PHOTOGRAPHIC LIGHT SOURCES 



285 



lighting, and care should be taken to ascertain that the wiring is adequate to carry 

 the current required by the lamps. House circuits are usually wired and fused to 

 carry currents not in excess of 15 amp. Consequently if this current is to be exceeded, 

 it will be necessary that several lamps be operated from separate circuits so that the 

 safe rating of any circuit is not exceeded. 



Carbon ^ res.— The carbon arc is still used as a source of luminous radiation for 

 photographic purposes, but the convenience of incandescent lamps and gaseous- 

 discharge lamps, together with their more quiet operation, are responsible for the fact 

 that incandescent lamps are displacing carbon arcs. 



The electrical characteristics of the carbon arc are quite different from those of 

 incandescent lamps. The resistance of the arc decreases as the current increases, 

 which in turn further decreases the resistance. In order to prevent a disruptive 



1.0 



0.9 



0.8 



^0.7 

 o 

 ? 0.6 



^ O.B 

 .^0.4 



0.3 

 0.2 

 0.1 



3.000 



7,000 



Fig. 17. 



4,000 5,000 6,000 



Wciveleng+h in Angstrom Unifs 

 -Visual and photographic effects (when using panchromatic emulsions) of daylight 

 and carbou-arc illumination. The solid curves are for the arc. 



discharge from taking place, a ballast resistance or other current-limiting device must 

 be used in series with the arc to maintain stability of operation. The voltage across 

 the arc is about 40 to 80 volts, depending upon the spacing between the electrodes, the 

 voltage being proportional to the spacing. Increasing the current through the arc 

 increases the size of the arc crater but does not materially affect the luminous output. 



The spectrum produced by the arc is the continuous spectrum produced by the 

 incandescent carbons, upon which are superimposed line and band spectra from the 

 vapors of the arc. In the ordinary arc with untreated carbons most of the light is 

 suppUed by the positive crater. This crater operates at a color temperature of about 

 3700°K. and produces a brightness of about 13,000 candles per sq. cm. The arc 

 produces a light which is more deficient in the long visible wavelengths and produces 

 greater radiation in the shorter visible wavelengths than incandescent lamps. 



The high-intensity carbon arc increases its efficiency over the ordinary carbon arc 

 by diminishing the diameter of the electrodes with a given current (which is the usual 

 method) or by increasing the current for a given diameter of carbon electrodes. The 

 high-intensity carbon arc produces a gain in efficiency and a light which is more nearlj^ 

 white and steadier than that of the ordinary enclosed carbon arc. In one high- 

 intensity carbon arc consuming 150 amp., the positive carbon is 16 mm. in diameter. 



The spectrum produced by carbon arcs is intermediate between that of mean noon 

 sunlight and that of incandescent electric lamps operated at temperatures in the 



