736 PERMANENT MAGNETS. 



current from two strong cells, it will become still more 

 magnetic ; and if a somewhat thick piece of cast steel, 

 4 or 5 cm long, which has been hardened by making it 

 red hot and then cooling it rapidly, be treated similarly 

 it will manifest an attractive force considerably greater 

 than that possessed by the thin needle. 



A piece of hardened steel which possesses the property 

 of attracting iron is called a 4 permanent magnet,' or 

 briefly a c magnet.' It is assumed that the Amperian 

 currents exist in hardened steel as well as in soft iron, 

 but that a special force, to which the name coercive force 

 has been given, tends to maintain the currents in the 

 precise position which they have at any instant. It 

 follows from this that the Amperian currents of a bar 

 of steel cannot so easily assume new positions as those 

 of soft iron, in which substance the coercive force is very 

 feeble or from which it is almost absent ; on the other 

 hand, if the Amperian currents have once been compelled 

 to assume a position of parallelism, they maintain this 

 position by the action of the coercive force. 



A permanent magnet behaves in every respect like 

 an electro-magnet: when freely suspended it takes up 

 the same definite north-south position, it exhibits the 

 same attraction between unlike and repulsion between 

 like poles, and it acts in the same way inductively on 

 soft iron which is brought near it. 



Magnetic induction does not take place so easily in 

 hardened steel as in soft iron. If a small piece of steel 

 be placed in contact with either a permanent magnet or 

 an electro-magnet, the attraction is much less than that 

 exerted on a piece of soft iron of the same size placed in 

 contact with the same magnet, and it will require some 



