excited by the ft Rays of Radium. 599 



excited 7 rays of uranium. Unfortunately the absorption 

 coefficient of the 7 rays of uranium X has not been measured 

 over the range used above. Soddy and Russell found an 

 absorption coefficient in iron of '34 cm. -1 within a range 

 of 2*52 cm. to 7*57 cm., but their results indicate a much 

 higher coefficient over the first 2 cm. 



The ratio of the amount of the excited 7 radiation to the 

 primary 7 radiation from the emanation was determined in 

 the following way. The lead screen L was taken away, and 

 the current in A due only to the emanation tube fC was 

 determined. This was 26*9 divs. per sec. The current due 

 to the radium R, w hen in its usual position, was then measured. 

 This was *43 div. per sec. Thus the ionization due to the 

 unabsorbed primary rays was 63 times the ionization due to 

 the radium R. Now the amount of excited 7 radiation from 

 uranium metal, referred to the same time, was 10 per cent, 

 of (he radiation from R, measured by the ionization produced 

 in A. Hence the 7 radiation excited when the ft rays fall 

 on uranium is about 0*16 per cent, of the primary 7 radiation. 

 This is produced by the ft rays which are concentrated on P 

 by the direct action of the magnetic field, together w T ith 

 those which are first reflected by the iron polepiece. Ac- 

 cording to McClelland and Schmidt (see Jalirbuck der Radio- 

 aktivitat, 1908) iron reflects from 30 to 40 per cent, of the 

 ft rays falling on it. Thus about 60 or 65 per cent, of the 

 total number of ft rays are directed on to the plate P. 

 Hence, assuming that the excited 7 rays are given out 

 equally in all directions, the amount of excited 7 radiation 

 from uranium metal due to the total number of ft rays is 

 about 0*3 per cent, of the primary 7 radiation, It is thus 

 not surprising that the earlier observers w r ere unable to detect 

 the 7 rays excited by the ft rays of radium 0, 



The proportion of ft rays directed on to the plate P can 

 be calculated when the magnetic field is known. For the 

 present purpose it is sufficient to consider the magnetic field 

 as due to a single magnetic pole, the position and strength 

 of the pole being given by the intensifies of magnetic field 

 at the polepieces. These were 1700 gauss at the broad pole- 

 piece and 3050 at the small one. The distance between the 

 polepieces was 7*6 cm. From this it can be found that 



ft rays of all velocities up to 2 9 x lO 10 ^ 1 strike the plate P. 



An attempt was made to observe the 7 rays excited by 

 ft rays of a definite velocity. Danysz * gives two groups of 



ft rays having velocities 2'85 X 10 10 — and 2'87xl0 10 ~- 

 ' J ° sec. sec. * 



* Danvsz, Le Radium, Jan. 1912 f 



2R2 



