VOLTMETER, an instrument which indicates the difference of the electric potential between its terminals on a scale graduated in volts. Legally, the (international) volt is the electromotive force which produces a current of one (international) ampere in a resistance of one (international) ohm. This volt is equal to absolute C.G.S. units. Voltmeters are always con nected in parallel across the points whose potential difference is required to be measured, and, since it is essential not to disturb this potential difference, they must have a high resistance so that they may pass only a very small current. They may be divided into two classes, (a) electrostatic, (b) electrokinetic. Electrostatic
voltmeters depend for their action on the fact that when two conductors are at different potentials they attract each other with a force which varies as the square of the potential difference between them. Such voltmeters have the advantage of possessing an infinite resistance, but they are not very suitable for the measurement of small voltages (e.g., ioo volts). Electrokinetic voltmeters are simply high resistance galvanometers, and measure potential differences in terms of the minute currents which pass through them when they are connected to the points whose potential differences are required. (See INSTRUMENTS, ELEC TRICAL.)