When you pop the hood of your welder, it should look something like this. The roller mechanism (A) pulls the wire from the spool (B), pushing it out to the torch nozzle. The tension adjustment knob (C) changes the amount of pressure the rollers apply to the wire. The electrodes (D) are adjustable, too, allowing the operator to switch the polarity of the machine. Most MIG welding is done in straight polarity mode-meaning the electrode (the MIG wire) is positive and the work is ground.
We've got 0.030-inch ER70S-6 steel wire mounted in place now, but what if we want to change to a different type or thickness of wire? First, put a piece of tape across the spool to keep it from unwinding, then cut the wire with a pair of side cutters anywhere between the spool and the rollers.