THE MAGNETISM OF IRON. 83 



a piece 01 iron in a magnetic field is always such as to suggest 

 that " iron is a better carrier of lines of force than air." The 

 lines of force tend to converge into the iron and pass through it. 

 Magnetic screening. A shell of soft iron forms a very effec- 

 tive screen which protects the region inside of the shell from the 

 action of outside magnetic influences. The lines of force, which 

 would pass through the region occupied by the shell if the shell 

 were not presen,t, pass into the iron and tend to flow around 

 through the shell and pass out on the other side without crossing 

 the region inside of the shell. This screening effect has been 

 used for the protection of watches against magnetic disturbances 

 by providing the watch with a thick case of soft iron. 



Note. The region surrounding a magnet is a magnetic field, it magnetizes any 

 piece of iron in the neighborhood, and the piece of iron is then attracted by the 

 magnet. 



46. Residual magnetism. Permanent magnets. An iron rod 

 retains much of its magnetism when it is removed from a mag- 

 netic field in which it has been magnetized ; or in case of an elec- 

 tromagnet, when the magnetizing current is reduced to zero. 

 Long slim bars retain a greater portion of their magnetism than 

 short thick bars, because of the fact that in short bars the poles 

 of the magnet are closer together and produce of themselves a 

 strong demagnetizing field along the bar. The magnetism which 

 is thus left in a bar of iron or in an electromagnet is called resid- 

 ual magnetism. Long slim bars of annealed wrought iron may 

 retain in this way as much as 90 per cent, of their magnetism, 

 but a very weak demagnetizing field or a very slight mechanical 

 shock is sufficient to cause such a bar to lose its residual mag- 

 netism almost completely. Cast iron, hard drawn iron wire and 

 mild steel retain a smaller portion of their magnetism but with 

 greater persistence, and hardened steel bars retain a portion of 

 their magnetism- very persistently even when roughly handled. 

 Magnetized bars of hardened steel are called permanent magnets. 



Aging of permanent magnets. A freshly magnetized bar of 

 hardened steel loses a portion of its residual magnetism rapidly 



