The lamp consists of a small diameter glass tube, bent into the shape of a "U" and having an electrode at each end.
A double contact cap made of porcelain is fitted, and the complete lamp fits into a detachable vacuum jacket and is held there by a spring loaded
pin. The vacuum jacket is a double skin glass tube, sealed at each end and exhaused.
For normal parallel connection, a leak transformer is used with each lamp. This transformer operates as a combined starting transformer and
also as a stabilising choke.
For the first one or two minutes the discharge is pink in colour, but as the sodium gradually vapourises so the colour changes to monochromatic
yellow. Should the lamp be subjected to a violent fluctuation of supply pressure, the lamp will not be extinguished. Similarly if the supply
ceases completely the lamps will relite upon restoration of the supply. These characteristics are quite an improvement on those of Mercury lamps.
Advantages Claimed For Sodium Lighting
- Efficiency. Initial efficiency 51 to 64 lumens per watt. Three to four times that of filament lamps.
- Life. The average life is 2,500 hours as aginst 1,000 hours for normal lamps.
- Colour. The colour is almost monochromatic yellow. All objects appear in neutral tints or black. This is no disadvantage for
street lighting as the eye is incapable of distinguishing colour at low levels of illumination, whatever the colour of the light. Chromatic
aberration is eliminated. The eye functions at almost the highest effciency under this colour.
- Visibility. Visibility under sodium light is approximately three times that under filament lighting. Comparison on a wattage
basis shows that equivalent visibility to that obtained from white light can be had for about one-ninth the wattage.
50 Watt "Philora" Sodium Lamp
|Alternating current 110 to 250 volts, 40-60 cycle frequency