Low mains voltage mercury burner

One key design requirement of self-ballasted mercury lamps is that the mercury arc tube ignites reliably at the mains voltage. In 220-250 V country this is easily achieved with an ignition probe which first initiates a glow discharge at one end of the burner. Electrons and energetic photons produced by this auxiliary discharge then ionize the space between the main electrodes, resulting in a full ignition of the lamp. The situation is a bit more difficult with 100-120 V mains as the voltage is not high enough to even guarantee a reliable ignition of a glow discharge using an auxiliary probe. A pair of short incandescent filaments are used instead, they are a source of free electrons when heated, which lowers the ignition voltage of the discharge tube sufficiently to ensure a reliable starting at 100-120 V. The picture above shows such an arc tube, taken from a 160 W Westinghouse self-ballasted mercury lamp for 120 V mains, operated with the filament electrodes connected in series with the main electrodes.