The importance of the stability of the welding arc on both the ease of welding and the quality of the weld is fully recognized by everyone concerned with arc welding, whether he be a welding engineer or operator. The term arc stability is used widely by the industry, although too often it is loosely or poorly defined.Qualitatively, arc stability is understood to mean the ability of an arc to remain fixed in position, with uniform electrical Characteristics, and uniform flow of heat to the weld. The arc should have no tendency to become extinguished and should have ample ability to recover from the frequent short circuits introduced by the deposition of weld metal.Quantitatively, it is difficult to express arc stability in terms that permit explicit definition or numerical measurement.Because of this, many decisions, such as the evaluation of the quality and performance of welding electrodes, machine characteristics, arid recommended voltage and current ranges, are left largely to the personal opinions of expert welders. Their judgment is frequently supplemented by visual examination of the welds, metallurgical studies of the weld metal, and mechanical tests.During the past two years, the Welding Laboratory at the Rensselaer Polytechnic Institute, under the sponsorship of the United States Navy Bureau of Ships, has been engaged in the development of instrumentation for the study of arc stability. A number of instruments have been developed, each of which measures some characteristic or parameter of the arc which is thought to influence arc stability, and which might serve as an indication of the degree of arc stability.These instruments have been designed to be simple in operation, easily read, readily reproducible, and portable enough to be taken from one laboratory to another and into the field to intake measurements under actual welding conditions.