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BELL SYSTEM TECHNICAL JOURNAL 



solid angles to the least practicable values. But in practice they can- 

 not be reduced to low values, because the ejection of a proton is so 

 infrequent an event that if one observed only those coming off within 

 say a degree of a certain chosen direction, they would be altogether 

 too few to be profitably observed during any reasonable period of time. 

 The same thing would happen, if one used a beam of alpha-particles 

 of similar narrowness; the impacts would be few because the impinging 

 corpuscles were few. It is therefore to be feared that under the best 

 of possible conditions, the horizontal segments in the curves will be 

 shorter, the descents broader and smoother, than under ideal conditions 

 they would be.* 



10 



15 



25 



30 



Fig. 9- 



20 

 CM AIR 



-Distribution-in-range curve obtained by differentiating the curve of Fig. 8 

 for boron (\V. Bothe and H. Franz). 



Another result emerges from the experiments of Franz, and those of 

 the Cambridge school: the mean speeds of the groups apparently 

 diminish with the speed of the a-particles. This is the effect which 

 Pose observed with the slowest of the groups which he detected, not 

 however with the faster and sharply-marked two; but from the ex- 

 periments of the others, it seems to be the rule— not that the others 

 tested all of the groups by varying the speed of the a-rays, far from it! 

 but rather, for all which they did test, they found that sort of a de- 

 pendence. Whether the constancy of speed of the groups which Pose 

 studied is a peculiar feature of these, or his were the better experiments, 

 I would not venture to say. At all events it is obvious that wherever 

 this effect enters in, the natural sharpness of the groups is bound to be 

 blurred by the differences in the speeds of the alpha-particles. 



Another of Pose's discoveries — the remarkable fact that with alu- 

 minium, certain groups of protons are ejected only when the speeds of 



' Whether the beauty of Pose's curves is to be ascribed to the smallness of the 

 soUd angles aforesaid is difficult to say. In one place he gives 0° and 58° as the range 

 of values of 6, in another a somewhat smaller amount. Bothe says that in each of^his 

 observations the values of d for 83% of the impinging a-particles lay within 15° of 

 the mean — an interval of 30°. The others are not so definite. 



