Abstract :
The paper describes a method that has been developed for measuring the current and voltage characteristics of a stable d.c. arc. The arc is formed between stationary electrodes by first breaking down the gap using a high-voltage capacitor discharge. This discharge is allowed to develop into a power arc which is then maintained by an accumulator supply. The arc current and voltage are measured on peak voltmeters which are switched into the circuit only after the arc has become stable. Switching is done automatically by a sequence controller which also opens the main circuit-breaker as soon as the measurements have been made. In this way, electrode wear is reduced. The methods of previous workers using oscillographic-fllm techniques are reviewed critically; the advantages of the new method of measurement are great saving in time and labour, increased accuracy due to the measurement of true stable arc characteristics, and freedom from the complication of moving electrodes such as is involved in the measurement of instantaneous gap length. This research is part of a general programme of investigation of the effects of reduced gas pressure on the behaviour of an arc. The paper gives the results for copper electrodes in air in the pressure range 4?760mm Hg at currents between 1 and 17 amp and for gaps up to 2mm in length; and for fine silver electrodes in air at pressures of 1?760mm Hg, at currents between 1 and 20 amp for gaps up to 2 in length. The investigation shows that, as the pressure is reduced below approximately 50mm Hg, the difficulties of interrupting a current appear rapidly. At lower pressures the voltage drop in the arc column becomes small compared with the drops at the electrodes, so that it is then possible to maintain an arc at a voltage very little in excess of the minimum arcing value for the electrodes. Under these conditions the length of the gap becomes unimportant, so that lengthening the gap is ineffective as a means of increasing the current-rup- - turing capacity of a pair of contacts. The experimental results for each electrode material are given in the form of a single expression, which is presented also as a nomograph.
