The Electromagnetic edge finder works on a very simple principle. The sensing head is an electromagnetic transformer primary. It induces a voltage  between the cutting tool and the workpiece. The circuit between the cutting tool and workpiece is the transformer secondary circuit. When there is no contact between the cutting tool and workpiece only magnetising current flows through the transformer primary. When the cutting tool makes contact with the workpiece additional current flows in the primary winding because there is now a short circuit on the transformer secondary. This additional current is detected  by a schmidt trigger circuit and indicates that an edge contact has been made.

This invention is covered by a provisional patent application No 607444 filed with the Intellectual Property Office of New  Zealand on the 22nd February 2013.