VOLTMETER. An instrument used to meas ure the difference of potential between two points on an electric circuit. The voltmeter is usually a galvanometer of portable form so arranged with pointer and scale that volts may be read directly. For direct-current work the most satis factory voltmeters are based on the principle of the Darsonval galvanometer (see GALVANO METER) and have the movable coil mounted on jeweled and kept in position by a fine spiral spring. In experiments with the galvano meter it is readily demonstrated that with a high resistance in the circuit the current will be proportional to the difference of potential. This idea is the basis of the Weston and similar volt meters and the resistances are so adjusted that the pointer deflects over an evenly divided scale. Any ammeter if given sufficient resistance can be used as a voltmeter. And there are different types of the latter which may depend on the movement of a small piece of soft iron on a coil or of one coil with respect to another. A volt
meter based on still another principle is that devised by Cardew where the difference of po tential is indicated by the expansion of a long fine platinum wire which is heated on the pas sage of a current. Voltmeters for alternating currents are either a form of dynamometer or based on the principle of the quadrant electro meter. (See ELEurttomETEn.) Where the dy namometer principle is employed the current passes through two coils connected in series, lint one being arranged so that it may rotate within the other. The attraction of one coil for the other produced by the current enables the difference of potential to be measured. See AMMETER; ELEC TRICITY.