874 



SCIENCE. 



[N. S. Vol. II. No. 52. 



following : Maxwell's theory is Maxwell's 

 system of equations. * Every theory which 

 leads to the same system of equations, and 

 therefore comprises the same possible phe- 

 nomena, I would consider as being a form 

 or special case of Maxwell's theory. * * * * " 

 Boltzman, one of Maxwell's most sincere ad- 

 mirers, introduces his lectures on Maxwell's 

 electro-magnetic theory f with the following 

 verse from Goethe's Faust, which he para- 

 phrased evidently with the intention of de- 

 scribing the desperate state of his mind : 

 So soil ich denn mit saurem Schwelss 

 Euch lehren, "was ich selhst nicht "weiss. 



maxwell's ELECTEO-MACiNETIO THEORY OF 

 LIGHT. 



These statements, coming, as they do, from 

 so high authorities, do not seem to present 

 a cheerful outlook to those who, like myself, 

 take upon themselves the burden of the pon- 

 derous task of popularizing Maxwell's elec- 

 tro-magnetic theory. But the outlook is 

 really not quite as gloomy as it appears at 

 first glance, provided, of course, that one 

 limits himself to the essential parts of Max- 

 well's story and leaves out the ornamental 

 detail. In other words, the story of what 

 Maxwell actually accomplished can be told 

 in a few and simple words; what he prob- 

 ably attempted to do, but did not accom- 

 plish, is a diiferent matter and does not 

 concern us at present. Maxwell's electro- 

 magnetic theory in its simple form and di- 

 vested of all unnecessary hypotheses can 

 be described briefly as the extension of the 

 meaning of certain well established experi- 

 mental facts. To state these facts it is well 

 to consider briefly the following well-known 

 experiments : 



First: Connect two metal plates, facing 

 each other and forming an electric con- 



*"Wliat Hertz calls 'Maxwell's system of equa- 

 tions ' means the same thing as the expression ' fun- 

 damental laws, ' mentioned above. 



tVorlesungen iiber Maxwell's Theorie, etc., publ. 

 by Barth, Leipzig. 



denser, to the poles of a galvanic cell. A 

 transient current takes place whose value 

 can be determined experimentally. Ex- 

 periment tells us that this transient current 

 is proportional to the electromotive force of 

 the cell, so that n equal cells in series will 

 produce n times the transient or integral 

 current. Having charged the condenser 

 we disconnect the cells and then join the 

 plates by a conducting wire and discharge ; 

 the integral discharge current is just as 

 large as the charging current, but in op- 

 posite direction. A charged condenser is, 

 therefore, the seat of an electromotive 

 force acting in opposite sense to the charg- 

 ing electromotive force. The old view 

 maintained that this electromotive force is 

 due to the accumulated electricities in the 

 plates ; the Faraday- Maxwell view denies 

 this and maintains that the electromotive 

 force is due to an action of the dielectric 

 separating the j)lates. According to the old 

 theories the current is a process confined to 

 the conducting parts; in our present case, for 

 instance, it is along the conducting wire and 

 stops at the boundarj^ separating the con- 

 denser plates from the dielectric. Accord- 

 ing to the Faraday-Maxwell view this pro- 

 cess continues through the dielectric, and 

 whereas it generates heat in the conduct- 

 ing parts it stores up energy in the dielec- 

 tric just as a compression stores up energy 

 in the body which is being compressed. 

 The charging cell supplies the action and 

 the dielectric reacts ; the ivork against this 

 reaction is the energy of the charged con- 

 denser, which is, therefore, in the dielectric 

 and not on the surface of the plates, as the 

 old theories supposed. The charging pro- 

 cess or current continues until the electro- 

 motive reaction of the dielectric is equal to 

 the electromotive force of the cell, and since 

 the integral current is proportional to the 

 electromotive force of the charging cell it 

 follows that the electromotive reaction of a 

 charged condenser is also proportional to 



