Voltmeters, Voltmeter basics, Simple voltmeter, electrostatic principle

Voltmeters operating on the electrostatic principle use the mutual repulsion between two charged plates to deflect a pointer attached to a spring. Meters of this type draw negligible current but are sensitive to voltages over about 100 volts.

An ideal voltmeter has infinite resistance: It is an open circuit. Although it is impossible to make a physical voltmeter with infinite resistance, a well designed voltmeter exhibits a very large internal input resistance. In some experiments, it is important to take into account the finite, non-ideal, internal resistance.

Voltmeter is an instrument that measures voltages of either direct or alternating electric current on a scale usually graduated in volts, milli volts (0.001 volt), or kilovolts (1,000 volts). The typical commercial or laboratory standard voltmeter in use today is likely to employ an electromechanical mechanism in which current flowing through turns of wire is translated into a reading of voltage. Other types of voltmeters include the electrostatic voltmeter, which uses electrostatic forces and, thus, is the only voltmeter to measure voltage directly rather than by the effect of current. The potentiometer operates by comparing the voltage to be measured with known voltage; it is used to measure very low voltages. The electronic voltmeter, which has largely replaced the vacuum-tube voltmeter, uses amplification or rectification (or both) to measure either alternating- or direct-current voltages. The current needed to actuate the meter movement is not taken from the circuit being measured; hence, this type of instrument does not introduce errors of circuit loading.

The instruments can provide readings in analogue form, by moving a pointer that indicates voltage on a scale. Modern digital voltmeters give readings as numerical displays. They also provide outputs that can be transmitted over distance, can activate printers or typewriters, and can feed into computers. Digital voltmeters generally have a higher order of accuracy than analogue instruments.