Arc welding by AC or DC:
Using a welding machine that has an AC, alternating current output is very ordinary as there are lots of these welding equipments in the world nowadays. But simply so that you know a mig welder is normally DC output. It is cheaper and easier to create an easy alternating current welding machine than it is to create a DC output machine. While, times are altering and there are a group of newer welding machines out there that are inverter driven and contain the capabilities to perform both AC and DC output.
If you use a DC or direct current machine you can decide which method you desire the polarity to be. You can contain the welding rod positive or negative. And this can be changed with no trouble using a rotary button on the front of the arc welding machine or by switching over the welding wires. Some welding rods will only work with AC power. And a few are only intended to work with DC power. It all depends on the welding application.