COERCIVE FORCE. 289 



actions, which they exert on each other, disappear at the same 

 time. 



More generally, when any magnetic body is placed in a mag- 

 netic field, it becomes itself a magnet. This is a magnetisation by 

 influence or induced magnetisation. 



The axis of magnetisation at each point is parallel to the direction 

 of the resultant force. This resultant arises from the action of the 

 field and of that which is produced by the induced magnetism itself. 

 If the body in question is infinitely small, the magnetisation is exactly 

 parallel to the force of the field at the point in question. 



This conclusion also follows, that the action of a magnet is 

 null on a neutral body, and that any action exerted by magnets 

 on magnetic bodies, is preceded by a magnetic induction on 

 the latter. 



Here again we see the analogy of this phenomenon with that of 

 electrostatic induction, and particularly the induction in dielectrics. 



The magnetism thus induced does not depend merely on the 

 strength of the field, but also on the nature of the substance in 

 question; magnetisation, which is very powerful with pure iron 

 and nickel, is far feebler with all other magnetic substances. 



302. SOFT IRON. COERCIVE FORCE. Iron is said to be abso- 

 lutely soft if, after having been placed in a very powerful magnetic 

 field, it loses its magnetisation when it is withdrawn from it. Soft 

 iron, in the magnetic sense, is also soft in the ordinary meaning 

 of the word ; it may be easily bent, worked, and it has but little 

 elasticity. Conversely, ordinary iron is not soft in the magnetic 

 sense of the word ; when it is impure, or has undergone mechanical 

 changes, it remains more or less magnetised, and this property is 

 designated by the somewhat barbarous term coercive force. A speci- 

 men of iron has so much the greater coercive force the greater is its 

 quantity of residual magnetism; at the same time, this kind of 

 iron is more readily magnetised by induction. Coercive force is 

 therefore a property analogous to friction. Within certain limits it 

 opposes the changes which external forces tend to produce in mag- 

 netisation, and hinders any one single state of equilibrium from 

 corresponding to given external conditions. 



The coercive force in steel is very great ; it becomes magnetised 

 by induction with more difficulty than soft iron, but it retains the 

 magnetisation, once acquired, so much the better. The magnetic 

 qualities of steel vary with the composition of the metal and with its 

 mode of preparation ; they depend greatly on the manner in which 

 the tempering has been effected, as well as on the degree of annealing 



