Volt Meter. The voltmeter measures the potential between two points in an electrical circuit. The voltmeter (containing two input terminals, leads, probes, or connectors) is connected to the two points of interest and the potential measured is presented on an analogue or digital display. Inside the meter, is a very high resistance inserted between one probe and the measuring circuit (ten or more meg ohms) to ensure only a very small current is taken from the circuit, otherwise a false reading is likely to be obtained. The measuring unit that determines the potential usually involves a high input impedance amplifier such as a field effect transistor (FET) or operational amplifier, which is designed to drive the display. The voltage range of thee voltmeter can be changed by choosing different input resistance values, which can be selected by a suitable switch.

Further Information on this Topic can be found in the books of R.P.W. Scott by following the listed links