The distance from the tip of the welding electrode to the adjacent surface of the weld pool. See STANDARD WELDING TERMS.
Arc length is the distance through the center of the arc from the end of the electrode to the point where the arc makes contact with the surface of the work.
The length of the arc for a coated electrode (SMAW) is usually greater than that of the bare electrode (GMAW) because with the latter, the greater heat intensity of the arc assures penetration and fusion, while the gas shield protects the molten metal from atmospheric contamination.
The arc length of the coated electrode is usually longer than the portion that may be apparent to the eye, because the end of the electrode core wire burns away more rapidly than the coating and allows the coating to come closer to the molten pool than the actual end of the core of the electrode.