Different Types of Beta Rays. 145 



them shows the manner in which the intensity o£ the 

 secondary radiation excited 'by the ft rays rose as the 

 thickness of the aluminium was increased. From this curve 

 it is evident that the maximum conductivity produced by 

 the ft and the reflected ft secondary rays attained a limiting 

 value when the aluminium-foil sheets reached a thickness 

 of *4 mm. 



It follows, then, from these results that a thickness of 7 mm. 

 of aluminium will completely absorb all the /3rays from radium 

 and the secondary rays which they excite in the metal. It 

 follows, too, that the secondary rays emitted by the front side 

 of a plate of the metal when bombarded by the ft rays from 

 radium do not come from a depth in the metal greater than 

 •4 mm. 



VI. Experiments on the Secondary Rays excited in 

 Aluminium by 7 Rays. 



It has been stated in Section V. A, that when a sheet of 

 aluminium 8*1 mm. in thickness, which was sufficient to 

 absorb all the ft rays and the secondary rays excited by them, 

 was placed over 'the opening at the top of the ionization- 

 chamber, the saturation-currents were not the same with a 

 magnetic field applied in one direction as those obtained 

 with the same field reversed. This lack of svmmetry in 

 the values of the saturation-currents obtained when screens 

 of aluminium were used, is illustrated by curves E and E' 

 in fig. 13. In the experiments with lead and tin screens no 

 effect of this kind was observed ; and in order to clear up 

 the matter, an additional series of experiments was carried 

 out to ascertain, if possible, the cause of it in the case of 

 aluminium. 



(1) In the first experiment a thickness of 4*73 mms. of 

 aluminium was placed over the opening at the top of the 

 chamber, and above this a thickness of '964 mm. of lead. 

 This thickness of lead, it will be remembered, was found in 

 the earlier experiments sufficient to absorb all the ft and the 

 ft secondary radiations. It follows, then, that with this screen 

 none but the 7 ray* of radium could enter the ionization- 

 chamber when this double thickness of lead and aluminium 

 was placed over the top. The conductivities in the chamber 

 for gradually increasing fields in both directions were taken, 

 and these are given in Table XVI. The second column of 

 Phil. Mag. S. 6. Vol. 17. No. 97. Jan. 1909. L 



