136 PHYSICS OF THE ELECTRON 



We easily see that the forces thus obtained, exerted on the electrons 

 by the ether, i. e. on the matter which contains them, do not satisfy 

 the principle of the equality of action and reaction, if we consider all 

 the forces which act at the same moment on all the electrons con- 

 stituting matter. In the case of a body which radiates in an unsym- 

 metrical manner, a recoil, an acceleration, is produced which is not 

 compensated at the same moment by an acceleration set up in 

 another portion of the matter. Later, at the time that the emitted 

 radiation meets an obstacle, the compensation is made (but only in 

 a partial manner if all the radiation is not absorbed) by means of 

 the pressure which the radiation exerts on the body which receives it; 

 a pressure whose existence is shown by experiment. 



The equality of action and reaction has never been verified in 

 similar cases, and it adds no difficulty to this subject if we do not 

 seek to extend the principle beyond the facts which suggested it. 



(20) Quantity of Electromagnetic Motion. If we could nevertheless 

 realize this extension of the principle, an extension somewhat arbi- 

 trary, we should be led not only to apply this principle to matter, 

 but to suppose the ether to have a quantity of motion which would 

 be that of a material system to which we compare it. 



Poincare has shown that this quantity of electromagnetic motion 

 ought to be, at every point in the ether, in direction and in magni- 

 tude, proportional to Poynting's vector, which gives at the same 

 time a definition of the energy transmitted through the medium. 



By starting with this idea of the quantity of electromagnetic 

 motion, Max Abraham has been able to calculate the terms, put to 

 one side by Lorentz, which depend on the motion of the electron 

 itself, its force of inertia, by the variation of the quantity of electro- 

 magnetic motion contained in its train. He was led for the first time, 

 by the form of the terms which represent this force of inertia, to the 

 notion of an unsymmetrical mass as a function of the velocity. 



(21) Quasi-Stationary Motion. The calculation can be completely 

 made only in the case, always realizable from the experimental point 

 of view, where the acceleration of the electron is so small that its 

 train can be considered at each instant as identical with that of an 

 electron having the actual velocity, but whose motion has been 

 uniform for a long time. This is what Abraham calls a quasi-station- 

 ary motion. In this case, the train is entirely determined at each 

 moment by the actual velocity of the electron, also the quantity of 

 electromagnetic motion which it contains, and consequently the 

 variation of this quantity which represents the force of inertia. The 

 condition of quasi-stationary motion is simply that in the neighbor- 

 hood of the electron, where the quantity of electromagnetic motion 

 is localized, the wave of acceleration may be neglected in compari- 

 son with the velocity wave. 



