All of it.
They sell these things as accessories for commercial MIG units, but they can also work great on 24 volts from two car batteries.
I have it loaded with .030" flux-core welding wire.
It cost me less than $100 on ebay.
The "Ready Welder" is a similar commercial product.
WARNING: The tip is electrically on all the time!
Cheap Welding for Punks compares this gizmo to other homebrew welding methods.
In the following video, notice that I'm controlling power with the distance to the work. Close in melts in more, further away increases the wire resistance. That decreases current and the weld is cooler and builds up metal in a taller puddle. Notice that my first weld is terrible because I forgot to do that. The next two welds get a lot better. These are two random batteries that have been sitting out in the yard for at least a month. I don't know how well charged they are.
Thanks to Star for videography and other good works.
Want more homebrew welders?
Try an AC stick welder from microwave oven transformers,
DC stick welding with car batteries,
Solar powered golf cart welder
Step 1: Gun Shopping
I bought this spoolgun on Ebay for under $100. There's no brand name on it and no provision for shielding gas. It's well made and works great for flux-core wire. I don't see any like it at the moment, but there are a number of Miller and other major brand spoolguns for sale. Do "show completed listings" to see how much they've been going for.