RELATIONS TO OTHER SCIENCES 131 



that all the internal forces of matter are of electromagnetic origin, 

 and that the energy is entirely divided between the two fields, elec- 

 tric and magnetic. 



We shall see, however, farther on that it Is difficult to eliminate 

 in this way all other forms of energy, all other forces, such as grav- 

 itation; and it would then be necessary to admit with Lorentz, in 

 order that the correspondence between the two systems should 

 actually subsist, that in the moved system the forces and masses of 

 different origins are modified exactly as the electromagnetic forces 

 and masses, an hypothesis too complicated and arbitrary in the 

 actual state of the question. 



But this does not seem to be a necessary consequence: it appears 

 probable that these actions, foreign to electromagnetism, and necessary 

 at the interior of the electron in order to give stability and in order 

 to represent gravitation, and which are probably connected with 

 one another, do not intervene in a sensible manner in the negative 

 experiments referred to above, and that everything transpires as if 

 the electromagnetic forces alone played a role, alone existed. 



We shall see farther on that perhaps experiments of another kind 

 than those referred to here, for example, some dynamic measure- 

 ments bringing in a relative motion of the system moved, or some 

 static experiments bringing in gravitation, would enable us to under- 

 stand the absolute motion, the axes bound to the ether, instead of 

 conceiving simply of their existence. 



(13) Electromagnetic Inertia. The problem of the electromagnetic 

 wake accompanying an electrified sphere or ellipsoid in the ether 

 has been taken up since J. J. Thomson by Heaviside and Searle. 



Max Abraham has shown their results to consist approximately 

 of a numerical factor when, instead of supposing the body to be a 

 conductor having a surface charge, we suppose its charge to have 

 a uniform volume distribution. 



Among the more important results contained in this solution of 

 J. J. Thomson's problem, I will point out these: that in the case of a 

 conducting sphere, the charge remains uniformly distributed on the 

 surface whatever may be the velocity, and that in all cases the electric 

 field at a distance tends to become more and more concentrated in the 

 equatorial plane with respect to the direction of the velocity in pro- 

 portion as this velocity approaches that of light. 



Moreover the kinetic energy which it is necessary to expend at the 

 moment of putting it in motion in order to create the electromagnetic 

 wake ceases to be proportional to the square of the velocity, and 

 increases indefinitely as the velocity approaches the velocity of light- 

 waves; the law of the increase of this kinetic energy with the velocity, 

 the energy of self-induction of the current to which the charged body 

 in motion is equal, may be easily deduced by Searle's solution. 



