Probe start

A probe-start metal halide lamp has three electrodes in the arc tube: a starting probe electrode and two operating electrodes. To start the lamp, a discharge is created across a small gap between the starting probe electrode (also called the starter electrode) and the operating electrode. Electrons then jump across the arc tube to the other operating electrode to help start the lamp. Once the lamp is started, a bi-metal switch removes the starting probe electrode from the circuit.