MODERN THEORIES OF ELECTRICITY AND MATTER. 105 



electrostatic i:)lienomena. An analoo-ous theory for magnetism may 

 be built up by assuming that the law of action between two mag- 

 netic poles is absolutely like the law of action between electrified 

 particles. The electric current was regarded as the flowing of an 

 electric fluid in a conductor. To establish a theory of electro-mag- 

 netics and electro-dynamic phenomena, it is necessar}'^ to bring in a 

 third law of action-at-a -distance between the magnetic poles and 

 the electric-current law of Laplace. All these theories in their 

 entirety are founded on the laws of forces acting at a distance, in com- 

 bination Avith the conception of electric fluids. 



Faraday, although contemporaneous with this development, looked 

 at the question from a different point of view. He did not believe 

 in the possibility or power of action-at-a -distance betM'een electrified 

 bodies, and thought that the forces which Avere exercised between 

 them resulted from elastic tensions which established themselves in 

 the intervening medium. These elastic forces comprise a tension in 

 the direction of the lines of force and a pressure at right angles to 

 them. In seeking to show the direct influence of the medium he was 

 led to the discovery of the inductiA'c power of dielectrics, and his 

 belief in the essential part played by the intervening medium Avas 

 thus strengthened. According to Faraday, the Surface of charged 

 conductors are to be regarded as surfaces of separation betAA^een 

 regions where an electric field exists and fields of zero intensity. 



He Avas struck by the barrenness of the efforts that had been made 

 to realize an absolute charge, and electric charges ahvays appeared to 

 him as the ends of tubes of force Avhich tra Averse the dielectric. 



MaxAvell, captiA'ated by the ideas of Faraday, endeaA^ored to ex- 

 plain them in mathematical language. He demonstrated that there 

 does not exist in a mathematical vieAv any incompatibility betAveen 

 theories based upon laws of action-at-a-distance and Faraday's theory 

 of continuous action; and that b}^ assigning a reasonable A'alue to 

 the tensions and pressures which Faraday conceived to exist in the 

 dielectric, an electrostatic theory could be constructed identical to 

 that Avhich is deriA^ed from the hiAV of action-at-a-distance. While 

 MaXAvell does not specify precisely the nature of electricity, he 

 treats of it generally as a fluid whose displacement in a conductor 

 giA^es rise to a resistance proportional to the A^elocit}^ of the flow, 

 while its displacement in a dielectric produces an elastic reaction. In 

 a dielectric, displacement can only occur at the time Avhen the field 

 changes. One of the essential ideas of MaxAvell was to consider the 

 displacement of electricity in the dielectric as an electric current to 

 which he gives the name of " currents of displacement." Currents of 

 displacement, according to MaxAvell, behaA^e like ordinary currents, in 

 the sense that they produce magnetic fields. EA^ery oj)en circuit in a 

 conductor, folloAving the opinion of Maxwell, is completed by a cur- 



