Resistance seam welding is a variant of the basic resistance spot welding process. In spot welding a weld nugget is produced by passing an electric current through the sheet materials to be welded while they are held together under a mechanical force between shaped copper electrodes in a lap configuration. Fusion is produced where the sheet surfaces are in contact because this is the point of highest electrical resistance and hence the point where the temperature is highest.
In seam welding a series of overlapping nuggets is produced, usually by replacing the conventional spot welding electrodes by wheels that turn as the parts to be welded are fed between them. The process thus produces a continuous leaktight seam.