520 JAMES CLERK MAXWELL. 



wondered to see that the subject stood it so well. I send by this 

 post another paper to you ; I wonder what you will say to it. I 

 hope however, that bold as the thoughts may be, you may per- 

 haps find reason to bear with them. I hope this summer to make 

 some experiments on the time of magnetic action, or rather on 

 the time required for the assumption of the electrotonic state, 

 round a wire carrying a current, that may help the subject on. 

 The time must probably be short as the time of light ; but the 

 greatness of the result, if affirmative, makes me not despair. 

 Perhaps I had better have said nothing about it, for I am often 

 long in realising my intentions, and a failing memory is against 

 me. Ever yours most truly, M. FARADAY. 



Prof. C. Maxwell. 



The paper, read before the Cambridge Philosophical 

 Society, and published in vol. x. of their Proceedings, is 

 confessedly only a translation of Faraday's ideas into mathe- 

 matical language, with illustrations and extensions, and it 

 makes no attempt at explaining the nature of the action in 

 the dielectric, or the mechanism by which the observed 

 effects are brought about. About five years later, in a series 

 of three papers communicated to the Philosophical Magazine 

 in 1861 and 1862, Professor Maxwell gave a simple sketch 

 of a system of mechanism, capable of producing not only the 

 electrostatic effects above alluded to, but also of accounting 

 for magnetic attraction, the action of electric currents upon 

 one another, and upon magnets, and electromagnetic induc- 

 tion ; but before giving an account of these papers it will be 

 necessary briefly to mention the principal phenomena, an 

 explanation of which was required. 



The ordinary phenomena of magnetism, including the 

 attraction between dissimilar and the repulsion between 

 similar poles, as well as the still more familiar phenomena 

 of the attraction of soft iron by a magnetic pole, are too well 

 known to require more than a passing mention. Coulomb 

 showed that the law of inverse squares obtained equally for 

 magnetic repulsions as for electrical, so that the stress between 

 two magnetic poles is proportional to the product of the 

 strengths of the poles and inversely proportional to the 

 square of the distance between them, provided the steel of 



