Voltmeter dial. Dial of an old voltmeter designed to measure 0-20 volts. The voltage of an electric current is measured in volts. This unit is named for the Italian physicist Alessandro Volta (1745- 1827). Voltage is the potential difference, also called the electromotive force, measured between two points in a flow of electric current. It can be thought of as a measure of the energy intensity of the current. Early voltmeters, with an analogue dial, were based on galvanometers modified with a high resistance. The voltmeter would measure the electric current, which is related to the electric voltage by Ohm's Law. The first dial galvanometers date from the 1880s.