CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 83 



explanation of the Zeeman effect, is the origin of the small negative moment possessed 

 by diamagnetic substances when subjected to a magnetic field. The smallness of the 

 diamagnetic susceptibility in general is due to the self-compensation of the forces 

 (produced by the oppositely rotating systems of electrons or any other magnetic 

 system in the molecule) which takes place at points whose distances from the molecules 

 are large in comparison with molecular dimensions. But quite close up to the 

 molecule these forces will not compensate and the local force will be comparable with 

 that due to an unbalanced revolving electron of other magnetic element of the 

 molecule. It is with this local field that the present communication is mainly 

 concerned. I shall call it the local molecular field of the diamagnetic substance. In 

 the crystalline state we may regard the space lattice as defined by the local forces 

 exerted between a system of electrons in one molecule upon another system in an 

 adjacent molecule and so on throughout the crystalline structure. 



As was shown in Part II., p. 142, the mean molecular field is proportional to the 

 intensity of magnetization of the substance, and we may write 



(2) 



where aj is the new constant of the molecular field, of the order 10 5 , N the number of 

 molecules per gram., AM the diamagnetic moment induced in a molecule by applying 

 an external magnetic field H, and p is the density of the substance. The local mole- 

 cular field will have the same constant of proportionality but we must replace the 



Fig. 1. 



diamagnetic moment AM of the molecule as a whole by the local moment associated 

 with a pair of molecules. We have no means of measuring directly the value of this 

 latter quantity, but it is probably very large in comparison with AM, and it was 

 suggested at the end of Part II. that it may be so large that the local molecular field 

 is comparable with the molecular field in ferro-magnetic substances.* On account of 

 bhe null initial moment which has been prescribed for the diamagnetic molecule the 

 local molecular field will be of an alternating character, the distance over which it is 

 unidirectional being comparable with the distance between the molecules, as the 

 diagram above shows. 



* It should be noted that this local molecular field will exist in all crystalline substances, whether they 

 arc subjected to an external magnetic field or not. On the other hand, the mean molecular field is a 

 differential effect and depends on AM, the diamagnetic moment which is induced by the external field. 



M 2 



