||Since the earliest days of projection apparatus, optical engineers had tried to create lamps having a near-ideal point source of very small dimensions. The favoured electric source was for many years the carbon arc, however this required constant attention and was often very noisy. In the mid 1930's a new concept was developed at the Ponders End Physics Laboratory of Royal Ediswan. Known as the Point O'lite, this totally enclosed arc lamp offered absolute stability with very long service life and was the favoured projection lamp for many years.
Although this is an electric discharge lamp, the light is actually created by the incandescence of the electrodes and a negligible amount of light is generated by the discharge itself. Lamps were usually made for operation on Direct Current and employed a tungsten bead as the incandescent body, upon which a DC arc would terminate. This particular lamp has been made for AC operation which relies on much simpler control gear, but has lower efficacy. The bulb is filled with a low pressure nitrogen-helium mixture and the arc is struck between two tungsten beads.
For ignition purposes, a 7V tungsten filament passes near to these beads, with a fine 'ioniser' tube of thorium oxide being slipped over one tail of the filament. Ignition is carried out thus : firstly, incandescence of the filament generates free electrons around the ioniser tube. A voltage is then applied between filament and one bead and an arc is struck. Once the latter is incandescent, the voltage is then applied across both beads and both become fully incandescent. No high voltages are required, a simple rheostat being adequate. A special 4-contact bayonet cap is used here.