voltmeter

Instrument for measuring the potential difference (voltage) between two points in a circuit. It should not be confused with an ammeter, which measures current. A voltmeter has a high internal resistance (so that it passes only a small current), and is connected in parallel with the component across which potential difference is to be measured – that is, the current divides and passes through both the voltmeter and the component at the same time.

A common type of voltmeter is constructed from a sensitive current-detecting moving-coil galvanometer placed in series with a high-value resistor (multiplier). To measure an AC (alternating current) voltage, the circuit must usually include a rectifier; however, a moving-iron instrument can be used to measure alternating voltages without the need for such a device.