566 



Sir E. Rutherford on Collision of 



H atoms due to a magnetic field with that due to a combined 

 magnetic and electric field. The glass tube carrying the 

 source and parallel plates was placed between the poles of a 

 strong electromagnet, the plane of the plates being parallel 

 to the direction of the magnetic field. The microscope was 

 fixed in the centre line of the plates A and B so as to count 

 the scintillations emerging from the plates, and the variation 

 of the number with strength of the magnetic field was deter- 

 mined. The reduction of the number with increase of the 

 magnetic field depended on two causes : — (1) the removal of 

 H atoms bent to the sides of the plates, and (2) the bending 

 of the H atoms emerging from the plates in the magnetic 

 Held in the short distance between the end of the plates and 

 the zinc sulphide screen. These two effects were difficult to 

 separate, but (2) was made as small as possible by reducing 

 to a minimum the distance between the end of the plates 

 and the screen. The relation between the number of scintil- 

 lations and strength of field with no electric field acting, is 

 shown diagrammatically in fig. 3. Suppose the magnetic 



Fig:. 3. 



\ 



R 



P 



Q 



H, H H 2 



Magnet/c field 



field to be of a strength H corresponding to a point P on 

 the curve. If a voltage be now applied so as to bend the 

 H atoms in the same direction as the magnetic field, the 

 number of scintillations on the screen decreases correspond- 

 ing to a point Q on the curve of field H 2 . On reversing the 

 voltage the two fields oppose each other, and the number of 

 scintillations correspond to the point R of field H^ Suppose 



