[pound] absorption OF THE DIFFERENT TYPES OF BETA RAYS 57 



In addition to the measurements just described, others were taken 

 for magnetic fields in both directions with absorbing layers 1.234 mms., 

 1.96 mms., and 3.136 mms. in thickness, and the results of these are 

 recorded in Tables I and II. The curves E, F, and G, Fig. 5, were 

 plotted from the numbers in columns VI to VIII of Table I 

 and represent the conductivities obtained with fields which 

 deflected the fi rays down towards the chamber. The numbers 

 corresponding to the saturation currents obtained with different 

 absorbing layers when the rays were deflected upwards and away 

 from the chamber are given in columns VI to VIII of Table II, and 

 curves E', F', and G, which were drawn from the numbers given in 

 this table are shewn in Fig. 5. and represent the conductivities when 

 the rays were deflected in the opposite direction. 



The short sharp rise in the curve E shews that with tinfoil 1.254 

 mms. in thickness the /3 rays were still able to penetrate the absorbing 

 layer. A slight rise, as can be seen from the figure occurs in the 

 Curve F, but with the Curve G no evidence exists of any rise in con- 

 ductivity. This curve, moreover, coincides with the Curve G' which 

 is drawn from values of the ionisation obtained when the rays were 

 deflected upwards, and this coincidence of the two curves G and G, 

 shew^s that with the absorbing layer with which the results illustrated 

 by these curves were obtained, a thickness of tinfoil was finally 

 reached which could not be penetrated by the fS rays and b}' the 

 secondary rays which were produced by them in the metal. 



In order to find the precise thickness of tinfoil necessary to stop 

 all the /5 and yS secondary radiations, a curve shewn in Fig. 6 was 

 plotted taking as ordinates the ionisation in the chamber due to the 

 maximum fS and fS secondary rays for each thickness of tinfoil, and 

 as abscissae the thickness of the tinfoil screen with which each maxi- 

 mum w^as obtained. The maximum /? and y^ secondary ionisation 

 for each thickness was determined in the following manner. Taking 

 the results for a particular thickness, the limiting value of the ordinate 

 of the curve drawn for a deflection of the ft rays upwards was sub- 

 tracted from the maximum value of the ordinates of the curve drawn 

 for deflections of the fi rays downwards. Inasmuch as the limiting 

 value of the ordinates of the former curve represented the ionisation 

 in the chamber due to y rays, and y secondary, together with that 

 due to natural causes, and the maximum value of the ordinates of 

 the latter, the ionisation due to the maximum ft, /? secondary, 

 Y and y secondary radiations with that due to natural causes, the 

 difference gave the ionisation due to the maximum ft and ft secondary 

 ionisation for the particular thickness of tinfoil over the top of the 

 ionisation chamber. 



