434 



ALTERNATING CURRENTS 



1 I 



people appear ghastly when viewed under this light, an object- 

 ionable feature in its use. Red light is tiring to the eye and 

 contributes little to the visibility of objects. Therefore, from 

 the physiological standpoint, the mercury-vapor lamp is excellent 

 for drafting rooms and for other places where fine detail is con- 

 cerned. Very satisfactory results are obtained if tungsten lamps 

 are used in conjunction with the mercury-vapor lamps, thus 

 doing away with the ghastly appearance of the workers. 



The mercury-vapor lamp has an 

 efficiency of about 0.75 watt per mean 

 spherical candlepower. It is funda- 

 mentally a direct-current lamp, but 

 with a compensator it can be used 

 on alternating-current circuits. The 

 connections when used with direct 

 current are shown in Fig. 390. 



A higher pressure mercury-vapor 

 lamp has been developed operating 

 at a higher temperature. The low- 

 melting point of glass necessitates the 

 use of quartz tubes with this type of 

 lamp. 



In the Moore tube, the light is ob- 

 tained from a luminescent gas at a 

 comparatively low temperature. This 

 is produced by high-voltage electric discharge 

 in vacuum tubes. These tubes are sometimes 



They have an 



Ballast 



Shifter 



-Starting Band 



FIG. 390. Connections 

 mercury vapor lamp. 



of 



luminescence 



taking place 



200 ft. long and are bulky per candlepower. 



advantage in that the color can be controlled by means of the 



gas used in the tube. 



If carbon dioxide be used, for example, a light closely approach- 

 ing daylight is obtained. This characteristic, together with its 

 uniform distribution, makes it an excellent lamp for color 

 matching, as with textiles. Its efficiency is about 2.5 watts per 

 candlepower. The power-factor is low. 



The Nernst lamp operates on the principle that porcelain, 

 at a red heat, is a conductor of electricity. This lamp has a 

 heating coil which makes a porcelain filament conducting. 

 The lamp gives a substantially white light and gives off no odor 



