Velocity of /3-par tides in passing through Matter. 631 



the maxima of the initial and displaced lines an attempt was 

 made to measure this distance instead of that between the 

 edges of the lines. 



A table of results is given below. The numbers of course 

 refer to mean and not to minimum diplacements. 







Stopping by Mica. ' 



Stopping by Tin. 



Stopping by Gold. 



Name 



of 

 Line. 













Bp. 



*(H P ) 

 100 X Mass -J - 



H 



*(Hp) 



I/3 3 - 



*(Hp) -! 



I/3 3 . 



100 X Mass — L 



100 X Mass 



Pi fl> 

 fl c 



1392 



1381 34-8 



89-2 



22-8 







1660 



101-4 34-7 1 



67-4 



234 



... 





.2 IB 



1 A 



1925 



78 331 



56-8 



24-1 



... 





2235 



72-6 



362 









... 





'K 



2960 



66-7 435 











Q 



H 



3260 



59-2 41 













(I 



4840 



47-3 399 



37 6 



31-7 



322 



27 3 



i< 



F 



5255 



49-3 422 



378 



32-5 



... 





TZ 



IS 



5880 



431 ! 38 



322 



28-6 



32 6 



29 



m 



D 



6160 



41 367 





1 









[o 



7060 



38-4 35-4 



302 



27-8 







The letters denoting the lines and the values of Up are 

 those given by Professor Rutherford and Mr Robinson in 

 their paper on the " Analysis of the /3-rays from Radium B 

 and Radium C"*. The magnetic fields had been previously 

 calibrated, but the actual values of the lines were used as a 

 check upon the calibration. 



The primary line was of course narrow and sharp, but the 

 displaced line was broadened out considerably, especially in 

 the case of gold. Neither edge of the displaced line was 

 really sharp, but the line appeared to have a distinct maximum 

 when viewed by the naked eye. This shows that the reduc- 

 tion in velocity is variable, unlike that for a-rays. The actual 

 measurements were exceedingly difficult, for the displaced 

 lines ceased to show a maximum if only slightly magnified, 

 and disappeared entirely if placed under a travelling micro- 

 scope. As the mean displacement, that is the distance between 

 the maxima of the two lines, was required the best available 

 means of measurement was found to bean ordinary millimetre 

 rule, but since the displacements were only of the order of 

 1 or 2 mm. the accuracy cannot be very great. The mean 

 value for several plates was of course taken. The measure- 

 ments were most difficult in the case of stopping by elements 



Phil. Mas. Oct. 1913. 



