After cooling, the welding object is glued together. At this point, due to the minimum gap between the electrode and the welding piece at the moment of lifting, the voltage between the electrode and the welding piece is higher (60--70v), and the preheating makes it easy to emit electr*** at the end of the electrode and where the welding piece is welded. As a result, the air in the gap is broken down and conductive, and a dazzling spark is produced, which is called arc discharge. The temperature of arc discharge can reach more than 2000k, the electrode and welding piece are melted, thus the welding is realized. After arc discharge starts, the voltage of electrode end and welding piece (arc voltage) is reduced by about 30 vs. the load formed by arc is resistive load.