Portable, pivoted-coil DC Milli-Voltmeter; black painted metal case on wooden base; 10-40C thermometer and a level indicator are also mounted on the wooden base; Glass window showing mirror and white dial.
The millli-voltmeter contains a coil of thin copper wire that rotates on an axis, that is pivoted on jewelled pivots, and is perpendicular to the magnetic field of a permanent magnet. 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. There is a soft iron cylinder (strengthening the field and making it approximately radial in the space inside the coi) fixed to a brass bar which forms a bridge between the pole pieces. 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.
A.B.
