The tungsten rod is the electrode that transfers all of the current to create the weld since its melting point (6,192 degrees) is far above anything else you’ll be welding. There are several types available, but to keep things simple you can start with lanthanated. Consider it an all-purpose tungsten that will work on carbon steels, stainless steel, nickel alloys, aluminum, magnesium, titanium, cobalt, copper alloys, pot metal, and just about everything else. Size should be proportionate to the amount of current you need to use. Common 1/16, 3/32, and 1/8 sizes are good for 70-140A, 70-140A, 200- 310A, respectively.