298 MAGNETISM 



will be given by M = tfA, where e is the elect ionic- charge and A i- 

 the area of the orbit swept out per second. 



Now suppose that a field II is put on perpendicular to the 

 plane of the orbit, and in the positive diivction, so that if the 

 equivalent current is going round clockwise' in the plane of tin- 

 paper, or the electron is going round counter-clockwise. II is from 

 above downwards. During the increase in II then- U a negatixc. 

 i.e. counter-clockwise, E.M.F., and this, acting on : nc 



electron, gives a clock \\i-e or retarding toree. \Yhcii II i>establi- 

 the angular velocity in the orbit is less than initial! 

 diminished, and it can be shown that 



A M/ M = - cllr!*- 



where r is the period of revolution and /// is the mass of tin- 

 electron. 



Now imagine a more complex atom in which there is a central 

 body with a large number of elect rons circling round it in orl 

 with their aspects indificrentlx (list i United in all direction-. On 

 the whole the moment of each atom, then, i- \\ 



field II is put on, the effect on each orbit is like ti 

 above, but less as the inclination of II to the perpendicular t< 

 orbit increases. Those going round one \\-.\\ \\ill ha\e their 

 positive moments decreased, tho-e going round the other 

 will have their negative moments increased, so that on the whole 

 the moment of the atom will become negatixe. SYe max suppose 

 that the ordinary dian atom is of this t\; 



formula for the change of moment shoxx.s that, in -Mill 



experience, it is very minute. Further. lineC tl D to 



siippo-e that temperature describes the agitation of the molecules 

 and atoms as wholes and not the motions x\ ithin the atoms. \\c 

 should not expect diamagnetic susceptibility to depend on 

 temperature. Curie showed that, excluding bismuth and antimony, 

 the susceptibility was constant through a MIX wide rang* 

 temperature. 



To explain paramagnetism. imagine a bodx to consist of atoms 

 in each of which the aspects of the electronic orbits are not 

 indifferently distributed, but that tl .,uuped more or 



about a particular axis. Each atom, then, will have a magnetic 

 moment, like the molecular magnets in \Y U-r's theory* \\hen 

 there is no external field and the body is unmagnctist d. the ax 

 the atoms and the molecules into which they are grouped will be 

 distributed indifferentlv in all directions. When a field II is put 

 on, it acts in txvo opposite ways, In the first place it tends 

 to decrease each atomic magnetic moment in the way already 

 explained for diamagnetic atoms. In the second place it tends to 

 pull the magnetic axes into its own line and so to give a positive 

 moment to a mass of molecules. This second effect is vastly 



