GENERATION, CONTROL, AND MEASUREMENT 167 



higher than 25 c mm-^, the G.E. photomicrographic lamp operates with 

 a brightness up to 48 c mm-^, which implies that the tungsten electrode is 

 within 100°C of its melting point. In such a lamp, life is determined 

 primarily by the blackening of the bulb. The photomicrographic lamps 

 are used principally with optical systems. The S-1 lamp (Taylor, 1931) 

 has sufficient mercury to produce considerable ultraviolet from the 

 mercury-arc stream and is used chiefly for the production of therapeutic 

 ultraviolet. 



CARBON ARC 



The carbon arc is a medium-pressure arc operating in air at a pressure 

 of 1 atm between electrodes of carbon compounded with various other 

 materials (Bourne, 1948; Finkelnburg, 1949, 1950; Forsythe, 1940; 

 Illuminating Engineering Society, 1952; National Carbon Company, 

 1944, 1948). It was the first electric-Hght source invented and is still 

 the most intense source available for continuous operation. Modern 

 lamps with well-designed feed mechanisms and magnetic-field stabili- 

 zation and using cored carbons of uniform composition have a high 

 degree of stability and are quiet. The brightness of a high-intensity arc 

 operating under favorable conditions will fluctuate less than 5 per cent 

 over considerable periods of time. The low-intensity arc (Rupert, 1952) 

 can have a fluctuation of less than 2 per cent. 



The spectrum of the carbon arc consists of thermal radiation, of 3800°K 

 color temperature, from incandescent carbon in the positive-electrode 

 crater upon which are superposed the fine and band spectra of core mate- 

 rials and substances formed by the reaction of air with carbon. Pure- 

 carbon electrodes produce an arc with a strong series of "cyanogen" 

 bands in the region between 380 and 400 m^ and another wide band at 

 250 m^ (Bowditch and Downes, 1938; Coblentz ci al, 1926; Greider and 

 Downes, 1932; National Carbon Company, 1944) (Fig. 3-11). 



In d-c arcs most of the incandescence comes from the crater formed in 

 the end of the positive electrode. The negative electrode is smaller in 

 diameter and burns at a much slower rate, and the tip becomes pencil- 

 shaped. Various mechanisms have been devised for controlling the rate 

 of feed of the individual carbons so as to keep the arc length constant 

 and the crater fixed in position. 



All modern carbon arcs employ cored-carbon electrodes which consist 

 of a tube of hard carbon filled with soft powdered carbon mixed with 

 various inorganic salts. The softer core tends to stabilize the arc stream 

 in the center of the electrode and provides a convenient means for adding 

 other compounds. Potassium salts are added as arc-supporting mate- 

 rials which contribute little to the radiation but, because they ionize 

 readily at high temperatures, stabilize the arc. Salts of the rare earths, 

 particularly those of the cerium group, produce an incandescent white 



