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. Electrical and Magnetic Instruments numbers 210 and 213. Mounted on a black box, with hole covered in frosted glass in wall, leading to a prism directing light through the bottom window.
Operation is similar to that of a quadrant electrometer.
It has a high sensitivity and a stable zero, and does not require levelling. The needle is attached to a torsion fibre, which is fixed under tension to fix the centre of rotation. Since no mirror is needed, moving parts only have a small inertia, therefore keeping the period low. An earth terminal eliminates problems with stray capacitance. Cambridge Electrical Instruments for D.C. Measurements, p70.