It is the displacement of this coil, when a current flows through the coil, against a mechanical restoring force that indicates the presence polarity and strength of a current. Hair springs are attached to the axis of the pivoted moving coil, where they establish a restoring couple, and to points on the framework insulated from one another. They also form leads for the current to and from the coil. Attached near the upper spring is a light counterpoised pointer which moves over a scale. There is a resistance in series with the coil to avoid errors due to rearrangement of current and potential drop in the circuit.
Voltage is indicated by a mechanical displacement of a pointer against a scale.
To use the instrument it must be placed in parallel with the points whose potential difference is to be measured. Weston's design remains basically unchanged up to the present day. W. Paul, London,N. No. 31. A circular ebonite base with three levelling feet carries a reflecting galvanometer with its coil suspended between the poles of an electromagnet. The suspension also carries a mirror to reflect a beam of light to a scale.