178 McCiLetLanp—On Secondary Radiation. 
of secondary effects when dealing with this type of radiation. Suppose we place 
the radium so that a pencil of 8 (and y) rays falls directly on the ionisation tube 7’ 
as in fig. 4. If, now, we proceed to measure the absorption produced by, say, a 
screen of cardboard, the result we get depends very much on where we place the 
cardboard in the path of the rays. 
In an experiment, the distance AB, fig. 4, was 26 cms., the internal 
diameter of the tube 7’ being 3:2 cms. 
The result was as follows :— 
lonisation in 7. 
No screen, ; : ; Poo 
Sereen at B, : ' al 
Sereen 5°5 cms. from B, : ‘ i 62 
Screen 10:5 ems. from B, : : : 54 
Screen 26:0 ems. from B, ; : ; 36 
The screen produces a different effect in different positions, because it is a 
source of radiation which travels in all directions from the part struck by the 
primary rays; and therefore the nearer the screen is to the testing-vessel 7’, the 
greater is the observed ionisation. If the secondary rays were of much less 
penetrating power than the primary, the observed ionisation would, of course, be 
increased by moving the screen towards the vessel 7’; but we have seen there is no 
very great difference in penetrating power between the primary and secondary 
rays, so that the chief cause of the effect noted must be the fact that the 
screen becomes a source of radiation spreading out in all directions. Of course 
we have also tertiary radiation from the air and from the end of the testing- 
tube. 
The numbers given above show that, after passing through the sheet of card- 
board, the secondary rays are of as great importance as the primary rays; for the 
ionisation with the screen close to the testing-vessel is twice what it is with the 
screen 26 cms. away. 
Another example of the importance of the secondary rays is given by results 
obtained in comparing the penetrating power of the secondary rays with that of 
the primary pencil consisting chiefly of B rays. The radium was first placed as 
in fig. 4, and the primary rays examined by placing different thicknesses of tinfoil, 
first at B and then at A. The apparatus was then arranged as in fig. 1; and the 
absorption of the secondary rays measured by placing sheets of tinfoil at the end 
of the tube 7. 
