474-477] Molecular Theories 407 



Weber supposes that, even in a substance which shews no magnetisation, 

 every molecule is a permanent magnet, but that the effects of these different 

 magnets counteract one another, owing to their axes being scattered at 

 random in all directions. When the matter is placed in a magnetic field 

 each molecule tends, under the influence of the field, to set itself so that 

 its axis is along the lines of force, just as a compass-needle tends to set 

 itself along the lines of force of the earth's magnetic field. The axes of the 

 molecules no longer point in all directions indifferently, so that the magnetic 

 fields of the different molecules no longer destroy one another, and the body 

 as a whole shews magnetisation. This, on Weber's theory, is the magnetisa- 

 tion induced by the external field of force. 



Weber supposes that each molecule, in its normal state, is in a position 

 of equilibrium under the influence of the forces from all the neighbouring 

 molecules, and that when it is moved out of this position by the action of 

 an external magnetic field, the forces from the other molecules tend to 

 restore it to its old position. It is, therefore, clear that so long as the 

 external field is small, the angle through which each axis is turned by the 

 action of the field, will be exactly proportional to the intensity of the field, 

 so that the magnetisation induced in the body will be just proportional to 

 the strength of the inducing field. In other words, for small values of H, 

 p must be independent of If. 



There is, however, a natural limit imposed upon the intensity of the 

 induced magnetisation. Under the influence of a very intense field all the 

 molecules will set themselves so that their axes are along the lines of force. 

 The magnetisation induced in the body is now of a quite definite intensity, 

 and no increase of the inducing field can increase the intensity of the 

 induced magnetisation beyond this limit. Thus Weber's theory accounts 

 quite satisfactorily for the phenomenon of saturation, a phenomenon which 

 Poisson's theory was unable to explain. 



477. In connection with this aspect of Weber's theory, some experi- 

 ments of Beetz are of great importance. A narrow line was scratched in 

 a coat of varnish covering a silver wire. The wire was placed in a solution 

 of a salt of iron, arranged so that iron could be deposited electrolytically 

 on the wire at the points at which the varnish had been scratched away. 

 The effect was of course to deposit a long thin filament of iron along the 

 scratch. If, however, the experiment was performed in a magnetic field 

 whose lines of force were in the direction of the scratch, it was found not 

 only that the filament of iron deposited on the wire was magnetised, but 

 that its magnetisation was very intense. Moreover, on causing a powerful 

 magnetising force to act in the same direction as the original field, it was 

 found that the increase in the intensity of the induced magnetisation was 



