Hi. I tried experimenting with a carbon arc the other day, and it did not work out as I hoped. I used a rewound microwave oven transformer with #10 wire in the secondary which gave 3.1 V out with an 11.8 V test input. A resistance soldering unit probe was used to test the function of the carbon arc. There were no active methods for current limiting, except for the use of #14 zip cord to the resistance soldering head, as well as the soldering probe resistance itself of0.14 ohms. The system was plugged into a computer surge protector strip with a 10 A circuit breaker, and fused with the original microwave oven fuse in an attempt to keep it from blowing a breaker in the (far away) breaker box.
Previous google searches of newsgroups have stated that these types of carbon arc torches are only useful for welding tin cans. This is probably due to their limited output of 1 kW, which would be about 30 A at the observed voltage. So, a test was carried out on 0.05" mild steel sheet to begin with.
When the grounded steel sheet was touched with the carbon rod, a horrible brush like purple flame appeared. This flame was not like an acetylene flame: it was very hot. It melted a radial front of metal away from it faster than I could react to move the carbon rod, then the arc went out. This is not even useful for cutting, since the melting front moves faster than I can respond with the carbon rod. The lip was verified to be melted steel with a file. This thing sure has a lot more action than that weak resistance soldering unit! It would just poke holes in tin cans, contrary to that post that said that these devices were quite limited.
It seems that the problem of lack of control comes from insufficient current limiting, or insufficient skill. There is probably a positive feedback problem that as the metal melts away lengthening the arc, there is more energy in the arc, causing the process to go faster, until the arc becomes too long and gets extinguished. Would it help to add current control (tapped inductor or phase control circuit) to make the arc easier to maintain in a steady fashion?