CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 269 



The fall off in the value of x as the temperature is reduced, may, in part, be 

 explained by the increased value of the molecular field, due to the nearer approach of 

 the molecules. As the local molecular field becomes very big, the induced diamagnetic 

 effect in each molecule will become big, in the same proportion, and this will tend to 

 reduce the value of x and make this quantity tend to a limiting value. If the 

 molecular field is of the order 10 7 gauss at ordinary temperatures, we have seen that the 

 ratio of the induced diamagnetic moment AM to the magnetic moment of the electron 

 orbit M, is of the order 1/100. If the molecular field at low temperatures approaches 

 10 9 gauss, owing to closer proximity of the magnetic elements in neighbouring 

 molecules, the diamagnetic effect would be comparable with the ferro-magnetic effect.* 

 On our view this does not imply that diamagnetic substances should acquire a large 

 diamagnetic susceptibility at very low temperatures. For taking the molecules in 

 pairs, locally they are paramagnetic and the action of the local molecular field is to 

 reduce this paramagnetic effect so that the. local magnetic moment becomes smaller 

 and the susceptibility to an external field tends to zero as in iron. 



According to EwiNGt experiments carried out to test this effect have neither proved 

 nor disproved this theory, probably because the external fields were not sufficiently 

 strong. But during crystallization we are applying unconsciously to each molecular 

 current a magnetic field 500 or 1000 times stronger than the largest field we can 

 apply externally, and probably even greater local intensities are attained at low 

 temperatures, since the interacting magnetic elements in adjacent molecules may be 

 almost touching one another. The mutual induction and temperature effects combine to 

 cause x to approach the limit zero at the absolute zero or in very powerful external fields. 



In diamagnetic media we have seen that the constant of the local molecular field a' c 

 (which corresponds to N in WEISS'S ferro-magnetic field) is of the order 2'5x 10 4 and 

 the reciprocal of this, viz., +4x 10~ 5 , is the order of magnitude of the local positive, 

 limiting susceptibility of a diamagnetic crystalline medium. (At ordinary temperatures 

 the diamagnetic susceptibility per unit volume is of the order 10""). The parts of 

 molecules adjacent to one another in a diamagnetic crystalline medium attract in a 

 similar manner to the adjacent parts of molecules of a ferro-magnetic or paramagnetic 

 medium. 



In a ferro-magnetic medium, as the temperature is raised, the susceptibility increases 

 up to a certain point just below the critical temperature and then falls off rapidly. 

 The temperature controls the susceptibility in two ways ; first, by helping the molecules 

 to overcome the difficulties of orientation, produced by the neighbouring molecules, to 

 a point just below the critical temperature ; second, by overdoing this effect and by 

 giving the molecules too much rotational energy, at the critical temperature and above, 

 so that the susceptibility to magnetization falls very rapidly. 



* In nickel the molecular field is 6 3 x 10 6 gauss. An applied field of 10 gauss would make a substance 

 as diamagnetic as bismuth have a saturation value equal to that of nickel. 

 t ' Magnetic Induction in Iron and other Metals,' p. 353. 



