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A voltmeter is a measuring instrument for measuring the voltage between two points in an electric circuit.
Many voltmeters are in fact very high resistance ammeters. Hence the design of the instrument is identical to that of an ammeter except that 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.
Other methods for measuring voltage include the potentiometer, which measures voltage by cancelling the flow of a current, and the oscilloscope, which measures voltage by its effect on a beam of electrons.
The first digital voltmeter was invented and produced by Andy Kay of Non-Linear Systems (and later founder of Kaypro) in 1954.
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