Abstract: Signals which contain components that vary with the lateral position of an electric welding instrument as it travels along a weld path between opposite work pieces are obtained from the arc or other heat source and are first processed to remove confusing or nonsignificant components. The processed signals are then compared or combined with each other, or with other signals of predetermined value, to obtain outputs which can be used to determine or control the lateral position or movement of the welding instrument along the weld path. Specifically, in electric arc welding of metals, signals from an electrical characteristic at the arc, such as arc current, which include a component indicative of lateral position of the welding instrument with respect to one or more sidewalls, are first filtered and processed to remove undesirable components.