The Corona Voltmeter

An instrument is described in which the first appearance of corona is used as a measure of the applied voltage. Three methods for detecting the first appearance of corona have been developed, in addition to the method of visual observation. These methods involve the use of the electroscope, the galvanometer, and the telephone respectively. For a given wire, in fixed relation to the opposite side of the circuit, corona-forming voltage depends on the density of the air, that is, on the pressure and temperature. The corona voltmeter consists of a grounded metal cylinder, with a central conductor on which corona is formed. Both cylinder and conductor are enclosed in a larger, air-tight cylinder, in which the pressure can be varied by a hand pump. This variation in pressure provides the means by which a wide range of voltage reading is possible. The calibration of the instrument is absolute, that is, can be calculated, or may be obtained by comparison with existing standards. The voltmeter is set for a given voltage by adjusting the pressure to a value calculated from the dimensions of the instrument and taken from a calibration table or curve. When the ascending voltage, reaches the value for which the voltmeter is set, corona begins, and this is sharply indicated by any one of the three methods mentioned. To measure an unknown voltage, the pressure is gradually lowered from some higher value and is read at the instant corona appears.