excited by the ft Rays of Radium. 595 



One would naturally expect that y rays would also be pro- 

 duced by the impact of the ft rays of radium C, but hitherto 

 no definite evidence of this has been obtained. 



Starke *, in 1908, showed that the amount of 7 rays 

 excited by the ft rays of radium (J must be very small com- 

 pared with the amount of primary 7 rays. He used about 

 6 milligrammes of radium bromide contained in a glass tube 

 thin enough to let out most of the ft rays, and measured the 

 ionization produced by this in an electroscope. The face of 

 the electroscope through which the 7 rays passed was a plate 

 of lead 2 cm. thick. He then placed plates of various 

 materials just behind the radium, but found no difference in 

 the electroscope reading. In another series of experiments 

 the face of the electroscope consisted of thin Al foil, and the 

 ft rays were prevented from entering by means of a strong- 

 magnetic field. He again found no effect. 



Davisson f used a slightly different arrangement. By 

 means of a converging magnetic field he directed the ft rays 

 on to a lead plate. Any 7 rays excited in the lead would 

 have caused an increase in the ionization of an electroscope 

 placed in a suitable position. He found some slight but 

 uncertain evidence of the production of 7 rays. 



These experiments show that the amount of 7 radiation 

 excited by the ft rays of radium C can at most be only 1 or 

 2 per cent, of the primary 7 radiation. 



It is of importance to settle definitely whether the ft rays 

 of radium C excite 7 rays or not, and to find the amount of 

 excited radiation under definite conditions compared with 

 the primary radiation. It is evident, from the above ex- 

 periments, that if the excitation of 7 rays is to be shown 

 quite clearly, a method must be used which will increase the 

 apparent proportion of excited 7 rays to primary 7 rays. 

 This has been done by means of the arrangement shown in 

 the figure (p. 596). 



A quantity of radium emanation contained in a very thin 

 glass tube is placed at E between the polepieces of a large 

 electromagnet. A converging magnetic field is obtained by 

 using polepieces of suitable shapes. 



Before the primary 7 rays can enter the ionization chamber 

 A they have to pass through the lead screen L, which absorbs 

 about 95 per cent, of the rays. 



The converging magnetic field concentrates about one half 

 of the total number of ft rays from the emanation tube on 



* H. Starke, Le Radium, Feb. 1908, p. 35. 



t Davisson, Phys. Kev. vol. xxviii. p. 469 (1909). 



