PROFESSOR AT HEIDELBERG 273 



from the case in which =o. The analytic treatment of the 

 problems of the motions of electricity can also be simplified if 

 k is not a very large number, by making =o, or assuming the 

 propagation of the longitudinal waves to be infinitely great, as 

 long as the dimensions of the conductor used are vanishingly 

 small in comparison with the wave-lengths of the oscillations 

 that come under observation. Thus with such electric motions 

 as are produced within a conductor by external forces after 

 a previous state of electric equilibrium, there can only be free 

 electricity (on the assumption that =o) at the surface of the 

 conductor, or at the limiting surfaces of different conductors. 

 The investigation of a very long wire as conductor, compared 

 with whose diameter the wave-length is very great, also shows 

 the influence of the constant k only in the small terms of the 

 higher order. Helmholtz concludes from this that in electrical 

 experiments in the laboratory the velocity of the electric 

 longitudinal waves depending on the constant k need not be 

 taken into consideration, unless we have the means of detecting 

 extraordinarily minute time-differences. 



After carrying out these experiments rigidly, without allow- 

 ing himself to decide on any particular hypothesis, and taking 

 the electrostatic and electrodynamic effects as action at a 

 distance, which did not affect the surrounding insulating 

 media and was not affected by them, he accepted the Faraday- 

 Maxwell theory, which replaces action at a distance by the 

 polarization of a medium, and assumes that the electric dis- 

 turbances propagate themselves across an insulating dielectric 

 in transverse waves, the velocity of which in air is equal to the 

 velocity of light. 



Faraday, like Newton, wholly rejected the hypothesis of the 

 existence of forces acting at a distance, according to which 

 there is direct and immediate action between two bodies 

 separated from each other in space, without any alteration of 

 the intervening media. He found that magnetism or dia- 

 magnetism exists in almost all the bodies previously held to 

 be non-magnetic, and that in the same way good insulators 

 suffered a change under the action of electrical bodies, which 

 he termed the electric polarization of the insulator; and in 

 virtue of this he sought to explain magnetic and electric action 

 at a distance as due to the agency of the intervening polarized 



