[ALLAN] EXCITED RADIOACTIVITY FROM ATMOSPHERIC AIR 75 
Fig. 3 shows the results for a lead wire charged for 190 mins. at 
a P.D. of about — 25,000 volts. This falls to one-half its value 
in about 46 mins. 




NOR | 
ee 
HS ee 
Rey 
Es = 
Q 20 40 Go 80 100 120 140 
160 

— rare of Leck 


These experiments were performed at long intervals between 
and under different conditions. They show that the rate of decay 
is the same wherever the radioactivity is produced. 
ABSORPTION OF RADIOACTIVITY. 
Several experiments were made to test the absorption of the 
excited radioactivity. For this purpose another apparatus was used. 
This is shown in Fig. 4 It consists of an upper plate A connected 
to the electrometer, a lower plate B connected to the battery C. The 
lead wire on which the radioactivity was collected was wound in the 
form of a flat spiral and placed on the lower plate. The whole was 
inclosed in a metal box connected to earth. The rate of leak was 
observed when the wire was bare, and when covered with various 
thicknesses of aluminum foil. From these two readings the amount 
of absorption could be obtained. The results are shown plotted in 
a curve, Fig. 5, along with some absorption curves for other radiations. 
It is seen that the excited radioactivity from air is of a more pene- 
trating character than any of those types shown. 
EFFECT OF WEATHER CONDITIONS. 
A long series of experiments were made to determine the amount 
of radioactivity that could be collected on a wire under different 
weather conditions, especially those of a Canadian winter. For this 
purpose the wire was suspended outside in the open air, and was 
