Carbon Arc Torch

Iron Man may have the arc reactor, but Dayton Diode has an arc torch. Electrically, the arc torch is about as simple as you can get. It consists of a voltage source, a resistor and an arc gap.
Arc Torch Circuit
The resistor is necessary to limit the current flow from the outlet. The resistance of the plasma in the gap between the electrodes is not zero, but it is small enough that it looks like a short circuit to a circuit breaker. The resistor in this case is a water resistor.

Water Resistor
The water resistor is dead-simple. Two electrodes hanging in a five-gallon bucket full of salt water (just normal table salt is fine). You just have to "salt to taste": use your multimeter to measure the resistance across the electrodes, and add salt until you get to the 8 to 10 Ohms range. This magnitude of resistance should limit current flow to acceptable levels so you don't flip the circuit breaker.

The torch electrodes are graphite rods from zinc-carbon D-cell batteries cleaned and neutralized with baking soda (the batteries have an acidic paste inside).
Arc Lit
As you can see from the picture a fairly standard looking welding arc is created between the electrodes. This means that the standard welding eye protection is required. DO NOT LOOK AT THE ARC UNPROTECTED.
Hot Graphite Electrodes

The electrode holders are straight 3/8" OD copper repair pipe that you can find in any home improvement store.

Enough heat is created by the torch to do light welding / brazing. Heating is proportional to current, so a step-down transformer would be a good addition to the circuit. Also, controlling the resistivity of the water resistor might make a good Arduino control project. We could measure the voltage drop and add salt or water to the bucket based on the resistance.
Tack-welded Soup Cans

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