Optical examination of spark erosion pits on some metal electrodes

Abstract

Optical methods have been used to examine the topography of the small erosion areas that are produced at the cathode and anode due to single, short duration, high current discharges. A plane parallel electrode arrangement has been used with gap separation of between one and two millimetres or less, and allowing electrical breakdown to occur under nearly uniform field conditions. The electrode erosion has been studied for discharges passed in oil dielectric, and also hydrogen, air, and argon at atmospheric pressure. High and low melting point metal electrodes have been used in the investigation. Constant current discharges have been produced by using pulse forming networks, but for slower rates of current rise in the external circuit, a single section condenser-inductance network was used. Some of the erosion areas have been suitable for examination by Fizeau reflexion interferometry, and the light profile techniques, which made possible an estimation of the volume of material lost from these areas. Observations were made using reflexion microscopy, which also served as a means for making erosion measurements. The results of the measurements of erosion loss at the anode for discharges in oil and gases, indicate that the electrode material cannot have been removed by a thermal evaporation process alone. From observations on the topography of the anode erosion areas, it would appear that the molten metal within the area is drawn into one or more peaks, and metal is lost from the area by some mechanical force action. The erosion areas at the lower melting point electrodes for discharges in oil and gas dielectric appear similar, but usually more intensive melting is produced for the discharges in oil. The molten multiple marks left on the anode and cathode, have been discussed in the light of recent research by other workers, studying the luminous areas formed at the electrodes during the discharge.