November io, 1921] 



NATURE 



341 



Faraday and the Quantum. 

 By Dr. H. Stanley Allen. 



IX the third volume of his "Experimental Re- 

 searches in Electricity " Faraday returns again 

 and yet again to the discussion of lines of magnetic 

 force and of their physical existence. The first paper 

 in the volume bears the suggestive title, "On 

 the Magnetisation of Light and the Illumination 

 of Magnetic Lines of Force." He defines the 

 latter by saying : " By line of magnetic force, or 

 magnetic line of force, or magnetic curve, I mean 

 that exercise of magnetic force which is exerted 

 in the lines usually called magnetic curves, and 

 which equally exist as passing from or to mag- 

 netic poles, or forming concentric circles round 

 an electric current." He then goes on to describe 

 his discovery of the magnetic rotation of the plane 

 of polarisation, a phenomenon which Lord Kelvin 

 regarded as a demonstration of the reality of 

 Ampere's explanation of the ultimate nature of 

 magnetism. In the celebrated letter to Richard 

 Phillips, published in the Philosophical Magazine 

 for May, 1846, under the title, "Thoughts on 

 Ray-vibrations," Faraday writes: — 



The view which I am so bold as to put forward 

 considers, therefore, radiation as a high species of 

 vibration in the lines of force which are known to 

 connect particles, and also masses, of matter together. 

 It endeavours to dismiss the aether, but not the 

 vibration. 



Again in 1852 he says : — 



Having applied the term line of magneUc force to 

 an abstract idea, which I believe represents accurately 

 the nature, condition, direction, and comparative 

 amount of the magnetic forces, without reference to 

 any physical condition of the force, I have now applied 

 the term physical line of force to mclude the further 

 idea of their physical nature. The first set of lines I 

 affirm upon the evidence of strict experiment. The 

 second set of lines I advocate, chiefly with a view of 

 stating the question of their existence. 



This question he regards as "both important 

 and likely to be answered ultimately in the affirma- 

 tive." 



The quantum theory seems to supply the 

 affirmative answer anticipated by Faraday. It has 

 long been recognised that this theory requires a 

 certain atomicity in nature which may be repre- 

 sented either by Planck's constant h, or bv some 

 combination of h with other fundamental con- 

 stants. Planck's constant, which has the value 

 6-558x10-2" erg sec, may be looked upon as a 

 quantum of action, but it is perhaps simpler to 

 regard it, in accordance with the suggestion of 

 J. W. Nicholson, as an angular momentum. The 

 first indication that the quantum may be essen- 

 tially magnetic appears in the work of S. B. 

 McLaren, who, in a letter to Nature (vol. 92, 

 p. 165, 1913), identified the natural unit of angular 

 momentum, /ly 2t, used by Bohr, with the angular 

 NO. 2715, VOL. 108] 



momentum of the magneton. " Rejecting entireh 

 the idea of magnetic or electric substance, th- 

 magneton may be regarded as an inner limiting 

 surface of the aether, formed like an anchor-ring. 

 The tubes of electric induction which terminal, 

 on its surface give it an electric charge ; thr 

 magnetic tubes linked through its aperture make 

 it a permanent magnet." It may be shown from 

 ordinary electrodynamic considerations that the 

 angular momentum of such a system is equal to 

 Ne/zTT, where N is the number of magnetic tubes 

 threading the magneton, and e is the electro- 

 static charge. According to the quantum theory, 

 the angular momentum must be nhjzv, where n 

 is an integer, and we may have a one-quantum 

 magneton, a two-quantum magneton, and so on. 

 Identifying these two expressions for the angular 

 momentum, we find 



or 



Ne = nh, 



N=n(;2/e). 



Thus it is seen that the number of tubes of mag- 

 netic induction passing tnrough the aperture of 

 the magneton is equal to an intoger, n, multi- 

 plied by the constant factor hjC. If we suppose 

 that the charge of the magneton is equal to the 

 electron charge 4-774x10-^'^ E.S.U., we find for 

 hje, which defines what must be the fundamental 

 unit magnetic tube, the value i-374x 10-^' E.S.L . 

 or 4-120 X lo"" E.M.U. Consequently one C.G.S. 

 magnetic tube contains nearly two and a half 

 million (2-43 x 10^) quantum tubes. The electro- 

 kinetic energy of the unit tube is ^/jv, where v 

 is the frequency of revolution. 



A magneton theory of the structure of the atom 

 has been developed with great ingenuity by A. L. 

 Parson, but his theorj^ has not met with accept- 

 ance, partly because it employs the notion of a 

 sphere of positive electricity in place of a positive 

 nucleus, but mainly because it is not based on the 

 atomic numbers of Moseley which are now gener- 

 ally accepted as representing accurately the 

 number of electrons in the neutral atom out- 

 side the nucleus. Ultimately it may prove 

 necessary to adopt some form of the mag- 

 neton hypothesis, which seems well adapted 

 to explain magnetic properties, and, by ad- 

 mitting the possibility of stationary electrons, 

 as in the Lewis-Langmuir theory, chemical pro- 

 perties also. But at the present time the results 

 obtained from the Bohr-Sommerfeld theory of 

 spectral frequencies seem to demand electrons 

 which are moving in certain orbits. As pointed 

 out in these columns by Dr. Norman Campbell 

 (Nature, vol. 106, p. 408, 1920), the difference 

 between the two views may be purely formal. 



It is, however, desirable to consider whether 



