PHOTOMETRY AND ELECTRIC LIGHTING. 341 



to provide each lamp with an automatic by-pass cut-out, as ex- 

 plained in Art. 140, but no by-pass or ballast resistances are 

 needed. Also when many lamps are operated in series by a con- 

 stant current the electromagnet which operates the mechanism of 

 each lamp must have a shunt winding connected across the arc 

 as well as a series winding in circuit with the arc. 



142. Luminous-arc lamps.* The light from the carbon-arc 

 lamp comes mostly from the intensely heated tips of the carbons 



Fig. 189 



and the arc itself emits only a pale violet light. The use of 

 carbons strongly impregnated with metallic salts, or the use of 

 rods of metal or of metal oxide instead of carbon, gives an arc 

 which is charged with metal vapor. Such arcs are as a rule 

 brilliantly luminous, and an arc lamp employing such an arc is 

 called a luminous-arc lamp. 



* See paper by C. P. Steinraetz, Trans. International Electrical Congress, Vol. 

 II., pp. 710-730, St. Louis, 1904 ; and also paper by Andre Blondel, ibid., pp. 

 731-767. 



