A voltmeter is an instrument used to measure the voltage or difference in electrical pressure between two points of an electrical circuit. The meter is placed across that part of the circuit where the voltage is to be measured.
Because direct current (DC) circuits have a steady voltage and alternating current circuits (AC) have a varying, pulsating voltage, voltmeter construction must be designed to allow for these differences.
Direct currents are usually measured by a moving-coil, permanent magnet voltmeter or by a fixed-coil, moving magnet meter. The moving-coil meters use the most common of electric meter designs, the D'Arsonval movement. The current from the external circuit flows through the moving coil of the meter and sets up a magnetic field around the coil. This field opposes the field due to the permanent magnet, and the coil is made to rotate. The coil rotates until the force of the opposing magnetic fields is just balanced by the mechanical force of the springs on which the coil is suspended. The larger the current, the greater the rotation of the coil. Since a pointer is attached to the movable coil, as the coil rotates the pointer indicates the voltage-value on a scale calibrated (marked off) in volts.
Fixed-coil, moving magnet voltmeters are, in principle, similar to moving coil meters except that it is the permanent magnet that rotates with the pointer attached. A fixed coil surrounds the magnet.
Alternating voltages are measured with moving-vane meters, THERMOCOUPLE meters, copper oxide rectifier meter, or vacuum tube voltmeters.
In the moving-vane meter, a moving vane of soft iron is pivoted within a circular coil. When current flows through the coil, the moving vane is attracted by the field of the coil. As the vane moves, the pointer attached to the vane moves across a scale.
In a thermocouple meter, the current passes through a coil which heats the junction of two dissimilar metals, usually bismuth and antimony. The electromotive force generated by the junction is fed to a D'Arnsonval meter.
In a vacuum tube voltmeter, a vacuum tube isolates the input circuit of the meter from the circuit being measured. The AC is sometimes rectified by the tube and the resultant DC is fed to a D'Arsonval meter. At other times, the AC is merely amplified and fed directly to an AC meter.
