104 McCietitanp—The Penetrating Radium Rays. 
APPARATUS USED. 
The brass tube a, fig. 2, is insulated, and fitted with a paraffin cork ¢c, through 
which passes the brass rod b, joined by a wire to an electrometer. This wire is 
led along an earthed metal tube filled with paraffin. The other end of the tube 
a is closed with a very thin metal cap. 
The radium is contained in a small vessel &, which is placed in a block of lead, 
so that a diverging pencil of rays travels along the tube a, ionising the air between 
aand 6. 
The tube a is kept at a high potential by joining it to a battery of small 
storage cells. The current through the ionised gas is measured by the elec- 
trometer, the tube @ being kept at a sufficient potential to produce the saturation 
current. 
Kie. 2: 
Sufficient thickness of lead is placed permanently between & and the tube a to 
absorb all the 6 rays, so that the ionisation observed is due entirely to y rays, 
and measures the intensity of these rays. The substance to be examined is placed 
in successive layers in front of the tube a. In this way curves are drawn showing 
the intensity of the y rays after passing through various thicknesses of different 
substances. 
Fifty milligrams of pure radium bromide were used, placed 10 cm. from the 
testing tube a. 
OBSERVATIONS. 
In the first place, the thickness of lead required to stop all 8 rays was deter- 
mined by observing the ionisation produced in the tube a, with nothing between 
the radium vessel R and the tube, and then putting successive layers of lead 
between; the vessel & itself stops most of the a rays. Fig. 3 shows that all the B 
