Mr. Norman Campbell on Delta Rays. 270 



less than that corresponding to ON. In the same way, i£ V 

 is negative, P'N' will represent the number of rays emitted 

 by A which have a speed less than that corresponding to ON'. 

 The curve will be divided into two parts by the line CD, the 

 upper half representing the properties of the rays emitted 

 from B and the lower half those of the rays emitted 

 by A. 



It is important to notice tliat this simple relation holds 

 only if the condition is fulfilled that, in the absence of the 

 field, all the rays emitted by one plate fall on the other. 

 For, if, when no field is acting, some of the rays emitted by 

 A can strike A or some third body not electrically connected 

 to B, a positive potential on B will change the current, not 

 only by diminishing the number of rays emitted by B which 

 strike A, but also by increasing the number of rays emitted 

 by A which strike B. The necessary condition is not very 

 easy to realize experimentally, but preliminary observations 

 showed, as might be expected, that the form of the curves 

 obtained varied considerably with small differences in the 

 arrangement of the apparatus unless it was fulfilled. Indeed, 

 at first sight it might appear that it is geometrically impos- 

 sible to fulfil the condition. For, on the one hand it is clear 

 that the emitting surfaces must be plane, or the rays from 

 one body might strike the same body again ; and, on the 

 other hand, that each body must surround the other com- 

 pletely, lest the rays emitted strike some third body. How- 

 ever, if the active surface itself is not employed as one of 

 the bodies, the conditions can be fulfilled. Fig. 2 (p. 20<^) 

 shows the form of apparatus finally adopted for the first part 

 of the investigation. The drawing is approximately correct 

 in scale. 



4. A and B are the bodies from which the delta rays are 

 emitted. A is a solid plate connected to the apparatus for 

 measuring the current by a rod passing through the brass 

 base plate and insulated by an amberoid stopper. B is a 

 brass cylinder ; in the end facing A is a circular hole which 

 is covered with a layer of some material thin enough to 

 transmit alpha rays. The polonium plate C is placed at 

 such a distance above this end of B that all the rays which 

 pass through the hole (and are not scattered considerably) 

 fall upon A. Accordingly delta rays can only affect the 

 measurements if they are emitted from either A or B. To 

 prevent these delta rays striking the cylinder D which 

 surrounds A and B, D is maintained at a potential of —80 

 volts. It is insulated from the base plate by the ebonite 

 plate, the surface of which is covered almost completely 



