A tool found in a machine shop that is used to attach two pieces of sheet metal together. The ones I've used look like a mutant cousin of a pair of scissors, with a kind of clamp replacing the blades, and a thick cord running to a big transformer.

  1. Put two pieces of sheet metal on top of each other.
  2. Clamp the two leads of the spot welder on the junction.
  3. Run a high current between the leads, across the junction. This causes the metal to melt and fuse together where the leads touch. This is usually done as a secondary mode of the clamping (i.e. squeeze once to clamp, and harder to weld).

Good spot welding is something of an art. Too little juice will create a weak weld. Too much current will make the junction brittle and blackened. Waaay to much current will burn a hole in the metal.