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Philips very soon realised that an AC driven lamp which created light from a long, thin positive column discharge was a much more efficient efficient arrangement than the former short arc low voltage lamps, because the efficiency of a sodium lamp increases as the current density in the discharge falls Thus a long tube was called for and as the gap between the electrodes was increased, this necessarily called for a high voltage to be applied across the lamp The high voltage had an added advantage, in that lamps no longer had to be operated in long series chains on the mains electricity supply, and each could be installed independently of the others in an installation.
Alternating current is absolutely essential for the high voltage lamps, because owing to the great length of the discharge tube, the sodium would all be drawn to the cathode end within a short period of time In the DC lamps discussed above, this was not a problem since the cathode is at the centre of a large bulb and it served to always maintained the sodium at the core of the discharge.
In the first design, again pioneered by Philips, the long discharge tube was double-folded into four parallel sections to give it similar dimensions to the earlier DC products, and it was operated inside a detatchable dewar jacket to maximise its efficacy Oxide-coated electrodes were sealed into either end of the tube on a stem assembly similar to that used for incandescent lamps of the time. A pin-type base supplied current to cathodes at each end of the tube which were preheated, and the new lamp was famously introduced in late 1932 on the Purley Way in Croydon, UK It was rated 100 watts and drew a current of 0.6A to deliver a much better luminous efficacy of 62lm/W (Figure 14).
Figure 14 - Philips Philora AC, the first positive column sodium lamp (1932)
It was soon discovered that as a result of the high voltage across the lamp, sufficient power was actually radiated at the electrodes that they kept themselves warm, and no heating current was required The lamp shown in Figure 14 was redesigned with a large E40s screw base, one contact simply being connected to each end of the discharge tube
Efficiency was again advanced the same year when the lamp was redesigned into a plain U-shaped discharge tube operating inside the dewar jacket Although the lamp was not so compact, it had advantages because sodium vapour absorbs its own radiation, and in the Purley Way lamps, some light was being lost due to the light from one tube being absorbed by its neighbours Thus luminous efficacy with the new arrangement was increased a few percent and a complete range of different wattage lamps was put on the market They were offered in ratings of 50W, 65W and 100W for operation at a current of 0.6A, and a 150W size for 0.9A The name SO/H was coined and these lamps set the standard for nearly all of the future low pressure sodium light sources which will be described.
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