CONTEMPORARY ADVANCES IN PHYSICS 247 



mostly the magnetic methods refer to atoms in a soHd, and so they 

 make available a new and broad domain for the operations of atomic 

 theory. 



There remains diamagnetism. The first thing to be said al)out the 

 theory of diamagnetism is discouraging; for it has the earmark of a 

 futile atomic theory — it involves the assumption that the individual 

 atoms behave exactly like the substance as a whole. Under all 

 tield strengths and all conditions, it is assumed that the diamagnetic 

 moment of a block of N atoms is A'^ times the diamagnetic moment of 

 a single atom. Nevertheless this is not a futile assumption, for strictly 

 it is not an assumption at all but an inference from atomic structure. 

 It was mentioned early in these pages that owing to the unbreakable 

 link between angular momentum and magnetic moment, a magnetic 

 atom precesses about the direction of the field. This motion of pre- 

 cession is an extra motion of the electrons of the atoms, a circulatory 

 motion around the axis supplied by the direction of the field. This 

 extra motion entails an extra current, which entails an extra magnetic 

 moment, which is the source of diamagnetism or which is diamagnet- 

 ism. Diamagnetism is precession. It is not confined, as the fore- 

 going words suggest, to atoms which have a net magnetic moment. 

 Consider an atom (a free atom of any noble gas will afford an example) 

 possessing two or more electrons, the orbits and the spins of which are 

 so oriented that the resultant magnetic moment is nil. Though some 

 of the orbits and spins are pointed oppositely to others, they all pre- 

 cess in the same sense, and the atom acquires a magnetic moment in 

 the field though it had none beforehand. The like is true, of course, 

 when the resultant of the orbits and the spins is different from zero; 

 the agents of orientation which were discussed above render it para- 

 magnetic, but the precession renders it diamagnetic, and it is para- 

 magnetic and diamagnetic — or ferromagnetic and diamagnetic — at 

 one and the same time. The moment due to the precession is pro- 

 portional to the field strength, and the factor of proportionality may 

 be calculated from the structure of the atom (it depends primarily 

 upon the areas of the electron-orbits). The agreement of the cal- 

 culated values with the data is generally satisfactory; and diamagnet- 

 ism, the least conspicuous of the three types of magnetism, takes 

 precedence over the others as being that one of the three of which our 

 understanding is most nearly perfect. 



