426 Dr. J. P. V. Madsen on 



which produce ionization in the testing chamber, as shown 

 in previous papers by means of — 



(a) " tertiary emergence " ft rays as at / ; 

 (K) tertiary incidence /3 ra} r s as at k ; 



(f) j3 radiation produced in the gas of the chamber from 

 the 7 rays as they pass through it : 



(d) possibly some ions produced directly in the gas by 



the 7 rays ; and 



(e) subsequent /3 and 7 ray effects of a higher order. 



The results of a similar set of experiments without a 

 Pb dome and with Zn as radiator are shown in fig. 3, 

 curve A. The maximum value of the effect measured is 

 considerably greater than in the case of Pb and occurs at 

 a thickness of about 2 cm. of the radiator; for increased 

 thicknesses the curve falls as in the case of Pb. It will be 

 noticed that with sufficient thickness of radiator in this case 

 the curve falls below zero. This was found to be due to the 

 fact that a considerable amount of secondary y radiation, 

 proceeding from the sides of the conical hole in the Pb block 

 containing the Pa, could enter the ionization-chamber as 

 shown by the line c, f (fig. 1). As any radiator is increased 

 in thickness, it not only sends out its own secondary but 

 absorbs that coming from the sides of the conical hole. 

 It will be shown later how pioper correction can be made 

 for this effect. 



A third set of experiments was carried out with Al as 

 radiator ; the effects were of a similar nature to those from 

 Zn 7 the maximum value being about the same in amount as 

 for Zn, and was obtained with a thickness of about 4*5 cm. ; 

 but it was found that an appreciable difference was made 

 when a plug of Al was placed at b in place of d as shown 

 in fig. 1, even though the total thickness of Al was kept the 

 same in both cases. It will thus be seen that some secondary 

 radiation can, after emerging obliquely from a depth of 

 several cm. of Al^ produce an effect in the ionization- 

 chamber. The present form of apparatus was therefore not 

 suitable for obtaining from Al all the results so far obtained 

 for Pb and Zn ; in these latter it was found that similar 

 errors were very small. It may, however, safely be said 

 for Al as for Zn that the effect produced is considerably 

 greater than for Pb. 



We have so far neglected all consideration of the secon- 

 dary radiation which comes from the sides of the conical 

 hole ; this it will now be shown is by no means small, but 

 the correction which it introduced was found not to be 



