Radioactivity resulting from oc-Ray Bombardment. 941 



except in the case of lead foil. After one accident it was 

 not thought advisable to ran more than § max. speed with 

 lead. 



With the available driving apparatus it was possible to 

 drive the steel disk at a speed of 250 revolutions per second. 

 Taking the circumference for the bombarded portion of the 

 wheel as 60 cm., this gives a velocity of 15,000 cm. per sec. 

 Since the distance between the edge of the active source and 

 the screen was 1*2 cm., the time for the wheel to move 

 between these points is —^ sec. But from the diagram it 

 is evident that the heavily though obliquely bombarded 

 portion of the wheel has to move through only about '5 cm. 

 in order to reach a position where it can in turn bombard the 

 screen. This gives a time-interval of ^-^ sec. 



The observations were first made with the wheel stationary 

 or running very slowly, and the numbers of natural scintil- 

 lations counted. Except in the case of lead, these never 

 amounted to more than 1 in 2 minutes. The wheel was then 

 gradually speeded up and the screen observed as far as 

 possible continuously. The roar of the wheel when running 

 at high speed was not, at first, conducive to perfect concen- 

 tration on the observer's part ; but, after one experiment 

 had been carried out in safety, no further difficulty was 

 experienced. 



The materials tried included a wide range of atomic 

 weights. The first tried was carbon in the form of paper. 

 No effect at all was found. Aluminium, because of its 

 proven emission of H particles, was considered a very likely 

 material, but from it also no effect was observed. The wheel 

 itself, of nickel steel, was next tried ; but, like the others, it 

 gave no observable emission. The lower limit of observation 

 in these experiments was probably one scintillation a minute. 

 Two would most certainly have been detected. In the case 

 of lead, however, the number of natural scintillations 

 observed was as high as three a minute, so that the limit 

 of detection of additional scintillations was probably not 

 better than three a minute. With lead no effect at all was 

 observed with the disk running about 160 revolutions a 

 second. 



A rough calculation shows that the period of decay of the 

 induced radioactivity, if it exists at all, must be extremely 

 short to have remained undetected. Sources of about 25 to 

 30 nio's. activitv were used. Assuming that A- of tl 



10 



lie 



a-particles could reach the disk and that 1 in 10 6 ejects a 

 portion of the nucleus, the number of abnormal atomic 



