RELATIONS TO OTHER SCIENCES 129 



III. Inertia and Radiation 



(11) The Electromagnetic Wake. 1 Before going farther it is im- 

 portant to point out what we can draw from the point of view to 

 which we have now come. Electrified centres, whose existence is 

 experimentally proven, whose charge we know in absolute units, 

 are movable with respect to a fixed ether defined according to the 

 equations of Hertz, without its having been necessary for us to have 

 recourse to dynamic principles to arrive at this point of view. 



To what extent can the known properties of matter be deduced 

 from these two ideas of the electron and the ether, and is it necessary 

 to add something to them in order to build up a synthesis? We are 

 going to see rapidly and definitely from our idea of the electron, 

 how it is sufficient to represent at the same time the inertia of matter, 

 its dynamic properties, also how it can emit and absorb the radi- 

 ations which the ether transmits. 



The possibility of conceiving of inertia, mass, not as a funda- 

 mental idea, but as a consequence of the laws of electromagnetism, 

 is a conception which owes its origin to an important memoir pub- 

 lished in 1881 by Professor J. J. Thomson. 2 He studies there, basing 

 his assumptions on the existence of the displacement currents of 

 Maxwell, the electromagnetic field accompanying an electrified sphere 

 in motion. This motion implies a change in the electric field at a 

 point fixed with respect to the medium, and this displacement current 

 immediately produces a magnetic field according to the ideas of 

 Maxwell. The necessity of a convection current is pointed out later. 

 The magnetic field thus produced, identical with that of an element 

 of current parallel to the velocity of the moving charge, is propor- 

 tional at each point to that velocity, at least, if it does not approach 

 too nearly to that of light. 



The creation of a magnetic field at the time of setting the charged 

 centre in motion implies an expenditure of energy, energy of self- 

 induction of the convection current, proportional to a first approxi- 

 mation to the square of the velocity, for those velocities which are 

 small compared to the velocity of light. It is thus an expression of 

 the same form as that of ordinary kinetic energy. A part, at least, 

 of the inertia of an electrified body, of its capacity for kinetic energy, 

 is thus a consequence of its electric charge. 



Moreover, the magnetic field thus produced, and the electric field 

 as well, modified by the velocity as it approaches more nearly to that 

 of light, constitute around the electrified centre in translation a wake 

 which accompanies it in its translation through the ether without 

 change so long as the velocity remains constant. It is besides neces- 



1 Le Sillage Electro-magmtique. 



2 J. J. Thomson, Phil. Mag. t. 11, p. 229. 1881. 



