168 



RADIATION BIOLOGY 



flame that greatly increases the brightness of the arc. Salts of other 

 metals are used to accentuate various portions of the spectrum. 



In order to reduce the voltage drop along the carbon and consequent 

 overheating at high current densities, the carbons are often copper-coated 

 to increase their electrical conductance. As the carbon burns, the copper 

 burns off at the same rate. The positive rotating carbon, in many high- 

 intensity lamps, is not copper-coated, but the carbon is clamped between 



150 



125 



100 



75 



< 



50 



25 



HIGH-INTENSITY PROJECTION ARC, 



1 185 amp, 75 v,-dc 



LOW-INTENSITY ARC 



30 amp, 50 V, dc 



_L 



J_ 



J_ 



I 



200 300 400 500 600 700 800 900 



WAVE LENGTH, m JLl 



1000 



1100 1200 1300 



Fig. 3-11. Relative spectral energy distribution of projection types of carbon arcs. 

 {Data from Greider and Downes, 1932.) 



metal jaws close to the tip so that the current path is short. In these 

 lamps only the negative electrode is copper-coated. 



The rate of burning of the electrodes is largely a function of current 

 density and air movement. The rate of positive carbon feed varies from 

 5 to 60 cm hr~^ in standard equipment (Illuminating Engineering Society, 

 1952; National Carbon Company, 1948). Standard carbon diameters 

 range 5-16 mm, and lengths, 20-55 cm. 



The a-c arc employs symmetrical cored carbons in which both elec- 

 trodes radiate equally. The a-c arc is less intense than the d-c arc of 

 equivalent power because each electrode is positive only half the time 

 (Joy and Geib, 1934). 



The arc is usually initiated by bringing the carbons together and then 

 separating. At the point of contact the carbon becomes incandescent 

 and is capable of emitting sufficient electrons to start the arc at the instant 

 of separation. The arc may also be started by a third high- voltage elec- 

 trode. All d-c arcs at low to medium power have a negative resistance 

 characteristic and require a stabilizing resistor. At very high current 

 densities the volt-ampere characteristic becomes positive, and less ballast 

 is needed in this region. In general, the most stable operation is obtained 

 when the supply voltage is 20-30 per cent above the arc voltage. The 



