Potential gradient measurements in argon-nitrogen mixtures using a wall-stabilized arc†

The potential gradient in wall-stabilized atmospheric pressure arcs in argon, nitrogen and argon-nitrogen mixtures has been measured over the current range 5 to 40 A.

The potential gradient was measured by four methods which were found to agree to within 3%. The values obtained were found to be a function of the state of the surface of the discs forming the arc channel and independent of the gas flow rate in the range l·-9·0 1/min.

The results demonstrate that the effect of a small trace of nitrogen in argon is much greater than that of a small trace of argon in nitrogen. For example, at 26 A 5% nitrogen in argon increases the potential gradient by ～30%, while 5% argon in nitrogen reduces it by ～ 3% from the pure argon and pure nitrogen values, respectively.

A simple two zone arc model is used to explain the results. This model indicates that the arc in argon entirely fills the channel for currents greater than 5 A. However, when nitrogen is added a central core is formed whose diameter decreases as the nitrogen concentration is increased, the current being kept constant.