Iv] NATURE OF THE RADIATIONS 131 
90. The increase of absorption of the a rays of polonium with 
the thickness of matter tra- 
versed has been very clearly 
shown in some experiments 
made by Mme Curie. The 
apparatus employed is shown 
in Fig. 26. 
Electrometer 
The saturation current was 
measured between two parallel | 
plates PP’ 3 ems. apart. The 
Fig. 26. 


polonium A was placed in the 
metal box CC, and the rays from it, after passing through an 
opening in the lower plate P’, covered with a layer of thin foil 7, 
ionized the gas between the plates. For a certain distance AT, of 
4 cms. or more, no appreciable current was observed between P 
and P’. As the distance AZ was diminished, the current increased 
in a very sudden manner, so that for a small variation of the distance 
AT there was a large increase of current. With still further decrease 
of distance the current increases in a more regular manner. The 
results are shown in the following table, where the screen 7’ con- 
sisted of one and two layers of aluminium foil respectively. The 
current due to the rays, without the aluminium screen, is in each 
case taken as 100. 



Distance AT in cms. 3°5 2°5 1:9 1°45 0-5 
For 100 rays transmitted by one layer| 0 0 5 10 25 
| For 100 rays transmitted by two layers| 0 0 0 0 0-7 





The metallic screen thus cuts off a greater proportion of 
the rays the greater the distance of air which the radiations 
traverse. ‘The effects are still more marked if the plates PP’ are 
close together. Results similar but not so marked are found if 
radium is substituted for the polonium. 
It follows from these experiments that the ionization per unit 
volume, due to a large plate uniformly covered with the radio- 
active matter, falls off rapidly with the distance from the plate. 
At a distance of 7 or 8 cms. the a rays from uranium, thorium, or 
9—2 
