\QQ RADIATION BIOLOGY 



constant-voltage source, the current increases rapidly until failure occurs 

 in the power supply. Therefore all arcs and other forms of gaseous dis- 

 charge sources require a stabilizing impedance to neutralize the negative 

 resistance characteristic of the discharge and to give the over-all system 

 a positive impedance. Such an impedance is known as a " ballast. ' ' Dis- 

 charge lamps operating on direct current require resistive ballasts, but on 

 alternating current they are usually stabilized with inductive ballasts. 

 Capacitive ballasts may be used only at frequencies above about 300 cps 

 (Campbell, 1948; Campbell and Bedford, 1947). At very high current 

 densities the resistance characteristics of the arc may become zero or 

 even positive. However, even when operating in the positive-resistance 

 range of the characteristic curve, the arc requires a hmiting impedance 

 for stable operation. 



If the discharge requires a voltage for starting which is above that of 

 the line voltage, the current-limiting inductance and the step-up trans- 

 former are usually incorporated into one unit as a high-reactance trans- 

 former. As more current is drawn from the secondary, the secondary 

 voltage of such a reactive transformer decreases nearly proportionately. 



Life. The life of enclosed gaseous discharge lamps is determined by 

 the rate of blackening of the envelope and deterioration of the cathode. 

 The blackening of the envelope is largely due to the sputtering and 

 evaporation of material from the electrodes and in some cases to chemical 

 decomposition resulting from traces of water vapor or oxygen in the lamp. 

 Deterioration of the oxide-coated cathodes results principally from evapo- 

 ration and sputtering of active material from the surface. The most 

 rapid rate of deterioration of oxide-coated cathodes occurs at the instant 

 of starting, when the potential gradient is high. Therefore the life of 

 such lamps is markedly influenced by the frequency of starting. 



TUNGSTEN ARC 



In an attempt to attain round small sources of high brightness, several 

 forms of the tungsten arc have been developed under such names as the 

 PointoHte, photomicrographic lamp, and the S-1 sunlamp (Bourne, 1948; 

 Illuminating Engineering Society, 1952; Roller, 1952; Taylor, 1931; 

 Weitz, 1950). These sources consist of tungsten electrodes, one or both 

 spherical or cup-shaped, which are brought to incandescence by ion 

 bombardment. The discharge is maintained in an atmosphere of argon 

 or argon and mercury. Although these lamps are operated as gaseous 

 discharge lamps, most of the radiant energy comes from the incandescent 

 electrodes, and the spectrum is that of incandescent tungsten upon which 

 is superposed a spectrum of the ionizing constituents. Since it is not 

 necessary to maintain the complex mechanical shape of a filament, a 

 somewhat higher temperature can be maintained than with filament 

 lamps. Whereas in the filament projection lamp the brightness is seldom 



