# What is the resistance of an electric arc between two graphite electrodes?

I want to make a mini arc furnace just for fun. I have two 4mm diameter graphite electrodes from an AA battery. I could calculate the loss of heat by radiation, but I dont know the resistance of the arc and I need it to calculate the windings of the transformer that will supply it since I dont have a power source with more than 10 A.

• I can't stand behind this as an answer, but another forum has a discussion on this topic here: physicsforums.com/threads/… . I think you want to know what the minimum resistance is (for the voltage or current you apply, depending on whether your application is limited by voltage or current). Or maybe you want to know the average resistance during arc if the system oscillates very fast. Also, please add to the question what is the voltage of your power supply, so someone can better tell you possible/impossible right away. Commented Oct 21, 2019 at 7:49
• Datapoint only: With a 30Vmp 300W solar panel I can draw and hold an arc in full sun. Not a large arc, but impressive that it can be done. And salutary when considering DC switching. Commented Oct 21, 2019 at 12:18

Arc lamps have a negative resistance characteristic, so they need some series impedance to stabilize them if you are planning on connecting them to a voltage source.

You can experiment with something like a 500~1500W (at the supply voltage) heating element such as a small room heater in series with the electrodes. You need a way of (safely) bringing the rods into contact and then separating them when the carbon gets hot enough to sustain an arc.

As a WAG I would say the resistance in normal operation is around 5~10 ohms, but has a negative slope so a series resistance of perhaps 10~20 ohms will stabilize the arc and control the current to something reasonable for the size of the rods.

It cannot be emphasized too strongly that there are many ways to hurt yourself playing around with this- the voltages on the rods are usually high enough to be lethal (perhaps with respect to ground if you're trying to use mains without an isolation transformer- not recommended) and the arc will likely emit strong UV light- that's why they were used in the old plate making machines for offset printing (now replaced by sealed mercury arc lamps).

Check out the venerable Don Klipstein's page for more info.

I've been in an electric arc steel mill as this is happening, and a real one requires thousands of amps, enough to power a small town. It works by conducting current through scrap metal until it heats up enough till it melts, and then the liquid steel becomes part of the circuit, not the actual arc radiating. You need a ton of power to convert steel from a solid state into a liquid state.

To put it in perspective, consider a stick welder from Lincoln electric which can melt the tip of a welding rod. It uses about 80 amps just for melting the tip of some 3/32" carbon steel rod. So, with a paltry 10 amps, don't expect too much to happen.

That being said, for experimental purposes, to see what you can accomplish with 4 amps, a high school experiment I've seen was to attach leads from a small battery to steel wool. Wear safety gear of course. Basically it will glow red hot and then oxidize before your eyes.