Wave form and amplification of corona discharge

Corona forms on a round wire or cable when the voltage is raised to such a point that the voltage gradient near the wire is sufficiently high to break down the insulating properties of the air. The larger the wire the higher the voltage to cause corona. The corona is luminous and ionizes the air, giving it electrical conductivity; thus corona has the effect of giving the conductor a larger diameter; and since a higher voltage is required for corona on a larger wire, a state of equilibrium is reached and corona is an equilibrium phenomenon not necessarily attended by spark-over. Since corona causes conductivity it is a cause of leakage and consequently of loss of power. It also increases temperature and decomposes the air into chemical constituents which are harmful to insulation. Engineers therefore have usually regarded corona as a dangerous phenomenon and one to be avoided by proper design. Transmission lines, for example, for the most part are designed so that their operating voltage is well below that at which corona would be formed on the conductors. Two suggestions have been made to make use of the properties of corona. The first is as a protective device for transmission lines. Since the corona is conducting and dissipates energy, it has been proposed to operate transmission lines relatively close to the corona-forming voltage. When abnormal rises of voltage due to lightning or other causes occur, corona begins, the air becomes conducting, and the high voltage is relieved. The exact value of corona in this connection is not known, although there is some evidence that it acts in the manner described. A second use of the corona, suggested by one of the authors of the present paper, is that it be used as a method for measurement of the crest values of high alternating voltages. The corona begins at a sharply marked definite value of voltage, and the laws governing the value are now well-known. The corona voltmeter is an instrument for detecting, either by the sound or by the conductivity of the air occasioned by corona, the exact voltage at which corona begins. The value of the voltage is given in terms of the dimensions of the instrument and the pressure and temperature of the air. The instrument has been described in two earlier papers, in which it is shown that the measurements of voltage with the corona voltmeter are more accurate than by any other method and that the instrument has several other advantages over methods at present in use. The present paper deals with methods for increasing the sensitivity of the corona voltmeter and for adapting the ordinary telephone receiver and standard portable instruments as indicators of the first appearance of corona. The corona discharge current is rectified and also amplified by means of hot cathode tubes. Incidentally the experiments on rectification included a study of the wave form of the discharge due to the alternating corona. It was also found that the sound due to the corona could be amplified so that the first appearance of corona would be indicated on a loud-speaking telephone.