,804 



SCIENCE. 



[N. S. Vol. VIII. No. 206. 



the action of the electric current upon the 

 magnetic needle had aroused the enthusiasm 

 of Ampere, with the result of the swift 

 production of his discovery of the laws of 

 electrodynamics and their representation in 

 mathematical form, by a process of reason- 

 ing characterized by Maxwell as ' perfect in 

 form and unassailable in accuracy.' Within 

 the next decade Faraday and Henry had 

 independently discovered the phenomena 

 of induction, and thus completed, with two 

 exceptions, the list of epoch-making dis- 

 coveries in the science. We may remark 

 in parenthesis that the dynamo and the 

 electric motor, which have wrought such a 

 change in our city and Association in very 

 recent years, were thus possible before the 

 birth of the Association. 



Not only had the quantitative laws of 

 induction of currents been formulated by 

 Faraday, but they had been obtained by a 

 remarkable mathematical process by Neu- 

 mann, using the then hardly recognized 

 principle of the Conservation of Energy. 

 The mathematical theory of the subject 

 was accordingly in a well advanced state 

 fifty years ago. It is to be noticed, how- 

 ever, that all the work then done had been 

 on the basis of action at a distance, the ex- 

 istence of which was then unquestioned by 

 mathematical physicists. 



Not so, however, by that prince of ex- 

 perimental philosophers, Michael Faradaj^ 

 Not less important for the theory of elec- 

 tricity than his discovery of current induc- 

 tion were his electro- statical researches by 

 which he first showed that the forces be- 

 tween electrically charged bodies were not 

 independent of the surrounding medium. 

 Thus Faraday was led to concentrate his 

 attention upon the medium instead of upon 

 the charges themselves, and daringly to 

 attack the notion of action at a distance. 

 In order to clothe his ideas in an intuitional 

 geometrical form, Faraday introduced the 

 idea of physical lines of force, an idea that 



was long in having its fruitfulness recog- 

 nized by others. 



It was not until 1861 that the note was 

 struck which has produced such a remark- 

 able change in the theory of fifty years 

 since. In Maxwell's papers on ' Physical 

 Lines of Force ' in the Philosophical Maga- 

 zine of that year he gave Faraday's ideas 

 a mathematical garb, and introduced to the 

 mathematical world the theory that the 

 energy was resident in the medium, rather 

 than in the charged bodies. In 1865 ap- 

 peared in the Philosophical Transactions 

 Maxwell's chef-d'oeuvre, the elaborate de- 

 velopment of his ideas in his paper on ' A 

 Dynamical Theory of the Electromagnetic 

 Field.' Here we find for the first time the 

 application of Lagrange's dynamical equa- 

 tions to obtain in a systematic and logical 

 way the laws of electricity and its connec- 

 tion with magnetism. Besides the notion 

 of the localization of energy, both electric 

 and magnetic, in the medium, we find the 

 other idea, foreign to the old theory, of the 

 magnetic action due to time- variations of 

 the electric field, these variations being 

 termed by Maxwell displacement currents. 

 It is not a little remarkable that Faraday 

 had considered the possible changes taking 

 place in the electrical state of the dielectric 

 medium by changes of the magnetic field, 

 and had attempted to make them experi- 

 mentally evident, but without success; 

 Whether Faraday contemplated the effect 

 of the changes of the electrical state on the 

 magnetic field I am not able to state. At 

 any rate the introduction of this hypothet- 

 ical magnetic eifect of the displacement 

 currents by Maxwell gave rise to a hitherto 

 unlooked-for possibility, namely, the estab- 

 lishment of an electro- magnetic theory of 

 light. This theory not only enjoyed the 

 advantage of novelty, but was free from 

 the fundamental diflBculties of the previous 

 dynamical theories of light, in that in it no 

 longitudinal wave appeared. 



