Diferent Types of Beta Rays. 131 



conductivity would be obtained with each thickness of tin- 

 foil, the value of the maximum would decrease with the 

 thickness of the absorbing layer. It is evident, too, since 

 with increasing thicknesses the first effective sheaves of 

 transmitted rays would possess higher and higher velocities,, 

 that the field required to deflect the axes of these different 

 sheaves into coincidence with the axis of the chamber would 

 increase. Hence the maximum conductivities, when absorbing 

 layers of increasing thicknesses were used, would be obtained 

 by fields excited by currents of greater and greater intensity, 

 and this, as the curves A, B, C, and D show, is actually what 

 happened. 



The numbers corresponding to the saturation currents 

 obtained with different absorbing layers when the rays were 

 deflected upwards and away from the chamber by the magnetic 

 fields, are given in columns II., III., IV., and V. of Table II., 

 and curves representing them are shown in fig. 4. From 

 these it will be seen that with each absorbing layer the 

 ionization fell away as the rays were deflected upwards, and 

 soon reached a value which was constant, and which repre- 

 sented the natural conductivity of the air in the chamber, 

 together with that impressed upon it by the undeviable rays 

 from the radium and by the secondary rays which they 

 excited. 



These limiting curves, it will be seen, exhibit an effect 

 already pointed out and emphasized by Mackenzie * and 

 others, that when the thickness of a plate or wall upon which 

 7 rays are allowed to fall is gradually increased, the gain in 

 ionization at the back of the plate from the secondary radia- 

 tion is at first greater than the loss produced by the absorption 

 of the primary rays. This result is well exemplified by the 

 curves A', B', C, and D', which correspond to absorbing- 

 layers of increasing thicknesses, and which show that the 

 limiting value of the ordinate of B' is greater than that of 

 A', that of C is equal to that of B', and that of D' is again 

 less than that of C 



In addition to the measurements just described, others 

 were taken for magnetic fields in both directions with ab- 

 sorbing layers 1*251 mms., 1*96 mms., and 3*136 mms. in 

 thickness, and the results of these are recorded in columns 

 VI., VII., and VIII. of Table I. The curves E, F, and G, 

 tig. 5, were plotted from the numbers in these columns, and 

 represent the conductivities obtained with fields which de- 

 flected the (3 rays down towards the chamber. The numbers 

 corresponding to the saturation currents obtained with 

 * Mackenzie, Phil. Mag. July 1907. 



K2 



