GASEOUS VAPOUR LAMPS About 1890 Dr. D. McFarlan Moore tested a large number of gaseous tube designs containing all available gases and a great many vapours. The ones then considered commercially promising were the relatively low-voltage, long-tube, nitrogen or carbon dioxide filled lamps. The former yield yellow-orange tinted light, the latter, white. With the former efficiencies of 6 lumens per watt are attainable ; with the latter, 2 lumens per watt. With the attain ment of efficiencies of the order of io lumens per watt in the tungsten lamp, the lamps were doomed as commercial sources for general illumination. However, where accurate colour match ing of objects is of interest, the carbon-dioxide filled tube still finds a very limited commercial application.
The Mercury Vapour Lamp.—The Cooper-Hewitt mercury vapour lamp was first exhibited in 1901. The present standard lamps range in size from 200 watts to 1,600 watts. They consist of a tube of glass containing mercury, mercury vapour and wires sealed into the ends of the tube and attached to a cathode elec trode of metallic mercury and a cup-shaped anode electrode of iron to conduct electricity to and from the current-carrying vapour.
The arc current may be considered as a continuous drift of electrons from the cathode to the anode and a relatively much slower movement of positive ions toward the cathode. The whiteness of the mercury-vapour light is due to the combination of the nearly complementary hues of the yellow-green lines with the blue and violet lines. On the basis that white light is one third each of red, green and blue, the mercury arc light gives the effect of being 29% red, 30% green and 41% blue. Green and red produce the sensation of yellow. This excess of blue and green is quite apparent. The variation from whiteness in comparison with other commercial illuminants and for sunlight is shown in Table III.
The Alternating Current Mercury Vapour Lamp.--The construction of the A.C. mercury-vapour lamp is identical with
that of the D.C. lamp except that there are two anode electrodes. The current in the lamp tube is a pulsating direct current of a frequency twice that of the alternating current. The mercury arc is essentially an unidirectional conductor because it is dependent upon the existence of a so-called cathode "hot spot" which forms on the mercury electrode but not on the iron electrode.
Neon Lamps.—A development of a gaseous conductor tube employing neon gas, which dates back to about 1911, has in recent years come into some prominence for certain special applications. By applying a high voltage to tubes fitted with electrodes and containing neon gas, the electrical discharge through the gas causes a luminous glow. The usual neon lamp consists of a section of glass tubing varying in length from io to 20 ft. and when used for electrical displays is bent to form the desired letters or pattern. Neon produces a characteristic reddish-orange glow at a medium efficiency of 5 to 15 lumens per watt. This colour is quite distinctive and is effective for electrical display even in day light. Because of the fact that orange rays are readily distinguish able in contrast to other yellowish-white colours, some applica tions of neon for limited signal purposes and for aviation have been made. By the addition of slight amounts of mercury a light blue colour is obtained. By adding other gases in conjunction with neon or by using coloured glass for the tubing, other colours are obtained, usually, however, at some sacrifice in luminous efficiency. The further development of theory of electric dis charges in vacuum as well as in the rare gases is likely to produce far-reaching results in the practical application of illuminants of this character. Some work, which is still in the experimental stage is being done on the so-called "hot cathode" tube which operates on standard iio volt circuits.