1951 Philips 9512M

The first gas discharge device developed at Philips in the Netherlands was the negative-glow neon lamp, which the company first released on the market in 1922, just five years after the invention of this technology by D. McFarlan Moore at General Electric (USA). Philips's first glow lamp was designed with a rather large beehive electrode structure and was intended as a nightlight. The Dutch released a smaller "half-watt" version for voltage signaling purposes in the early 1930s in the form of the lamp featured here, initially available in various types for mains voltages ranging from 110 to 650 V. By the 1940s the voltage range was reduced to 75–260 V (extended to 380 V in the early 1950s) and this lamp type became eventually known as model 9512, a popular workhorse in its application. The electrode structure in the first variant consisted of an iron disc with an opposite tubular ring made of the same metal.

Since the distance between the two electrodes is too small to allow the formation of a positive column in the low-pressure atmosphere, only the negative glow can exist. Interestingly, while the discharge spreads over the entire surface area of the disk electrode and its lead wire, the negative glow covers only the much smaller internal surface of the ring electrode. Since the lamp operates in the normal glow regime, one should expect that both negative glows cover similar surface areas. The phenomenon visible here is caused by the so-called hollow cathode effect where light and charges trapped inside the symmetrical structure of the tubular electrode results in a significantly higher electron current density emitted by the inner cathode surface, which leads to a smaller discharge coverage at a given drive current (2–3 mA in the present case), confined inside the electrode. This effect is most effective in the space close to the planar electrode, resulting in the discharge being the brightest there.



Another intriguing feature of the present lamp is the fact that its gas fill consists of a helium-neon mixture, used to ensure a low ignition voltage and a low cathodic sheath voltage drop (to limit sputtering). A neon-argon mixture is usually used for that, so it is possible that early Philips neon lamps employed a glass type which caused argon cleanup via physisorption, a problem which was known to occur in the company's "SO" sodium lamps made at that time. Using helium instead of argon solved this problem but results in a less effective (albeit more stable) Penning mixture and it gives the emitted light a distinctive pinkish-orange color. Philips eventually improved the lamp with better concentric electrodes, in a cup and wire ring and later in a bowl and curved ring configuration, so as to direct more light forward. By the late 1950s the glass bulb featured a front lens and the gas fill was a neon-argon mix resulting in a saturated orange light color. This latter design became an industry standard and remained unchanged to this day.