A "Wagner Hammer" or "Neeff Hammer" is quite simple and essentially consists of an electromagnet and a breaker switch. The electromagnet pulls the hammer inward as soon as a current flows through the coil. If that current is interrupted, the hammer falls back down. The trick is to let the current through the coil flow via this hammer so that it acts as a breaker switch for the coil. Now, connect one terminal to the coil and one to your workpiece and you are ready to do spark erosion.

It is a simple add-on for any CNC or 3D printer and a great way to teach numerical control. You can engrave a piece of metal for your students without sharp, rotating tools or dangerous laser light.