished hy developing 
and fixing, shows curves corresponding to the variation of 
electric potential, and both the time and extent of these can 
at once be obtained from them. The beginning and end of 
each day's curve is marked by a short mark, distinct and 
somewhat removed from the general curve; this is the 
_ earth reading at the beginning and end of each day and is 
the photographic registration of the pendulum’s position 
when the earth electrode only was connected ; a line drawn 
from one to the other may be assumed as the line of earth 
xio2 
