A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit.

The voltage can be measured by allowing it to pass a current through a resistance; therefore, a voltmeter can be seen as a very high resistance ammeter. One of the design objectives of the instrument is to disturb the circuit as little as possible and hence the instrument should draw a minimum of electric current to operate. This is achieved by using a sensitive ammeter or microammeter in series with a high resistance.

A voltmeter may also be realized using a potentiometer, which is a length of uniform resistance material (wire or carbon film, for instance) and a “wiper” that can short-circuit any portion of the material, thereby changing effective resistance between the wiper and an end terminal of the potentiometer.

Another popular form of voltmeter is the electronic voltmeter, or vacuum tube voltmeter, frequently referred to as a VTVM. This kind of voltmeter uses a tube (or valve in British English) or transistor circuit to amplify the input voltage, which facilitates two objectives: increased sensitivity, and/or increased input impedance (this equipment usually has an input resistance of 10 to 20 megohms).