130 PHYSICS OF THE ELECTRON 



sary that an external action should intervene in order to modify the 

 energy of this wake and consequently to increase or diminish the 

 velocity. This implies, in the absence of all other kinetic energy than 

 this of electromagnetic origin, corresponding to the production of 

 the wake, by the law of Galileo on the conservation of the velocity 

 acquired, in the absence of action of all external fields of force, 

 that an electrified centre possesses inertia by the fact alone that it is 

 electrified. 



It is the immovable ether, the electromagnetic medium, which 

 serves as a fixed support for the axes with respect to which the prin- 

 ciple of inertia is applicable, and of which the ordinary mechanics 

 limits itself in affirming the existence by saying: there exists a sys- 

 tem of axes, determined by a nearly uniform translation with respect 

 to which the principle of Galileo is exactly verified. 



(12) The Absolute Motion. If we are able, from the actual point 

 of view, to conceive of the ether as supporting these Galilean axes, 

 it does not necessarily follow that the electromagnetic phenomena 

 enable us to arrive at this absolute motion. It seems, on the contrary, 

 so far, that static experiments, carried on in a material system by an 

 observer carried along with it with a uniform motion of translation, 

 do not allow, whatever may be the degree of accuracy of observa- 

 tion, the detection of a relative motion of the ether with respect to 

 matter. 



Larmor, and more completely Lorentz, have shown that there 

 exist in the system actions of electromagnetic origin; it is possible 

 to establish in a complete manner a static correspondence (relating to 

 the positions of equilibrium or to the black fringes in optics) between 

 the system in motion and a system fixed with respect to the ether, by 

 means of a change of variables which preserves for the equations of 

 the medium for a moving system the exact form which they possess 

 for a system at rest. 



The two systems differ from one another in that the moving system 

 is slightly contracted compared with the fixed system in the direction 

 of the resultant motion by an amount always very small, propor- 

 tional to the square of the ratio of the velocity of motion to the veloc- 

 ity of light. This contraction affects equally all the elements of the 

 moving system, i. e. the electrons themselves, if we admit with Lorentz 

 that the interior actions of these electrons are solely electromagnetic 

 actions or are modified in the same manner by the translation, with 

 the result that observation cannot prove this contraction any more 

 than it can prove the general dragging of the ether. These elements 

 behave as though they belonged to a corresponding fixed system. 

 Thus is found an explanation of the negative results of experiments 

 undertaken to show the absolute motion of the earth, by Michelson 

 and Morley, Lord Rayleigh, Brace, Trouton, and Noble, if one admits 



