New Self-Registering Electrometer. 297 
a mirror), with two horizontally projecting arms, into 
which are screwed the two delicate steel cones on 
which the pendulum is poised; these points are in a line 
with the centre of the mirror-ring; ninety degrees from 
these points the top and bottom stems of the pendulum are 
fixed into the edge of the ring; the upper stem is of light 
aluminum wire, carrying at its top, about 44 inches from the 
centre of the ring, a piece of sheet aluminum about half-inch 
broad and three inches long, bent to the curve of a circle of 44 
inches radius ; this is fixed at right angles to the plane of the 
mirror frame; the lower stem is a piece of light brass wire 
screwed over its whole length, on which a small weight to act 
as a counterpoise can be screwed upand down. On one of 
the arms of the mirror-frame a stud is fixed, through which 
is inserted a piece of screwed brass wire to act as a balancing 
arm. ‘The mirror, which is a circle of silvered. parallel glass, 
is fixed into the ring with cement. The steel suspension- 
points of the pendulum rest upon flat polished steel surfaces 
let into the arms of the frame attached to the inner coating 
of the Leyden jar, and when these surfaces are level, the pen- 
dulum properly balanced is counterpoised so as to vibrate 
once in two seconds ; it is then extremely sensitive to the 
slightest influence. A cradle to lift the pendulum off its 
bearings and drop it gently and accurately in its place again 
is adapted to the jar-frame, and is worked by a lifting screw 
_at the back. 
The jar and frame is so fixed to the base, that when the 
pendulum is in its place the small segment of sheet alumi- 
num on its top swings freely and symmetrically under and 
about one-eighth of an inch distant from the electrode arch. 
Two leaden trays, holding lumps of pumice stone soaked in 
sulphuric acid, rest on the slate base, and partially surround 
the bottom of the jar. Over all this is a cylinder of copper, 
closed at the top, and fitting on to the base with a ground 
flange. Through the top of this cylinder there are two 
tubular apertures, fitted with plugs of vulcanite, through 
which two stout brass wires are inserted with a sliding 
air-tight fitting, and pass into holes in the upper surface of 
the electrodes ; one wire ends outsidethe cylinder in a kind of 
hook, the other carries a sliding piece, so that it can be 
brought in metallic communication with the covering 
cylinder. In front of the cylinder is a window of parallel glass, 
fitting air-tight. At the back, and ona level with the centre of 
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