Michael Faraday, his Life and Works. 4*27 



tion which accompanied his fundamental discovery. I shall 

 only refer to the fact that in 1834 he discovered a new important 

 fact — namely, the production of an induced current in the very 

 wire that conducted the inductive current, and which takes place 

 at first at the moment when the latter current begins to circu- 

 late, and then at that when it ceases passing. If this wire is 

 rolled in a coil round a cylinder of soft iron, the effect produced 

 acquires great intensity by the fact of the alternate magnetiza- 

 tion and demagnetization of the iron which accompanies the 

 passage and interruption of the current in the wire. We all 

 know the advantage that has been taken of this combination in 

 the construction of very powerful apparatus. We also know how, 

 from one improvement to another, we have come to find in in- 

 duction, and consequently in the simple mechanical movement 

 which gives birth to it, the most simple and economical prin- 

 ciple for obtaining electricity, especially with regard to its appli- 

 cation to therapeutics and illumination. 



The discovery of electrodynamical induction (that is to say, 

 the production of a current by the influence of an exterior cur- 

 rent) led Faraday to examine more closely than had previously 

 been done into the phenomenon of statical induction — that is to 

 say, the development at a distance of tension-electricity in an 

 isolated conductor by the influence of an electrized body. He 

 ascertained, what no one had previously suspected, that the 

 nature of the body interposed between the source of electricity and 

 the conductor submitted to the action of this source had a great 

 influence upon the effect produced — that, of the various bodies, 

 some facilitated the development of electricity at a distance, 

 whilst others completely stopped it. He named the former di- 

 elecmcs ; and he proved that these dielectrics, which are essenti- 

 ally resins, sulphur, shellac, oils of turpentine and naphtha, &c, 

 enjoy this property of transmitting electricity by influence in 

 different degrees, whilst there is not in this respect any differ- 

 ence between the gases, which have the same dielectric power 

 whatever their nature or their density may be. On the other 

 hand, none of the metals are dielectric; they are subject to the 

 electrical influence, but do not transmit it. 



From the investigation which we have just summarized, Fara- 

 day drew the conclusion that induction does not take place at a 

 distance, but that it is effected by the intermediation of the par- 

 ticles interposed between the inductor and the inducted body. 

 He assumed that these particles are polarized one after the other, 

 which M. Matteucci afterwards demonstrated directly by expe- 

 riment ; that consequently the mode of propagation of electri- 

 city, is the same in insulating as in conducting bodies; and that 

 the various substances only differ from each other by the greater 



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