Resistance welding uses heat and force to create a bond between two base metals. There are several different forms of resistance welding such as spot, seam, and projection welding. The differences between the three involve the types and shapes of weld electrodes used and the actual process involved. Weld electrodes are used to conduct the heat to the base metals as they are forged together. These electrodes are usually made of copper because of its conductivity and low resistance. Generally, water is used to cool the electrodes during the process. Water flows inside the cavity of the electrode, thus removing excess heat. Many different types of welding machines can be built to fit various types of applications.