Maker's Name: 
James White, Glasgow, No. 1405
Where made: 
20 × 20 × 34 cm

Cylindrical brass case, mounted on square wooden base with three levelling screws. Circular 20 cm diameter bevelled glass at top showing silvered dial, horizontal quadrant scale.
Non-linear paper-scale allowing readings up to 160 volts. The scale which extends over 70o, is calibrated between 40 and 160 volts, and approximately linear between 60 and 100 volts. Smallest divisions 1 volt. The moving element below has attached an aluminium pointer needle.
Central brass suspension tube extends vertically from the glass. The tube is a modern replacement.
A zeroing screw is found in the base, and two terminals, on the side of the brass case, one of which is labelled 'CASE' and a shorting switch for discharge of quadrants.
A specially-shaped dumbell needle is rotated by electrostatic attraction to charged conducting quadrants, between which it is suspended. Opposite quadrants are connected to earth (one set), and the potential of the needle (other set), which is the voltage being measured. This causes a deflection of the needle proportional to the square of the difference in voltages of the quadrants - hence the non-linear scale.
As the deflection is an even function of the voltage difference, i.e. always in one direction, the voltmeter can be used for AC measurements. Since no current flows through the device (the two terminals are isolated), the voltmeter is almost ideal.
Practically all electrostatic voltmeters are founded on the types developed by Lord Kelvin. The voltmeters were first produced in 1888, and later models were able to measure voltages up to about 20,000 volts. This type of voltmeter was in use for laboratory of industrial measurements as late as 1922.