2 Introduction 



a reciprocal relation : it is found that magnets in motion produce the same 

 effects as electricity at rest, while electricity in motion produces the same 

 effects as magnets at rest. The third division of Electromagnetism, then, 

 connects the two former divisions of Electrostatics and Magnetostatics, and 

 is in a sense symmetrically placed with regard to them. Perhaps we may 

 compare the whole structure of Electromagnetism to an arch made of three 

 stones. The two side stones can be placed in position independently, neither 

 in any way resting on the other, but the third cannot be placed in position 

 until the two side stones are securely fixed. The third stone rests equally 

 on the two other stones and forms a connection between them. 



3. In the present book, these three divisions will be developed in the 

 order in which they have been mentioned. The mathematical theory will be 

 identical, as regards the underlying physical ideas, with that given by 

 Maxwell in his Treatise on Electricity and Magnetism, and in his various 

 published papers. The principal peculiarity which distinguished Maxwell's 

 mathematical treatment from that of all writers who had preceded him, was 

 his insistence on Faraday's conception of the energy as residing in the 

 medium. On this view, the forces acting on electrified or magnetised bodies 

 do not form the whole system of forces in action, but serve only to reveal 

 to us the presence of a vastly more intricate system of forces, which act 

 at every point of the ether by which the material bodies are surrounded. 

 It is only through the presence of matter that such a system of forces can 

 become perceptible to human observation, so that we have to try to 

 construct the whole system of forces from no data except those given by the 

 resultant effect of the forces on matter, where matter is present. As might 

 be expected, these data are not sufficient to give us full and definite knowledge 

 of the system of ethereal forces; a great number of systems of ethereal 

 forces could be constructed, each of which would produce the same effects on 

 matter as are observed. Of these systems, however, a single one seems so 

 very much more probable than any of the others, that it was unhesitatingly 

 adopted both by Maxwell and by Faraday, and has been followed by all 

 subsequent workers at the subject. 



4. As soon as the step is once made of attributing the mechanical 

 forces acting on matter to a system of forces acting throughout the whole 

 ether, a further physical development is made not only possible but also 

 necessary. A stress in the ether may be supposed to represent either an 

 electric or a magnetic force, but cannot be both. Faraday supposed a stress 

 in the ether to be identical with electrostatic force, and the accuracy of this 

 view has been confirmed by all subsequent investigations. There is now 

 no possibility, in this scheme of the universe, of regarding magnetostatic 

 forces as evidence of simple stresses in the ether. 



