The ionic wind voltmeter and thermo-electrostatic relay

The electric wind observed in the neighbourhood of a highly charged point in air is caused by the repulsion of ions produced at the surface of the point by the intense field there. In their movement the ions collide with uncharged molecules of air, thus giving to them velocity and, in effect, causing a wind from the point. When the field is alternating this wind has two components, a steady stream of air and a movement of alternation in synchronism with the field. The effect is observed at both high-tension and earthed poles. By enclosing the latter in an insulating vessel and using as the electrode a bent wire with suitable anti-corona shields, the wire is found to be cooled by the alternating component of the wind, and this cooling to be remarkably steady. The effect was investigated by varying the temperature of wire, the gas pressure in the enclosure, the nature of the gas, and the dimensions of the field, and the law for each of these variants is given. An instrument based on this action, termed an ¿ionic wind voltmeter,¿ was designed to indicate the voltage of any system above a few thousand volts by making the earthed electrode part of a hot-wire bridge. The balance of the bridge is disturbed when the field is applied, and the out-of-balance bridge voltage is a measure of the high-tension voltage, which can be observed at any distance. An indoor form is made to indicate up to 300 kV, an outdoor form up to 132 kV, and portable forms from 3 kV to 150 kV for general testing and X-ray work.