722 Prof. W. H. Bragg on the 
pencilof a rays to be emitted from the material, and confined by 
suitable lead stops as in the diagram. Let the pencil cross an 
ionization chamber AB, A being 
a sheet of gauze, Ba metal plate. 
Let B be connected to an electro- 
meter and a saturating potential 
applied. Let us now calculate the 
ionization. Suppose the e rays 
to be all emitted with the same 
velocity. Let a be their range in 
air, d the distance from the sur- 
face of the radioactive material 
to the gauze, p the ratio of the 
density of the material to the 
density of the air, and let # 
be measured down into the material from its surface. 
Remembering that when an a@ particle comes from a depth 
« in the material, it only has a path w—pw in the air, we see 
that all the a particles belonging to the pencil and coming 
from a depth (a—h)/p will enter the chamber. If the depth 
of the chamber be of, and if it be assumed that each particle 
makes ions whose number is proportional to the distance 
traversed, then we may put the ionization equal to n(a—h)bdh/p, 
where 7 is aconstant. Weare here supposing that the ioniza- 
tion does not depend on the speed, and this is reasonable ; 
for Durack found that each 8 particle moving ata speed 
approaching that of light made a new pair of ions in every 
6 cm., whereas the slower particle of the Lenard ray made a 
new pair in every 2°3 cm., the air traversed being in each case 
ata pressure of 1mm. The speed of the a ray varies between 
far narrower limits than these. 
If the depth of the chamber be 0, the ionization is 
This supposes that part of the stream is strong enough to 
cross the chamber. If not the expression becomes 
(", Be O77, 
p 
ane : 
x 
Thus, if the ionization is measured, and a curve plotted 
showing its relation to h, the curve should in the former case 
be a straight line whose slope is nb/p, and in the latter a 
parabola. 
It should be observed that in this form of experiment a 
0 ee ee, ee ee —- 
