A.O. Smith Arc Length Monitor circa 1939

Here’s something you don’t see everyday. Actually, you may not have ever seen one, I know i hadn’t. It’s an arc length monitor produced by the A.O. Smith company, who incidentally also made several welding machines.



In stick welding (SMAW) with a constant current welder (CC), the arc length has a direct influence on the welding voltage output. This device apparently was just a voltage meter that provided feedback to the welder via two lights that were mounted inside the welder’s hood.



When the correct arc length was attained, both lights would be off, and not lit up. If you pushed your rod too close, the voltage would drop, and one of the lights would turn on, signaling the welder that he was out of the accepted arc length range and that he needed to lengthen his arc. The same was true if he pulled the rod too far away from the weld pool. The opposite light would turn on letting him know that he needed to shorten up his arc.



I have no idea whether this was something that was used by welding schools/training centers, or if in fact this was to be used in the field on critical welds – high pressure steam or chemical lines for instance. Either way, it’s a pretty interesting device. It seems like it would have been a really cumbersome and annoying device, hence the reason it probably disappeared from the marketplace. If anyone ever used one of these, or had experience with one, please let us know!