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.


Starting Characteristics
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

1. Efficiency. Initial efficiency 51 to 64 lumens per watt. Three to four times that of filament lamps.

2. Life. The average life is 2,500 hours as aginst 1,000 hours for normal lamps.

3. 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.

4. 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.