Dbcembeb 27, 1895.] 



SCIENCE. 



871 



in Prof. 0. Lodge's charming book on 

 'Modern Views of Electricity.'* Suffice it 

 to state that these mechanical models were 

 temporai-y structures only, as it were, mere 

 scaffolding, which Maxwell tore down as 

 soon as the building which he started to 

 raise reached its completion, f 



In the third essay the description of the 

 physical properties of the medium is not so 

 specific as in the first two. The mechani- 

 cal models are replaced by broad mechani- 

 cal hypotheses. 



"It appears, therefore," says Maxwell, 

 in the introduction to the famous third es- 

 say, " that certain phenomena in electricity 

 and magnetism lead to the same conclusion 

 as those of optics, namelj^, that there is an 

 setherial medium pervading all bodies, and 

 modified only in degree by their presence ; 

 that the parts of this medium are capable 

 of being set in motion by electric currents 

 and magnets ; that this motion is communi- 

 cated from one part of the medium to an- 

 other, by forces arising from the connections 

 of those parts ; that, under the action of 

 these forces, there is a certain yielding de- 

 pending on the elasticity of these connec- 

 tions, and that, therefore, energy in two 

 different forms may exist in the medium, 

 the one form being the actual energy of 

 motion of its parts, and the other being the 

 potential energy stored up in the connec- 

 tions, in virtue of their elasticitj'." 



This paragraph contains the keynote of 

 the essay. Its meaning may be illustrated 

 in a simple manner, as follows : Consider a 

 charged Leyden jar. Its energy is poten- 

 tial and stored up in a sort of elastic defor- 

 mation of the dielectric, principallj' in that 

 part of the dielectric which separates the 

 metallic plates of the jar. Connect the two 

 plates by a conducting wire ; a current is 

 set up and a magnetic field accompanying 

 this current appears. Magnetic force is 



* Published by Macmillan & Co. 

 t Treatise, Vol. 2, p. 427, 2d. ed. 



due, according to Maxwell's mechanical 

 hypotheses, to some kind of motion in the 

 medium, and, therefore, the appearance of 

 the magnetic field means that the discharg- 

 ing process in the jar consists in a transfor- 

 mation of the potential energy of the charge 

 into kinetic energy of the magnetic field. 

 At the moment when the jar is completely 

 discharged, all the potential energy of the 

 charge, except that part which has been 

 ti'ansformed into heat in the conducting 

 wire, appears as kinetic, that is, as mag- 

 netic energy of the field. From that moment 

 on, this kinetic energy begins to diminish, 

 because, owing to the peculiar connection of 

 the conducting wire to the moving parts of 

 the medium, the current in it will persist and 

 charge the jar in the opposite sense, which 

 means a retransformation of the magnetic 

 energy of the field into the potential energy 

 of the charged jar, and so on. These cyclic 

 transformations continue until the total 

 initial energy of the charged jar is trans- 

 formed into heat in the conducting pai'ts of 

 the system. We have electric oscillations. 

 These oscillations were observed by Joseph 

 Henry, nearly twentj^ years before Maxwell 

 wrote his famous third essay ; Sir William 

 Thomson discussed their theory in 1853, 

 and Feddersen subjected this theory 

 to crucial experimental tests from 1857 to 

 1862. But that which, in the estima- 

 tion of the tendencies of modern electrical 

 research, is the most essential element in 

 our physical view of these oscillatory phe- 

 nomena is entirely absent from these 

 early investigations. Maxwell, guided by 

 the visions of Faraday, was the first to in- 

 troduce this element. It is this. If the 

 forces of the electromagnetic field are due, 

 as Maxwell assumed and illustrated by 

 mechanical models, to the reactions of the 

 moving parts of the field, elastically con- 

 nected to each other, then, since these 

 reactions must necessarily consume time in 

 passing from any part of the field to any 



