These are metal marking devices and run at about 6 volt AC. 
The "metal marking" thread in july 04 refers

The first picture is the Actograp device. The long screw is used to adjust the gap.

                    The second picture is a home brew device that I made before I picked up the Actograp tool at a car boot sale. It sparks a finer, less deep, line than the Actograp. In practice I use both depending on the size of the job.

                  The picture is pretty self explanatory. It's based two bits of 1/4" square steel screwed together. Strictly speaking, the lower bit should be spring temper silver steel but I'm pretty sure that I used mild steel for both pieces. The spriCan't remember the number of turns but it's 20SWG (0.036") and measures about 1/4 ohm - gets pretty hot if it's used for a long time. Air gap is 0.015" set by a shim at the fixed end.