The Followers of Maxwell. 345 



rapid, the. conductor has not time (so to speak) to display 

 the impSfection of its conductivity, and the magnetic field 

 is therefore unable to extend far below the surface. 



The same conclusion may be reached by different reasoning.* 

 When the alternations of the current are very rapid, the ohmic 

 resistance ceases to play a dominant part, and the ordinary 

 equations connecting electromotive force, induction, and current 

 are equivalent to the conditions that the currents shall be so 

 distributed as to make the electrokinetic or magnetic energy a 

 minimum. Consider now the case of a single straight wire of 

 circular cross-section. The magnetic energy in the space outside 

 the wire is the same whatever be the distribution of current in 

 the cross-section (so long as it is symmetrical about the centre), 

 since it is the same as if the current were flowing along the 

 central axis ; so the condition is that the magnetic energy in 

 the wire shall be a minimum ; and this is obviously satisfied 

 when the current is concentrated in the superficial layer, since 

 then the magnetic force is zero in the substance of the wire. 



In spite of the advances which were effected by Maxwell 

 and his earliest followers in the theory of electric oscillations, 

 the gulf between the classical electrodynamics and the theory 

 of light was not yet completely bridged. For in all the cases 

 considered in the former science, energy is merely exchanged 

 between one body and another, remaining within the limits of a 

 given system ; while in optics the energy travels freely through 

 space, unattached to any material body. The first discovery of 

 a more complete connexion between the two theories was made 

 by Fitz Gerald, who argued that if the unification which had 

 been indicated by Maxwell is valid, it ought to be possible to 

 generate radiant energy by purely electrical means; and in 

 1883f he described methods by which this could be done. 



Fitz Gerald's system is what has since become known as 

 the magnetic oscillator : it consists of a small circuit, in which 



* Of. J. Stefan, Wiener, Situungsber. xcix (1890), p. 319 ; Ann. d. Phys. xli 

 (1890), p. 400. 



t Trans. Roy. Dublin Soc. iii (1883) ; Fitz Gerald's Scient. Writings, p. 122. 



