Volt Meters

A voltmeter is a measuring instrument for measuring the voltage between two points in an electric circuit. more...

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.

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.

A voltmeter may also be realized using a potentiometer, which is a length of uniform resistance material (wire or carbon film, for instance) and a "wiper" that can short-circuit any portion of the material, thereby changing effective resistance between the wiper and an end terminal of the potentiometer. The unknown voltage source may be connected to a current detector, which is in turn connected to the potentiometer's wiper, while the known voltage source is connected to an end terminal of the potentiometer. Then the wiper position is adjusted to change the potentiometer's effective resistance until a balance is obtained and no current is detected. At this time, record the potentiometer's wiper position. For example, if our potentiometer were a length of very long wire and our wiper were some sort of metal wand in contant with that wire, record the length of wire between the wiper and the end of the wiper that is in our circuit. Now replace the unknown voltage supply with the known voltage supply and repeat the procedure. The unknown voltage is then given by the product of the known voltage and the recorded used length of wire corresponding to the unknown voltage, divided by the recorded length of wire corresponding to the reference voltage.

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