McCrriLanp—On Secondary Radiation. 181 
Magnetic Deflexion of the Secondary Rays. 
The nature of the secondary rays has been examined by passing them through 
a strong magnetic field. The apparatus, fig. 1, was modified by interposing between 
the plate P and the ionisation vessel 7’ the poles of an electromagnet, so that the 
rays had to pass for a distance of 7:7 cms. through a horizontal magnetic field ; 
the total distance from plate P to vessel 7 was 15 cms. Ii the secondary rays 
consist of charged particles, they should be deflected up or down, and thrown off 
the end of the tube 7. It was found that, as the magnetic field was gradually 
increased, the ionisation in 7’ gradually diminished, 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 interposing in the path of the 
rays a thickness of 7-7 ems. of iron, no further reduction of the ionisation took 
place, showing that this residual ionisation was not due to radiation from P, but 
simply to the normal ionisation in 7, perhaps increased by some direct radiation 
from the radium. This residual ionisation was, therefore, subtracted from that 
observed at each stage of increase of the magnetic field, and the remainder repre- 
sented 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 6 Rays. Primary 8 Rays. 
No field. > LOW : . 100 
160 : > te , ll 
310 , > =O : ; 39 
460 ; 5 2B : ; > I 
600 ; ; , 1G ; =. ; 4 
960 : U 0 
1250 ; : 0 : : 0 
2300 : : : 0 ; : , 0 
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 7, and then 
applying a magnetic field, first in one direction, and then in the opposite, that the 
direction of deflexion of the rays showed them to be negatively charged. The 
fact that the residual ionisation with strong magnetic fields was not diminished by 
interposing several cms. of iron showed that there was no non-deflectable radiation 
of the nature of y rays, or at least that it was too small to be detected by the 
apparatus used. 
TRANS. ROY. DUB. SOC., N.S., VOL. VIII., PART XIV. 21 
