8 



NATURE 



[March 3, 192 1 



Letters to the Editor. 



\The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications .] 



Amplifying the Optophone. 



It may be of interest to record some experiments 

 that 1 have recently been making on the application 

 of a thermionic amplifier to increase the volume of 

 the sounds produced by Dr. Fournier d'Albe's very 

 wonderful optophone so as to render these sounds 

 audible to everyone in a room without the necessity 

 of each listener being furnished with a separate tele- 

 phone receiver. 



The experiments were carried out at the instance 

 of Mr. J. M. McCarthey, who is teaching blind soldiers 

 to read with this instrument, and who asked me 

 whether it would not be possible to magnify the 

 sounds sufficiently to enable a class of a dozen or 

 more to hear them simultaneously. 



The Fournier d'Albe optophone instrument employed 

 was one of the improved type designed and manufac- 

 tured by Messrs. Barr and Stroud, and the amplifier I 

 found to work best out of several I tried was an audio- 

 frequency one with three " R " valves, transformer- 

 coupled, of the French military type. This was used 

 with a Brown loud-speaking telephone with consider- 

 able success. 



In Mr. McCarthey's opinion, and so far as a 

 person such as myself, who has no experience with 

 the optophone, could judge, the best results were ob- 

 tained when the optophone was arranged for what is 

 technically known as "black sounding," when the 

 white paper is represented by sihnce and notes are 

 sounded as the beam of light passes over the black 

 letters. 



I have very little doubt that still better results could 

 be obtained with an amplifier specially designed for 

 the purpose. Further experiment is desirable in order 

 to obtain the best results, but, so far, what has been 

 accomplished is quite encouraging. 



A. A. Campbell Swinton. 



66 Victoria Street, London, S.W.i, 

 February 25. 



Molecular and Cosmical Magnetism. 



Dr. Chapman's important letter (Nature, Novem- 

 ber 25, 1920) bases a theory of cosmical magnetism 

 on the presence of gyroscopic magnetic elements 

 proved to exist in ferro-magnetic substances by my 

 investigations on magnetisation by rotation. But he 

 considers my fundamental theory to require serious 

 modification. As I understand his letter, however, 

 his theory is identical with mine (see Science, 

 vol. xlviii., p. 304, 1918, and references) except as 

 to paramagnetic and diamagnetic bodies. He has, I 

 think, confused my treatments of magnetic intensity 

 and intensity of magnetisation. 



While in my paf)ers electron rings or orbits have 

 been assumed, the fundamental theory is essentially 

 the same if ring electrons or magnetons of other 

 types, preferable for Dr. Chapman's purpose, are 

 assumed instead ; and I have referred to this equi- 

 valence before the Physical Society and elsewhere. 



The gist of the theory is this : A magneton or 

 electron orbit, being a gyroscope, tends to take an 



NO. 2679, VOL. 107] 



orientation with the direction of its revolution co- 

 incident with that of any rotation impressed upon it. 

 Being a magnet, it also tends to set with its axis 

 parallel to an impressed magnetic intensity. Ultimate 

 coincidence in either case may be prevented by 

 extraneous forcives. But, in given circumstances,, 

 whatever the forcive towards alignment, and whatever 

 alignment of the magneton is produced by a magnetic 

 intensity, H will be produced by rotation about the 

 direction of the intensity with velocity £t=H/R, 

 where R is the ratio of the angular momentum of 

 the magneton to its magnetic moment. The general 

 idea has been applied to cosmical magnetism by 

 Schuster (1912), by Einstein and by de Haas (1915), 

 and by myself (1909 and 1915), though not with Dr.. 

 Chapman's detail. 



// all the magnetons within a body are alike, 

 rotating it at velocity Q. will produce the same mag- 

 netisation as would be produced by applying a uniform 

 magnetic field of strength H = RQ. 



For weak fields the ferro-magnetic bodies rotated all 

 receive intensities of magnetisation proportional to 

 the intensities of the fields applied, and are thus 

 magnetised by rotation proportionally to velocity. This 

 proportionality exists only for elastic displacements to 

 which Dr. Chapman refers (and to which I have 

 referred, comparing the molecular forces to those due 

 to springs). 



If the magnetons in a body are of two kinds, positive 

 and negative, with constants Ri and Rj, rotating the 

 body will have the same effect as if a magnetic 

 intensity Hi = R,Q were applied to the positive 

 magnetons and an intensity Hj^RjQ were applied 

 to the negative magnetons. If the effect on the 

 negative magnetons is preponderant, the rotation will 

 thus produce an intensity of magnetisation in the 

 direction of H,, but of magnitude less than that which 

 would be produced by the intensity RoQ, if all the 

 magnetons were negative. 



When the displacements are not elastic my theory 

 gives results analogous to those of Voigt for a swarm 

 of magnetons in an ordinary magnetic field. If there 

 are N similar magnetons per unit volume, if the 

 rotations are damped only about the axes per- 

 pendicular to the magnetic axis, and if the effects of 

 collisions and the molecular field are negligible, all 

 the magnetons, even in the weakest magnetic field 

 of strength H, will ultimately become oriented with 

 their axes in the direction of the field. In this case, 

 if C and U denote the moment of inertia and initial 

 (permanent and undamped) angular velocity about the 

 magnetic axis of a magneton, the intensity of mag- 

 netisation will be 



I-NC/R.(U-H/R). 



The first and principal term is entirely independent of 

 H. The orientation is produced by the field, but only 

 the time taken to arrive at the steady state is affected 

 by its magnitude. If collisions are not absent, or the 

 molecular field becomes appreciable, the intensity of 

 magnetisation will not reach saturation, but will in- 

 crease with the field strength, being greater for a 

 given applied field strength the greater the time 

 between collisions and the weaker the molecular and 

 demagnetising fields. 



For the same swarm of magnetons subjected to an 

 angular velocity Q instead of a magnetic field with 

 intensity H, we have, when the effects of collisions 

 and the molecular and demagnetising fields are 

 negligible, 



I = NC/R.(U+0). 



The first and only important term is independent of 

 Q. Here the orientation is produced by the velocity 



