Resistance welding is one of the oldest of the electric welding
processes in use by industry today. The weld is made by a
combination of heat, pressure, and time. As the name resistance
welding implies, it is the resistance of the material to be welded to
current flow that causes a localized heating in the part. The pressure
exerted by the tongs and electrode tips, through which the current
flows, holds the parts to be welded in intimate contact before, during,
and after the welding current time cycle. The required amount of time
current flows in the joint is determined by material thickness and
type, the amount of current flowing, and the cross-sectional area of
the welding tip contact surfaces. In the illustration below a complete
secondary resistance spot welding circuit is illustrated. For clarity, the
various parts of the resistance spot welding machine are identified.