[pound] absorption of the different types of beta rays 63 



III A, and was given as a reason for drawing the curve shewn in Fig, 

 6 with an additional rise, although no determinations were made with 

 which it could be confirmed. 



Curve C shews that while the more deflectable of the // radiations 

 were absorbed by 1.184 mms.of aluminium foil, the more penetrating 

 still passed through it. The slight rise in curve D also indicates that 

 some of the /^ radiation was still able to penetrate 4.73 mms. of 

 aluminium. With a thickness of S.14 mms. of aluminium, however, 

 no rise in the conductivity occurred, and as curve E, Fig. 13 shews, 

 this thickness was sufficient to cut off all the fi ray effect. 



It will be seen that the curves which are drawn on a large scale 

 for deflections of /^ rays downwards, and for deflections of these rays 

 upwards, corresponding to a thickness of 8.14 mms. of aluminium over 

 the opening at the top of the chamber and denoted by E and E' do 

 not coincide. It will be recalled further, that the curves drawn for 

 the limiting thicknesses of tin and lead under the same conditions 

 shewed an exact coincidence. This peculiarity in the behaviour of the 

 aluminium screen was investigated at considerable length and was 

 finally shewn by some experiments which are described later in Sec- 

 tion VI to be due to the action of the secondary f3 rays excited on 

 the far side of the thicker aluminium screens by the y rays entering 

 the chamber. 



In Table XIII there is given in row I the maximum saturation 

 currents in the chamber due to the ft, ft secondary, y and y second- 

 ary radiations and that due to natural causes for the different thick- 

 nesses of aluminium foil, in row II, the saturation currents due to the 

 y and secondary radiation and that due to natural causes and in row III 

 the maximum ionisations due to the ft and ft secondary radiations 

 deduced as explained in Section III A, from the Tables above and 

 their corresponding curves. On looking at the figures given in row 

 in of this table, it is seen that there is apparently a ft ray ionisation 

 of .5 or about one-seventh of one per cent of the greatest ft ray 

 ionisation in the chamber when the top of the chamber is covered by 

 8.14 mms. of aluminium. This conductivity, however, represents 

 really a x ray effect, due as said before to the thickness of the alum- 

 inium used and should be deducted from the last three of the 

 numbers given in row III of the table. These corrected values of 

 the maximum ft and ft secondary ionisations are given in row IV, and 

 a curve, Fig. 14, is plotted from these values. An examination of 

 this curve makes it evident that a thickness of approximately 7 mms. 

 of aluminium foil was amply sufficient to absorb all the ft rays, and 

 the secondary rays excited by them. 



