GENERATION, CONTROL, AND MEASUREMENT 169 



difference between supply and arc voltages is consumed in the stabilizing 

 resistor. 



There are three classes of carbon arcs, based upon the nature of the 

 coring material and the current density. These are the low-intensity, 

 high-intensity, and flame arcs. 



Low-intensity Arc. Solid or neutral-cored carbons containing potas- 

 sium salts for supporting the arc are used in the low-intensity arc. The 

 current density is 8-30 amp cm"^. Most of the energy comes from the 

 crater formed in the positive carbon which has an actual temperature of 

 about 3900°K, the temperature of sublimation of carbon. Increasing 

 the current density does not appreciably increase the core temperature 

 or the brightness of the low-intensity arc, since carbon sublimes without 

 melting; increasing the current merely increases the size of the crater 

 and the rate of burning. The crater brightness is relatively constant at 

 175-180 c mm~^ and is nearly independent of current density. 



The low-intensity arc is frequenth^ semienclosed for industrial oper- 

 ation. The arc quickly uses up the oxygen within the enclosure, and 

 the atmosphere becomes chiefly carbon dioxide and nitrogen at atmos- 

 pheric pressure. This greatly accentuates the cyanogen bands in the 

 blue and near ultraviolet and retards the rate of burning of the electrodes 

 but decreases the luminous efficiency. 



These arcs are operated in current ranges of 10-30 amp. Increasing 

 the arc voltage by lengthening the arc gap accentuates the ultraviolet 

 cyanogen bands. Arcs of this type are useful for blueprinting and other 

 applications requiring high blue and near-ultraviolet flux. The low- 

 intensity arc is probably the most uniform and reproducible of the three 

 classes and has been used as a source for infrared spectroscopy (Rupert, 

 1952). 



High-intensity Arc. In the high-intensity arc the core contains flame- 

 producing materials, principally cerium salts, in addition to the arc- 

 supporting compounds (Bowditch and Downes, 1938; Finkelnburg, 1949; 

 Gretener, 1950; Jones and Bowditch, 1949; Zavesky et al., 1945). The 

 current densities range from 60 to 200 amp cm~^. Most of the radiant 

 energy (70-90 per cent) comes from a highly incandescent vapor cloud 

 of cerium salts in the crater. The incandescence of the vapor cloud is 

 due to its extremely high temperature of 7000° 8000°K (Finkelnburg, 

 1949, 1950; Forsythe, 1940). Unlike the low-intensity arc, the bright- 

 ness of the crater increases with current density; values up to 2500 c mm~^ 

 have been produced experimentally (Finkelnburg, 1949; Gretener, 1950); 

 commercial lamps produce values of 350-1200 c mm^^. 



The current requirement of standard high-intensity-projection-arc 

 equipment is from 150 to 225 amp (Illuminating Engineering Society, 

 1952; National Carbon Company, 1948). The arc voltage is relatively 

 constant and varies from 70 to 80 v, depending upon the current. The 



