The Work Process of MIG Welder:

A mig welder runs off electricity and uses the electrical current to raise the temperature of the base metal and fuse the filler metal (MIG wire) together in an electrical arc. This is your welding process.

The electrical current is passed from the power point on the wall, into the welding power supply, through the welding torch. When the trigger is squeezed the current then travels through the contact tip onto the wire making the welding wire live and conductive.

On the part you want weld, you attach the "welding earth" lead. Then when the MIG wire touches the earthed part, the electrical circuit is complete. As a result, the electricity starts to flow making a flash of sparks and an arc is created.

This arc is welding, without it you have nothing. Remember that "arc welding" is the process of welding using electricity. Mig welding is still arc welding as it uses electricity to make an arc, it's just that we call it MIG welding.