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The Electric Stress at which Ionisation Begins in Air

1910; IOP Publishing; Volume: 23; Issue: 1 Linguagem: Inglês

10.1088/1478-7814/23/1/309

1747-3845

Alexander Russell,

Power Transformer Diagnostics and Insulation

In the Proceedings of the American Institution of Electrical Engineers for July, 1910, Prof. J. B. Whitehead publishes the values of the electric stress at which ionization begins in air. The electrodes he used in his experiments consisted of a metal tube and a cylindrical wire coaxial with it. Alternating pressures were employed, and the inner wires used had diameters varying in size from 0.089 to 0.475 cm. If a be the radius of the inner wire, the Author has found that the expression 32 + 13.4/radical a gives all Whitehead's experimental results for the maximum electric stress in kilovolts per centimetre with a maximum inaccuracy of less than 1 per cent. The experiments show that the electric stress at which ionization occurs is independent of the metals used for the electrodes and of the inner radius of the outer tube. It depends merely on the radius of the inner wire. It is shown that Steinmetz's experimental results on the sparking distances between parallel rods are in substantial agreement with Whitehead's figures. Possible reasons for the discrepancies are given. An empirical formula is given which is based on experimental results recently published by Kowalski and Rappel for the sparking voltages between equal spherical electrodes. The results indicate that the electric stress at the moment of discharge has a minimum value when the distance between the electrodes is a certain function of their radius. The author has neglected the currents of electrified air which stream round the electrodes before the discharge takes place. These currents probably modify appreciably the values obtained for the disruptive stress at the moment of discharge. The striking similarity between the formula for the heat emitted per unit area at the surface of a hot wire cooling in air and the formula for the electric stress at the surface of an electrified wire at which ionization first begins is pointed out.