130 Mr. V. E. Pound on the Absorption of the 



also, that when the ft 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 tinfoil over the 

 top of the chamber. In columns II., III., IV., and V. of 

 Table I. are given the results obtained with layers 0*0196 mm., 

 0-0781 mm., 0*1568 mm., and 0-3136 mm. 'in thickness re- 

 spectively, and curves A, B, C, and D, corresponding to the 

 results given in these columns, are shown in fig. 3 (PI. II.). 



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

 dowmvards, 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 thick- 

 ness of the layers was increased it required a stronger and 

 stronger field to produce the maximum ionization. 



The explanation of these results is found in the fact that 

 the ft rays issued from the radium in a number of approxi- 

 mately homogeneous sheaves or pencils possessing a maximum 

 intensity in a direction at right angles to the axis of the 

 ionizing-chamber. On applying the magnetic fields, these 

 sheaves or pencils would undergo different degrees of de- 

 flexion, those of high velocity being less affected by the field 

 than the more slowly moving ones. 



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

 chamber first, the ionization would increase and reach a 

 maximum when the axis of this sheaf of rays coincided w T ith 

 the axis of the ionizing-chamber. Still higher fields would 

 deflect the slow moving rays past the opening of the ionizing- 

 chamber and introduce others possessing still higher velocities. 

 Inasmuch as Bragg* and others have shown that the ionizing 

 power of ft rays of high velocity is not so great as that of 

 those moving more slowly, it follows that with increasing 

 magnetic fields the ionizing power of the rays introduced 

 would be less than that of the rays cut out, and hence a drop 

 in the ionization values would occur. This drop in the con- 

 ductivity would continue until ultimately all the deflectable 

 ySrays were swept past the chamber. As the layers of tinfoil 

 were gradually increased in thickness, the more slowly moving 

 ft rays would be absorbed, and the first effective sheaves 

 transmitted would consist of rays possessing higher and 

 higher penetrability, and consequently of rays with less and 

 less ionizing power. It follows, then, thai; while a maximum 



* Bragg, Phil. Mag-. Oct. 1907. 



