voltmeter instrument used to measure differences of electric potential , commonly called voltage, in volts or units that are multiples or fractions of volts. A voltmeter is usually combined with an ammeter and an ohmmeter in a multipurpose instrument. Most voltmeters are based on the d'Arsonval galvanometer and are of the analog type, i.e., they give voltage readings that can vary over a continuous range as indicated by a scale and pointer. However, digital voltmeters, which provide voltage readings that are composed of a group of digits, are becoming increasingly common. Since an oscilloscope is capable of giving a calibrated visual indication of voltage, it can be called a voltmeter. See also potentiometer .