902 Mr. A. L. McAulay on the Recoil of 



the equipotentials must be closer than this to some of the 

 electricity. It is evident that to make fig. 4 in any way 

 a correct picture of the field the scale must be reduced 

 at least ten times in the direction of the arrow, and the 

 positive electricity must be pictured as lying close under 

 the 6 million volts equipotential. 



A consideration of the difficulties in the way of con- 

 structing such a diagram suggests arguments against the 

 purely electrostatic field. The electricity must be spread 

 over a relatively large area, and therefore the density of 

 charge must be low, and the nuclei must approach extremely 

 close to attain the required potential. At the same time the 

 Inertia of the whole of the a particle must Come into play 

 during the collision ; consequently there can be no disruption, 

 and the forces between the nuclei at their points of intimate 

 contact must be transmitted to the rest of the mass. If the 

 hydrogen nucleus is pictured as a point charge flung at a 

 stationary a. particle, the structure of the latter would seem 

 to be analogous to a strong heavy flexible sheet. This would 

 offer a large target, yet oppose all its inertia to a small 

 missile, exerting an impulsive force on it in a direction almost 

 directly opposed to its line of flight. 



If the recoil is not due to a simple electrostatic field, it 

 would seem that the choice of structures for the nuclei would 

 be much larger. At the same time, it is difficult to picture a 

 mechanism which gives sufficiently large electromagnetic 

 forces. Perhaps the simplest combined field would be one 

 in which the magnetic field deflected the nuclei towards 

 one another, in which case they may both be of small volume 

 and the actual recoil be due to electrostatic repulsion. The 

 same effect might be produced electrostatically by an a particle 

 composed of two electrons and four hydrogen nuclei, the 

 electrons deflecting the recoil nucleus towards the positive 

 part of the a particle. 



It must be remembered that the field considered is probably 

 enormously different from that of the normal a particle. At 

 the same time the great energy of the collision will ensure 

 that the geometry of the system is that which gives minimum 

 potential energy, and is therefore almost certainly the same 

 in every collision, so that conclusions arrived at represent 

 more than merely a statistical average. 



I wish to thank Dr. A. Pontremoli, of the University of 

 Rome, for his assistance in the counting operations necessary 

 to obtain the relation between the a- and 7-ray activities of 

 thorium C, and also Mr. Chadwick and Mr. Bieler of the 



