THE MAGNETISM OF IRON. 85 



have at the utmost about 800 units pole per square centimeter 

 of section. 



48. The molecular theory of the magnetization of iron. When 

 a magnet is broken in pieces, each piece is found to be a complete 

 magnet having a north pole and a south pole. This fact sug- 

 gests the possibility that each molecule of iron may be a magnet. 

 Indeed, the hypothesis that each molecule of iron, or any sub- 

 stance capable of being magnetized, is a permanent magnet leads 

 to a very useful conception of what takes place in a bar of iron 

 when it is magnetized. 



Explanation of magnetization. In unmagnetized iron or steel 

 the molecular magnets are thought of as pointing at random in 

 all directions, thus neutralizing each other. When the iron or 

 steel is placed in an intense magnetic field, the molecular mag- 

 nets are turned with their axes parallel to the field, their north 

 poles all in one direction, and the iron or steel is completely 

 magnetized or saturated. If the magnetizing field is weak the 

 molecular magnets are only partially turned and the iron is only 

 partially magnetized. 



Explanation of retention of magnetization. A bar of iron 

 which is strongly magnetized, does not return to its original 

 state when the magnetizing field ceases to act. This is analo- 

 gous to the production of a permanent set when an imper- 

 fectly elastic substance is greatly distorted. This persistence of 

 a portion of the magnetization in a strongly magnetized bar may 

 be ascribed to a friction-like opposition to the rotation of the 

 molecular magnets. In annealed iron this friction is small, in 

 hard drawn iron wire it is greater, and in hardened steel it is very 

 great. Mechanical vibration and rise of temperature both act as 

 if to decrease this frictional resistance, thus enabling a given 

 magnetizing field to produce more intense magnetization and 

 causing the residual magnetism to disappear. 



Behavior of iron and steel when subjected to slight changes of 

 magnetization. When a bar of iron or steel is placed in a weak 



