CHAP, v.] ELECTRICITY AND MAGNETISM. 269 



" diamagnetic " bodies (see Art. 339). Bodies which 

 have a high coefficient of magnetisation may be re- 

 garded as good conductors of magnetism. When a 

 piece of soft iron is placed in a magnetic field the lines 

 of force gather themselves up and run in greater quan- 

 tities through the space now occupied by iron ; whereas, 

 if a piece of bismuth or copper is placed in the field, 

 fewer lines of force than before pass through the space 

 occupied by the diamagnetic metal. The intensity of 

 magnetisation through the substance of a magnet is 

 measured by dividing its "magnetic moment" 1 by its 

 volume. A permanent steel magnet has a certain per- 

 manent intensity of magnetisation ; a piece of soft iron 

 laid along the lines of force in a magnetic field has 

 induced in it a certain temporary intensity of magnetisa- 

 tion equal to the product of the "intensity" of the field H 

 into the coefficient of magnetisation of the iron k. 

 Intensity of magnetisation = m . ' l = k H. 



volume 



It is, however, found that there is a certain maximum 

 of intensity of magnetisation for each magnetic metal, 

 which cannot be exceeded, no matter how powerful the 

 field in which the metal is placed. According to Row- 

 land, the following are the maximum intensities for 

 different metals : 



Iron and Steel . . . 1390 

 Cobalt .... 800 

 Nickel .... 494 



Steel will not retain all the magnetism that can be 

 temporarily induced in it, its permanent maximum of 

 intensity being only 785.2 Everett has calculated (from 

 Gauss's observations) that the intensity of magnetisation 

 of the earth is only 0-0790, or only rnhnr of what it 

 would be if the globe were wholly iron. The fact that 



1 The "magnetic moment" is the product of the strength of either pole 

 of a magnet by its length, or = m X 1. 



2 According to Weber, it is 400 ; according to Van Waltenhofen, 470 ; 

 according to Schneebeli (in thin wires) from 710 to 1060. 



