504 Sir Oliver Lodge on the 



which was to explain electricity by electrons and not to explain 

 electrons by electricity. The only thing experimentally known in 

 this domain seems to be Kauffmann's result, which requires that an 

 electron is some structure expressible analytically as formally 

 equivalent to a spherical aggregation of ' electric charge ' with in- 

 variable volume relations, but with ' density of charge ' that may 

 vary, according to any law whatever, in terms of the distance from 

 the centre. Por other electrodynamic purposes this structure can 

 be treated simply as if it were an aggregate charge concentrated in 

 a point. 



" The case has here been put at its worst, and this aspect of it, 

 along with unusual features in the phenomena of mechanical re- 

 action, has in fact led M. Poincare* and others following to ascribe 

 insuperable difficulties to a rotationally elastic representation of the 

 aether, notwithstanding the direct and natural representation which 

 it gives of the field of an electron, as a self-locked stress-system 

 keying on to it. 



" But as I have been in the habit of explaining in academic 

 lectures for some years, though I have not found time to explain 

 the point in print, the case is far from being so hopeless. It may 

 or may not be true that a permanent magnet (so-called), if abso- 

 lutely at rest in aether, would through lapse of time have its 

 magnetic field gradually choked : there is no direct means of 

 knowing, so long as we must rush through space along with the 

 earth at 20 miles per second. Take, however, your own supposi- 

 tion of an electron rushing through space and having a circulating 

 movement of aether relative to itself which is of the same order of 

 velocity close up to the electron as its velocity of translation. If 

 this were steady actual circulation of the aether, instead of merely 

 apparent circulation relative to the moving electron, such an idea 

 of an electron would be blankly impossible on a rotationally elastic 

 theory. But on consideration it will appear there is in fact no 

 cumulative circulation at all. The electron is constantly shedding 

 off and leaving behind it, as a sort of wake in the stationary aether, 

 these beginnings of circulatory motion which constitute its magnetic 

 field, and thus the choking process never gets a start : in fact in 

 the nature of the case it can never get a start (if the inertia of 

 aether is sufficiently great), for the very essence of a magnetic field 

 is that it is a disturbance which arises from the electrons moving 

 on, and thus of necessity leaving it to collapse behind them. 



" Problems in hydrodynamics of the general character suggested 

 by these remarks have been dealt with long ago by Clerk Maxwell 

 (" On the Displacement in a Case of Fluid Motion," Proc. London 

 Math. Soc. iii., or Scientific Papers, vol. ii. pp. 208-214). lie 

 there considers the special case of a circular cylinder progressing 

 with uniform velocity through quiescent frictionless fluid : the 



* See the discussion at the end of the second edition of his Electriclte 

 et Optique, where the theory was by misunderstanding, doubtless due to 

 its mode of presentation, contrasted with that of Lorentz, the two being 

 in fact formally equivalent, as is now recognized. 



