A charged electrode is brought near the workplace. Between them is insulating oil, known in EDM as dielectric fluid. The electrical field is strongest at the point where the distance between the electrode and workpiece is least, such as the high point shown. Voltage has reached its peak, but current is still zero. A current is established as the fluid becomes less of an insulator.
Voltage begins to decrease.
The heat vaporizes some of the fluid, workpiece, and electrode, and a discharge channel begins to form between the electrode and workpiece. A vapour bubble tries to expand outward, but its expansion is limited by a rush of ions towards the discharge channel. These ions are attracted by the extremely intense electro-magnetic field that has built up. Current continues to rise, voltage drops. Near the end of the on-time, current and voltage have stabilized, heat and pressure within the vapour bubble have reached their maximum, and some metal is being removed.