Which factor can be used to judge the proper length of an arc when welding?

The sound of the arc is a critical factor in judging the proper length of the arc when welding. The sound produced when the arc is at the correct length creates a distinct, steady "buzzing" or "hissing" noise, which indicates an optimal and stable arc. If the arc length is too short, the sound may become harsh and crackling, while a longer arc may produce a softer, less focused sound. This auditory feedback helps the welder adjust their technique on-the-fly, ensuring that they maintain the appropriate arc length for effective fusion of the materials being welded.

Other factors such as the welder's position, the color of the smoke produced during welding, and the type of metal being used do not provide the same reliable and immediate feedback as the sound of the arc does. While these factors can influence the welding process, they are not as directly indicative of the arc length as the sound made by the arc.