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



out any metallic covering over the opening at the top of the ionising 

 chamber, and with only the single sheet of aluminium foil over the 

 opening at the bottom. In taking these the magnetic field was first 

 applied in such a direction as to deflect the fi rays down into the cham- 

 ber and observations were made on the saturation currents correspond- 

 ing to various field strengths. The field was then reversed and a 

 second set of readings taken as the (^ rays were gradually deflected 

 upwards and away from the chamber. Both sets of readings are given 

 in column I of Tables I and II and curves representing them are shown 

 in Fig. 2. From these curves it is seen that as the (i rays were de- 

 flected down into the chamber by the magnetic field, the ionisation 

 in the chamber rapidly increased to a maximum value and then 

 decreased as the different pencils of rays were swept past by the 

 increasing magnetic' field. It is seen, also, that when the^ rays were 

 deflected upwards and away from the chamber by gradually increasing 

 magnetic fields the corresponding saturation currents decreased rapidly 

 until a constant limiting value was reached. 



As already stated similar sets of readings were taken for different 

 thicknesses of absorbing layers of tin foil over the top of the chamber. 

 In columns II to V of Tables I and II are given the results obtained 

 with layers .0196 mms., .0784 mms., .1568 mms., and .3136 mms. in 

 thickness respectively and curves A, B, C, and D, corresponding to 

 the results given in columns II to V of Table I are shown in Fig. 3. 



Here again, it will be seen, when the ft rays were deflected down- 

 wards that with each absorbing layer the saturation current passed 

 through a maximum value. It will be seen, too, that the maximum 

 saturation current fell away as the absorbing layer was increased, and 

 further that as the thickness of the layers was increased it required a 

 stronger and stronger field to produce the maximum ionisation. 



The explanation of these results is found in the fact that the ft 

 rays issued from the radium in a number of approximately homoge- 

 neous sheaves or pencils possessing a maximum intensity in a direc- 

 tion at right angles to the axis of the ionising chamber. On applying 

 the magnetic fields these sheaves or pencils would undergo different 

 degrees of deflection, those of high velocity being less afïected by the 

 field than the more slowly moving ones. 



As the rays from a sheaf of low velocity would enter the chamber 

 first, the ionisation would increase and reach a maximum when the 

 axis of this sheaf of rays coincided with the axis of the ionising chamber. 

 Still higher fields would deflect the slow moving rays past the opening 

 of the ionising chamber and introduce others possessing still higher 

 velocities. In as much as Bragg,* and others, have shown that the 



1 Phil. Mag., Oct. 1907. 



