Absolute voltmeter. This absolute voltmeter measures up to 500,000 volts. The voltage to be measured is applied between two parallel discs, which results in a force of attraction between the discs. This electrical attraction is balanced by weights, which are used to give a measure of the exact voltage. A steel tank (lower centre) is filled with nitrogen under high pressure to increase the range of voltage detected by the instrument. Photographed at the National Physical Laboratory, Teddington, UK, in 1951.