Straggling of a Particle} 



225 



It is easy to see that on account of these two causes the 

 density of the silver grains will be inversely proportional 

 to WX 3 . If, therefore, N be the number of grains observed 

 at any point X, the variation of the product N(WX) 3 gives 

 the required falling off in the number of a particles in their 

 passage through the air intervening between the wire and 

 photographic film. 



The results of a series of experiments is shown in fig. 3, 

 curve I., which has been arbitrarily drawn to give an extreme 



•5 -6 



Centimetres. 



range of 7 cms. 



The absolute measurements of the distances 

 involved have not been made with sufficient accuracy to 

 improve upon the accepted value of the range for radium C. 

 Each point on the curve represents the counting of a large 

 number of silver grains, varying from about 7000 for the 

 points at the maximum to some 400 near the end of the 

 range. The number of grains outside the a-ray photograph 

 which amounted to about 3 per cent, of the maximum 

 intensity was subtracted from each observation. 

 In the same figure (curve II.) is given the 



ds. 



theoretical 

 lr s formula 



1 



P V7T 



the probability that the range R has a value 



s) and R (l + s-f ds). The value of p has 



qual to 1*6 x 10~ 2 . Curve III. represents the 



experimental results of Greiger. Those obtained by Taylor 



have not been plotted, for he states that his results obtained 



for air agree with those of Greiger, though it should be stated 



Phil Mag. S. 6. Vol. 32. No. 188. Aug. 1916. Q 



curve calculated from Bohr 7 

 W(s)ds = 



which gives 

 between R (l 

 been taken e 



