350 ELEMENTS OF ELECTRICAL ENGINEERING. 



mounted in a small glass bulb which is filled with inert gas to 

 protect the iron from oxidation. 



The Nernst lamp gives a beautiful white light and its efficiency 

 is claimed to be greater than that of the carbon-filament glow 

 lamp. 



147. The mercury- vapor lamp,* The mercury-vapor lamp con- 

 sists of a highly exhausted glass tube into the ends of which are 

 sealed two platinum lead-wires. One of these wires carries a 

 large electrode (the anode) of iron or graphite, and the other 

 makes contact with a pool of mercury (the cathode). Either a 

 very high electromotive force or a special starting device is re- 

 quired to start a current flowing through the tube, but when once 

 started a current of several amperes, is maintained by an electro- 

 motive force of from 30 volts to 100 volts and the mercury vapor 

 throughout the tube becomes brilliantly luminous. The mer- 

 cury-vapor lamp of Cooper-Hewitt is started by tilting the glass 

 tube, thus causing the mercury to extend momentarily from end 

 to end, and the current which starts to flow through the fluid 

 mercury continues to flow through the mercury vapor when the 

 thread of mercury breaks. It is necessary to use a ballast re- 

 sistance in series with the lamp. 



The mercury-vapor lamp gives a light which is deficient in the 

 longer wave-lengths (red) and it produces an unpleasant distor- 

 tion of color values. It is, however, the best source of artificial 

 illumination for photographic purposes and it is used extensively 

 for lighting machine-shops, ware-houses and drafting rooms. 



*See von Recklinghausen, Transactions A. I. E. E., Vol. XXII., pp. 71-90; 

 C. P. Steinmetz, Transactions International Electrical Congress, Vol. II., pp. 710- 

 730, St. Louis, 1904 ; E. Weintraub, Transactions American Electrochemical 

 Society, Vol. VII., pp. 273-289, 1905. 



