||The Philips tungsten arc lamps operate on a rather different principle than competitive designs. They are intended for AC operation, and as such employ two identical electrodes. This of course generates two points of light, and the lamp must be specially oriented such that one obscures the other. AC operation simplifies the control gear requirements, and the lamps are intended for operation on the mains with only a simple series resistance to limit the current.
In order to ensure ignition of the arc at the relatively low mains voltage, the lamp is filled with a neon-argon penning mixture. Even this may not always be sufficient to result in ionisation of the gas between the electrodes, so an auxiliary ignition arc gap has been provided near the neck of the bulb. This takes the form of small sharply-edged rectangular metal strip, positioned a short distance away from a metal rod. This arc gap is connected in parallel with the main electrodes, but in series with a small resistor inside the lamp cap. When power is applied the discharge strikes first here, producing photons and free electrons which then facilitate the ignition of the main arc. The high current flow from the relatively inefficient neon filling brings the electrodes to incandescence. A getter is also provided in the bulb neck, presumably to reduce hydrogen impurities that may otherwise impair ignition.
Neon, being a light gas of high thermal conductivity results in a rather inefficient discharge, with much heat being lost from the electrodes. It is also generates some light, as evidenced by its lines in the spectral power diagram, as well as the red halo that is clearly visible around one electrode in the photograph of the illuminated lamp.