298 New Self-Registering Electrometer. 
the mirror is an aperture lined with vulcanite, leaving an 
opening of about half-an-inch, over which is screwed a close 
fitting cover; this opening is for admitting the charging 
rod for charging or discharging the jar. The rod consists of 
a brass wire terminating at one end in a ball, at the other 
in a square filed on the wire, the intermediate stem being 
covered with gutta percha. When the rod is inserted, the 
square end readily finds its way into a trumpet mouth, 
formed in the head of the lifting screw, and forms an insu- 
lated connection with the inner coating, and at the same 
time serves as a key for turning the lifting screw for raising 
or lowering the pendulum. To charge the jar the rod is 
inserted, and a few good sparks from an electrophorus 
passed on to the ball end; the rod is then carefully with- 
drawn and the opening closed; the air within being dried 
by the sulphuric acid, and the jar and sensitive pendulum 
protected from all external electrical influence by the cover- 
ing cylinder; the charge of the jar remains sufficiently 
constant over a considerable period. 
The pendulum need never be lifted off its bearmgs except 
to adjust it, or in case it has got out of position, when the 
lifting and lowering it puts it imto its place again.— 
The pendulum being adjusted, and the jar charged, the 
former is highly sensitive to electrical intluences presented 
to it through the electrodes; and if we now connect one 
electrode with the earth by slipping down the sliding piece 
(S) till it touches the cover, the pendulum is immediately 
moved towards the earth electrode if the jar is charged 
positively, and vice versa if negatively. The mirror moving 
with the pendulum gives a means of ascertaining the 
amount of its deviation from the vertical or zero position ; 
to this end a scale is set up in front of the window and 
mirror, so that its reflected image can be seen—this image 
is viewed by the aid of a telescope, as the mirror moves 
the scale appears to move, and a different position 18 seen In 
the telescope. 
Suppose before the jar is charged the scale reading corre- 
sponding to the zero position of the pendulum is m—this 
is the zero reading, after charging and connecting earth 
electrode m+ %, this is called the earth reading. The 
difference between the two readings, or ”, 18 a measure of 
the charge of the jar, and varies with it: and as the stronger 
the charge the more sensitive the peudulum, a correction to 
