A voltmeter is an instrument used for measuring the voltage 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.
The moving coil galvanometer is one example of this type of voltmeter. It employs a small coil of fine wire suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current that is flowing through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.
