Arc characteristics are summarized and relationships to the welding arc are brought in.The welding arc is so called as a matter of convenience.Actually, it differs from other short, high-pressure, heavy-current arcs mainly in application. The welding arc is a relatively steady-state phenomenon which is used to melt or transfer molten material. The basic phenomena involved in this arc are not essentially different from those occurring in many circuit interrupters or the short steady-state arcs used for illumination. The material presented in this paper relates to the high-pressure, short arc in general, but is equally applicable to the welding arc.It is, of course, quite impossible to cover adequately the complex phenomena occurring in an arc in a paper of this length, and the reader is referred to books on the subject.