242 On Secondary Radiation. 



that the rays had to pass for a distance o£ 7*7 cms. through a 

 horizontal magnetic field ; the total distance from plate P to 

 vessel T was 15 cms. If the secondary rays consist of charged 

 particles, they should be deflected up or down and thrown off 

 the end of the tube T. It was found that as the magnetic field 

 was gradually increased the ionization in T gradually dimi- 

 nished^ reaching a minimum which was not reduced by a 

 further increase of the field. When this stage was reached, 

 if the poles were allowed to come close together, thus inter- 

 posing in the path of the rays a thickness of 7*7 cms. of iron, 

 no further reduction of the ionization took place, showing that 

 this residual ionization was not due to radiation from P, but 

 simply to the normal ionization in T, perhaps increased by 

 some direct radiation from the radium. This residual ioni- 

 zation was therefore subtracted from that observed at each 

 stage of increase of the magnetic field, and the remainder 

 represented the effect due to the radiation from the lead 

 plate P. 



The result is shown in column 2 of the following table : — 



Magnetic Field. Secondary /3 Rays. Primary /3 Rays. 



No Field 100 100 



160 73 81 



310 52 39 



460 23 12 



600 16 4 



960 7 



1250 



2300 



It was easy to show, by placing a thick lead screen in the 

 path of the rays so as to cut off the upper or lower half of the 

 pencil falling on the tube T, and then applying n magnetic 

 field first in one direction and then in the opposite, that the 

 direction of deflexion of the rays showed them to be nega- 

 tively charged. The fact that the residual ionization with 

 strong magnetic fields was not diminished by interposing 

 several centims. of iron, showed that there was no non- 

 deflectable radiation of the nature of 7 rays, or at least that 

 it was too small to be detected by the apparatus used. 



The primary /3 rays were examined in the same way, so as 

 to get a comparison of the velocities of the primary and 

 secondary, the mass and the charge being no doubt the same 

 in the two cases. The radium with the accompanying lead 

 screen was placed so that the primary rays fell directly on 

 the tube T, passing between the poles of the magnet on the 

 way. In this case a correction had to be applied for the 



