556 Sir E. Rutherford and Mr. H. Robinson on Mass and 



clone the radium A on the wire had practically disappeared, 

 and the « radiation was due entirely to the product radium C. 

 The experiment then consisted simply in maintaining the 

 magnetic field constant and reversing its direction at intervals 

 for a few minutes. With the apparatus described, satis- 

 factory photographs could be obtained with 20 minutes' 

 exposure to a wire which had an initial 7-ray activity cor- 

 responding to about 30 milligrams of radium. 



The electromagnet used was one of a large type of which 

 several have recently been constructed in the Department, 

 for the purpose of giving a strong and uniform magnetic 

 field over a considerable area. The pole-pieces were 

 10 x 16 x 15 cm. Using the 10 x 16 cm. faces 2*5 cm. apart, 

 a field of over 6000 gauss, sensibly uniform over the whole 

 distance between the source and photographic plate, viz. 

 13 cm., was obtained with an exciting current of 5 amperes. 

 In any experiment the definiteness of the magnetic field 

 corresponding to a given exciting current was ensured by 

 submitting the iron to a series of magnetic reversals so as to 

 bring it into a cyclic state. 



The theory of the magnetic deflexion of an a particle in a 

 field which varies from point to point is as follows : — 



Fig. 1. 



o represents the source, a the slit, and b the photographic 



plate. The direction of the magnetic field is supposed to be 



perpendicular to the plane of the paper; hence at any point 



the path of the a. particle projected from in the plane of 



the paper will be an arc of a circle of radius p where 



nzv 

 H/9=^-, H being the value of the magnetic field at the 



point. Take the straight line oab passing from the source 

 through the slit (i. e. the path of an a particle when no 

 magnetic field is acting) as the axis of a\ and let the y axis 

 be in the plane of the paper and perpendicular to oab. Now 



