Recoil of Radium D from Radium C» 813 



The wire mounted at at right angles to the plane of the 

 paper acted as a line source 3 cm. distant (/ : ) from a slit S 

 of width 0*14 mm., and the radiation which passed through 

 was allowed to impinge on a Schumann plate placed at P. 

 The distance SP (l 2 ) was 2'8cm. The whole apparatus was 

 enclosed in a brass box which could be completely evacuated, 

 and was situated between the pole- pieces of a strong electro- 

 magnet, which when excited produced a uniform field of 

 about 13,000 gauss at right angles to the plane of the 

 paper. 



To carry out an experiment the active wire, upon which 

 was deposited some 30 milligrams of active deposit, was 

 placed in a tube which could be evacuated and heated to 

 drive off any adhering emanation. The wire was then left 

 for twenty minutes, during which time the radium A on it 

 had decayed to a small fraction of its original value. It 

 was then mounted at 0, and the box containing it was 

 exhausted as rapidly as possible. An interval of eight 

 minutes was then allowed to elapse, during which the 

 radium A still remaining on the wire decayed to an insig- 

 nificant amount. Daring this time the radiation from 

 struck the centre of the Schumann plate P and produced a 

 photographic impression. The magnetic field was then 

 excited and kept at a constant value for 75 minutes. The 

 photographic record obtained in this way is shown in fig. 2. 



Fiir. 2. 



On the left of the photograph is a strong line giving the 

 undeflected position of the beam of a rays coming through 

 the slit before exciting the electromagnet. On the right is 

 another somewhat stronger line fixing the position of the 

 beam of a rays when deflected by the magnetic held. Between 

 these two lines is a much fainter one due to the deflected 

 recoil stream. It is evident that this line must have been 

 produced by the direct action of the recoil stream, since 

 radium D emits no a rays and its period of transformation 

 is so long that any photographic action due to this or any 

 subsequent disintegration product is precluded. 



