of u-P articles per Second emitted by Thorium C. 1041 



wheel. The recorder writes down this series o£ small 

 numbers , and they are added by minutes later. 



The power of the method lies in the fact that calling out a 

 seen number is merely a reflex action, whereas addition 

 requires mental effort. The best observer cannot count with 

 certainty above 65 or 70 scintillations a minute by the 

 ordinary method; but with this wheel method up to 160 a 

 minute can be recorded. Practice is, of course, required 

 before the actions involved become quite mechanical. For 

 a beginner, the wheel should be run as low as 50 a minute 

 for counts averaging about 75 a minute, or about 1*5 per 

 revolution. Later, the observer should have no difficulty 

 calling out the numbers at a rate of 80 a minute for counts 

 averaging two a revolution. 



Ability to judge rather than to count the number of 

 scintillations in a group is essential. An experienced 

 observer can judge accurately up to groups of 7, mainly, in 

 our experience, by the geometrical figures formed by the 

 scintillations. Beyond 7 it is not usually possible to be 

 absolutely certain of the number, but one must hazard a 

 guess. Such large numbers should appear, however, 

 extremely infrequently if the speed of the wheel is properly 

 adjusted. We found that it was quite unsafe to count when 

 the scintillations were appearing at a rate of over 2" 5 per 

 group on the average. When counting the particles from a 

 decaying source and the number drops below 60 per minute, 

 we found it easier to run the wheel about 200 revolutions 

 per minute and to add the numbers mentally. 



It is of interest to see the operation of the law of pro- 

 bability in the following examples of sequences of numbers 

 called out during this work. A certain number of zeros 

 should appear, but they were not noted. 



h minute:— 1 231142211314 4 322233133312141 



4 2 3 111. 

 1 minute:— 2 4222233322211521133223534432 



12 2311131132121111134225413. 

 | minute:— 1 222322221222221121313122112. 



y-Ray Measurements. 



Since the comparison of the a-ray activities of the radium 

 C and thorium C was to be based on equal 7-ray activities, 

 it was necessary to have accurate 7-ray measurements at the 

 same time as the counts. A 7-ray electroscope was mounted, 

 therefore, about 30 cm. from the active source when in its 

 counting position. The activity and decay of the source 

 could then be measured by taking groups of readings while 



Phil. Mag. S. 6. Vol. 43. No. 2z>8. June 1922. 3 X 



