64 BIOLOGICAL EFFECTS OF RADIATION 



the paths of the force Hnes, and it is, therefore, necessary to know 

 exactly which hnes terminate on the grounded guard plates and on the 

 insulated plate C which is connected to the electrometer. 



Since there must be a finite spacing between the edges of plates C and 

 G, and the lines of force are supposed to be parallel in these regions, the 

 plate C has an effective width greater than its actual width by an amount 

 equal to half the total gap between the collector C and the two guards G. 

 The effective width of C is also affected by its potential with respect to 

 the guard plates, being that given above only when the potential of C 

 is the same as G (42). On the other hand, according as C is above or 

 below the potential of G, its effective width may be less or greater than 

 the correct value. This is caused by field distortion, where, owing to the 

 small spacing between C and G, the field strength may be comparatively 

 large. In accurate measurements a potential difference of 0.5 volt 

 between guard and collector spaced 0.5 mm. apart may introduce a 

 noticeable error in the results even though the field strength between C 

 and H be as much as 200 volts/cm. 



In order to maintain the collector plate at the same potential as the 

 guards (earth) during measurements, it is necessary to employ a null or 

 other equivalent current-measuring device (2, 3, 12, 13, 25, 45). A 

 sensitive galvanometer is suitable for such measurements, although for 

 very weak X-ray beams the ionization current is usually too small to 

 permit the use of this method. Various electrostatic and vacuum-tube 

 measuring devices have been successfully used for the whole practical 

 range of X-ray intensities, but space does not permit their discussion 

 here (see pages 75 and 77). 



Effective Volume of Air Ionized.— The volume of air ionized by a 

 limited beam of X-rays cannot be determined accurately with the 

 requisite accuracy, the reason therefor being evident from Fig. 12. The 

 focal spot F is usually at least 8 mm. in diameter; hence a beam after 

 limitation by a diaphragm D consists of an umbral region u of uniform 

 flux density and a penumbral region p of nonuniform flux density. 

 Moreover, ions are formed along the secondary electron tracks e, and form 

 scattered X-rays both of which may be outside of the defined beam. 

 It is obvious from these considerations alone that the direct volume deter- 

 minations are rendered impracticable. 



Its indirect determination is rendered simple and exact by the fact 

 that all of the X-ray flux incident on the area of the aperture D tra- 

 verses the chamber, a fraction of which is absorbed in producing ions 

 between a and a' (3, 11, 41). The only assumption involved is that 

 absorption of the X-rays by the air between D and a is negligible m 

 comparison with total flux. This is valid except for the very soft X-rays 

 —below 60 kv.— where a correction must be made. The effective volume 



