THE MAGNETISM OF IRON. 87 



when the field ceases to act. If the field is increased in intensity 

 more and more the magnets turn more and more until the con- 

 figuration of the system becomes unstable, when the magnets 

 suddenly fall, as it were, into a new configuration.* If now the 

 field is slowly reduced in intensity the magnets tend to persist in 

 their new configuration. 



49. Paramagnetic substances and diamagnetic substances. Cobalt and nickel 

 are similar to iron in their magnetic properties except that the limit or saturation value 

 of their intensity of magnetization is not so great. Many other substances, such as 

 manganese, chromium, platinum, and oxygen, show similar properties but to a lesser 

 degree. Such substances are said to be paramagnetic, or simply magnetic. On the 

 other hand, substances such as bismuth, antimony, zinc and lead, when they are near 

 a magnet, are magnetized in such a way as to be repelled f by the magnet. Such sub- 

 stances are said to be diamagnetic. 



Paramagnetic substances are better carriers of lines of force than air and diamag- 

 netic substances are poorer carriers of magnetic lines of force than air, that is to say, 

 when a paramagnetic substance is placed in a magnetic field the lines of force converge 

 towards it and pass through it, and when a diamagnetic substance is placed in a mag- 

 netic field the lines of force tend to spread out and go round it. 



A paramagnetic substance when placed in a non-uniform magnetic field is drawn 

 towards the region where the field is most intense, whereas a diamagnetic substance 

 when placed in a non-uniform magnetic field is drawn towards the region where the 

 field is least intense. This behavior of a diamagnetic substance in a non-uniform 

 field may be shown by suspending a very small bar of bismuth between the pointed 

 poles of a strong electromagnet. If the suspending fiber is sufficiently flexible the bar 

 of bismuth sets itself at right angles to the lines joining the two pointed poles. J 



* A group of magnets mounted on pivots may be in equilibrium in a great variety 

 of configurations. 



| See note in Art. 45, page 83. 



J A bar of bismuth tends to place itself parallel to the lines of force in the uniform 

 magnetic field the same as a bar of iron. This apparently similar property of bismuth 

 and iron may be explained as follows : If a bar of iron (or bismuth) were magnetized to 

 the same degree irrespective of its direction in a uniform field, it would stand indif- 

 ferently in any position, but as a matter of fact, an iron rod is more strongly magnetized 

 when it is parallel to a magnetic field than when it is at right angles to the field, 

 because of the demagnetizing action of the free poles on the rod, and the result is that 

 the rod takes up the position in which it is most strongly magnetized. On the other 

 hand, the effect of the free magnetic poles on a rod of a diamagnetic substance is to 

 increase the negative magnetization, so that the negative magnetization of a rod of 

 bismuth is least when it is parallel to the magnetic field in which it is placed. A rod 

 of iron tends to place itself in the direction in which it is most strongly magnetized by 

 the field, and a rod of bismuth tends to place itself in the direction in which it is least 

 strongly magnetized by the field, and in each case this position is parallel to the lines 

 of force if the field is uniform. 



