134 NATURE OF THE RADIATIONS [CH. 
A simple method of determming the absorption in gases is 
shown in Fig. 28. The maximum 
current is measured between two LBP BIOTIC 
parallel plates A and 6B kept at a 
fixed distance of 2 cms. apart, and 
then moved by means of a screw to 
different distances from the radio- 
active surface. The radiation from 
this active surface passed through a 
circular opening in the plate 4A, 
covered with thin aluminium foil, 
and was stopped by the upper plate. 
For observations on other gases be- 
sides air, and for examining the Fig. 28. 
effect at different pressures, the apparatus is enclosed in an air- 
tight cylinder. 
If the radius of the active surface is large compared with the 
distance of the plate A from it, the intensity of the radiation is 
approximately uniform over the opening in the plate A, and falls 
off with the distance # traversed according to an exponential law. 
Thus 
Active Material 
ee ee a eS ee el ee ee 

TE 
= g—he 
== 6 
where A is the “absorption constant” of the radiation for the gas 
under consideration’. Let 
« = distance of lower plate from active material, 
1 = distance between the two fixed plates. 
The energy of the radiation at the lower plate is then [,e~*, 
and at the upper plate Jje"*. The total number of ions pro- 
duced between the parallel plates A and B is therefore proportional 
to 
eat ema (+2) — eae qd —_ (Sa 
Since the factor 1 —e- is a constant, the saturation current 
1 Since the ionization at any point above the plate is the resultant effect of the 
a particles coming from all points of the large radio-active layer, \ is not the same as 
the coefficient of absorption of the rays from a point source. It will however be 
proportional to it. For this reason } is called the ‘‘ absorption constant.” 
