During resistance welding, part of the heat generated is lost to the surroundings by conduction (heat transfer through solids), convection (heat lost from exposed surfaces by air-cooling), and radiation (does not require a medium). Heat balance is a function of part material and geometry, electrode material and geometry, polarity, and the weld schedule.
The goal of good resistance welding is to focus the heat generated close to the weld interface at the spot where the weld is desired. For more information, read our blog Heat Balance: the Key to Successful Resistance Welding.
And for more information about the resistance welding process read our Fundamentals of Resistance Welding.