Electrometer: Brook's Steelyard

The purpose of an electrometer is to establish the magnitude of an electric charge or voltage. This specimen was made by the instrument maker John Cuthbertson, who worked in Amsterdam. The instrument looks like a balance and is therefore called an electric balance. The sphere at the end of the horizontal arm is extremely light. The other sphere contains the pillar of the balance and a short balance arm with a lead counterpoise. As soon as an electric charge is applied to the instrument, the long balance arm with the light sphere will tend to rise. However, this is prevented by moving the sliding collar on the balance arm to a point where it counterbalances this tendency. Then the position of the sliding collar is read from the scale on the balance arm. The result is a measure for the electric charge.