44 



BIOLOGICAL EFFECTS OF RADIATION 



High-voltage Generators. — The Coolidge tube also has the properties 

 of a rectifier — permitting current to pass through it in only one direction 

 — and hence, for accelerating the electrons, it may be used directly on a 

 high-voltage transformer. However, larger electron currents and higher 

 voltages may be used with less stress on a given tube when the voltage is 

 rectified before being applied to the tube. A number of such rectifying 

 circuits with the corresponding resultant voltage wave forms are shown in 

 Fig. 1. The voltage relationships indicated on the figures are approxi- 

 mate. If, in the full-wave circuit A two diametrically opposite rectifying 

 tubes, a and c, are removed, a half-wave generator results, and although 





-»l)00»- 



-tAfii>- 



■tT' 



V 



r?l 



© 



L 



(D 



± 



^ 



Fig. 1. — Typical X-ray generator circuits, a, Full wave; b, constant potential 

 (Hull); c, voltage doubling {Villard); d, voltage tripling {Witka). (See Table 7 for 

 comparative outputs.) 



the voltage stress per valve tube is reduced to half, the circuit is not so well 

 adapted for large currents (over 100 ma.). In circuit B the ripplage is 

 approximately proportional to the tube current and inversely proportional 

 to the square of the capacity and the cube of the frequency. In circuits 

 C and D, the effect of decreasing the capacity or increasing the current is 

 to effectively raise the time axis, cutting off the bottoms of the waves. 

 Circuits of types C and D are, in general, therefore, only economical for 

 relatively small currents — less than 10 to 20 ma. — the cost of condensers 

 becoming prohibitive beyond that point. 



Owing to space charge in the X-ray tube, the electron current is not, 

 in general, proportional to the voltage but approaches a saturation value 

 depending principally upon the electron emission of the filament and 

 the geometrical structure of the tube. Without going into detail, it is 

 therefore evident that the X-ray emission varies with the wave shape of 

 the applied voltage — a factor which at once complicates the control of 

 X-ray output as shown below. 



