Contact resistance is an important parameter in the control of electrical spot welding. Here, a large current is passed through two water cooled copper electrodes which also clamp the sheets to be welded . The contact resistance should not be too high, otherwise too much current is diverted to previously formed, neighbouring spot welds, and can make nugget formation difficult to control in the early part of the welding cycle. However, if it is too low, nugget formation is not completed at the interface. Therefore, the parameters that affect the contact resistance must be known and it must be compatible with the welding conditions .