FARADAY'S DISCOVERY. 491 



CHAPTER IV. 

 INDUCTION. 



507. FARADAY'S DISCOVERY. The electromagnetic actions 

 studied in the preceding chapters are purely mechanical ; they are 

 exerted on conductors traversed by currents, and correspond to a 

 permanent condition of currents, and of the magnets near them. In 

 all cases in which the systems experienced relative displacements, we 

 have implicitly assumed that those displacements had no influence 

 on the electric condition of the conductors. Faraday discovered 

 in 1831 a class of phenomena of a totally different kind which 

 corresponds to the variable condition of the system ; these 

 phenomena, which he comprised under the term induction^ are 

 of an electrical character, and are manifested by the production of 

 temporary currents in conductors. 



The currents which are formed are called induced currents ; the 

 induced circuit is that submitted to induction ; the term inductor is 

 applied to the current, the variation in which has been the cause of 

 the induced current. 



508. The phenomena discovered by Faraday may be classed 

 under several heads : 



i st. A closed circuit becomes the seat of a temporary current 

 whenever a magnet is displaced near it ; or if the magnetisation is 

 varied ; or still more generally when the magnetic field is modified 

 in which the circuit is placed. This is magnetoelectrical induction. 



2nd. Analogous effects are obtained by substituting a system of 

 currents for the magnetic system. The circuit in question is 

 traversed by an induced current whenever the distance, strength, or 

 form of the external current is altered. The effect is the same as 

 that which would produce the corresponding modification of the 

 equivalent magnetic system or the current. This is electro dynamic 

 or voltaic induction. 



3rd. The change of form or of relative position of a closed 



