NATUEE 745 



the definition of magnetic force to have any meaning, the action exer- 

 cised by an open current on an indefinite solenoid would only depend 

 on the position of the extremity of that solenoid i.e., that the action 

 of a closed solenoid is zero. Now we have just seen that this is not 

 the case. On the other hand, there is nothing to prevent us from 

 adopting the second definition which is founded on the measurement 

 of the director couple which tends to orientate the magnetic needle; 

 but, if it is adopted, neither the effects of induction nor electro- 

 dynamic effects will depend solely on the distribution of the lines of 

 force in this magnetic field. 



III. Difficulties raised by these Theories. Helmholtz's theory is 

 an advance on that of Ampere; it is necessary, however, that every 

 difficulty should be removed. In both, the name " magnetic field " 

 has no meaning, or, if we give it one by a more or less artificial con- 

 vention, the ordinary laws so familiar to electricians no longer apply; 

 and it is thus that the electro-motive force induced in a wire is no 

 longer measured by the number of lines of force met by that wire. 

 And our objections do not proceed only from the fact that it is diffi- 

 cult to give up deeply-rooted habits of language and thought. There 

 is something more. If we do not believe in actions at a distance, elec- 

 tro-dynamic phenomena must be explained by a modification of the 

 medium. And this medium is precisely what we call " magnetic field," 

 and then the electro-magnetic effects should only depend on that field. 

 All these difficulties arise from the hypothesis of open currents. 



IV. Maxwell's Theory. Such were the difficulties raised by the 

 current theories, when Maxwell with a stroke of the pen caused them 

 to vanish. To his mind, in fact, all currents are closed currents. 

 Maxwell admits that if in a dielectric, the electric field happens to 

 vary, this dielectric becomes the seat of a particular phenomenon 

 acting on the galvanometer like a current and called the current of 

 displacement. If, then, two conductors bearing positive and negative 

 charges are placed in connection by means of a wire, during the dis- 

 charge there is an open current of conduction in that wire; but there 

 are produced at the same time in the surrounding dielectric cur- 

 rents of displacement which close this current of conduction. We 

 know that Maxwell's theory leads to the explanation of optical phe- 

 nomena which would be due to extremely rapid electrical oscillations. 

 At that period such a conception was only a daring hypothesis which 

 could be supported by no experiment; but after twenty years Max- 

 well's ideas received the confirmation of experiment. Hertz succeeded 

 in producing systems of electric oscillations which reproduce all the 

 properties of light, and only differ by the length of their wave 

 that is to say, as violet differs from red. In some measure he made 

 a synthesis of light. It might be said that Hertz has not directly 

 proved Maxwell's fundamental idea of the action of the current of 



