196 ELECTRICAL PHENOMENA [CH. xxi. 



Is there no inconsistency here ? 

 The appearance of inconsistency vanishes when we 

 come to calculate and realise how extremely local and 

 concentrated the intense part of the field of an electron 

 is. There is a sense in which it can be said that a 

 moving body, for instance a vortex ring, disturbs the 

 whole atmosphere ; but any perceptible disturbance 

 resides very near the ring. So it is with an electron. 

 The magnetic field falls off inversely as the square of 

 the distance from the moving nucleus ; hence at a 

 distance far less than a micro-millimetre, less even 

 than the size of an atom, it is quite inappreciable. 

 The whole magnetic field on which its inertia depends 

 lies practically very close to the electron itself : it is 

 just its extremely small size that enables this concen- 

 tration to be possible, and even in a closely packed 

 mercury atom there is practically no encroachment of 

 the field of one electron on its neighbour's. They are 

 all independent, each with its own inertia, almost 

 isolated from the others : for if it were not so, the 

 mass of a body in close chemical combination would 

 not continue constant, but would diminish. Whether 

 it does diminish, in the least degree, is a question 

 perhaps worthy of attack.* Minute effects in this 

 direction have been announced, by Heydweiler and by 

 Landolt ; but the results are doubtful. 



The momentum of a moving charge at ordinary 

 speeds is simply inversely as the radius of the sphere 

 which holds it, as stated in Chap. II. , but the 

 localisation of this momentum, which is the point we 

 are now considering, may be realised approximately 

 as follows: 



The momentum depends on the co-existence and 

 product of the electric and magnetic fields. Each 



*Cf. Rayleigh, British Association Belfast, 1902. 



