In the context of resistance spot welding, the bulk resistance of a workpiece is a function of temperature. All metals exhibit a Positive Temperature Coefficient (PTC), which means that their bulk resistance increases with temperature. Bulk resistance is a factor in longer welds. Combined with contact resistance, the bulk resistance value of the workpieces, electrodes, and their interfaces both cause and affect the amount of heat generated.
The diagram illustrates three contact and four bulk resistance values, which, combined, help determine heat generated.
For more information, read our Fundamentals of Resistance Welding