908 Supplement to NA TURE of May 21,1938 been to help to restore them to their rightful place electrostatic field afforded experimental verification as the greatest scientific generalization we have seen. of the formula H=VJ.The two equations E= But there has been a decided tendency to neglect VB and H=VJ may therefore be regarded simply them of late,and I am convinced that this has been as direct expressions of the results of Faraday's and due to the complexity of the mathematical processes Rowland's rotating disk experiments,and my associated with Maxwell's equations, which has original derivation of them by transformation of the obscured their simple physical basis and conclusions. Maxwellian equations,which I quite admit was not Dr.Wilcken is to be congratulated on being able to general,is of no importance except as showing a make them easily intelligible to students,but quite connexion between them.The equationsE =-[VB] recently a professor of physics at a leading London and H=[V]are completely general,and I hope to college told me frankly that he had never been able show that they are more fundamental. to understand Maxwell's electromagnetic theory of As Dr.Wilcken does not like my employment of light,and I have found that considerable confusion the quadrantal versor j=√】to indicate the exists among all with whom I have discussed it.The directions of the induced forces,on the grounds that simplified treatment has given me a much clearer it leads to confusion with its use in alternating realization of the physical processes involved and has current theory and with vector analysis,there is no convinced me that Maxwell's conclusions were objection to writing the four equations for an isotropic entirely sound although his equations were not com- dielectric as J =sE,B=uH,E=VB,and H =VJ, pletely fundamental. and leaving the directions to be determined by the Dr. Wilcken commences by objecting to my ordinary rules.I fail to see,however,why students “abrupt”substitution of for Maxwell's curl who are familiar with the use of j for alternating current vectors should be confused by its application function,and suggests that it was introduced as a to any other vectors. quasimathematical device to give colour to a pre- I dissent entirely from Dr.Wilcken's remarks con- conceived bias in favour of the flux-cutting principle. cerning the flux-cutting principle,but to deal with I admit and am sorry for the abruptness,but my them and his complete misapprehension of my article was a highly condensed epitome of a long method of explaining the generation and propagation unpublished paper and was intended merely to give of an electromagnetic wave would take too much an outline of the theory in the shortest possible space. space for a note;and I propose shortly to submit a According to my recollection and notes,the inspira. further article,which I hope will make the flux- tion for the modified (apparently I must not say cutting principle and its application quite clear and "simplified'')equations arose from wrestling with the indisputable. array of differential equations in Drude's 'Theory of C.V.DRYSDALE. Optics"';and becoming tired of the effort I decided on Athenaeum Club, examining the problem of electromagnetic wave Pall Mall,S.W.1. propagation from first principles.The most simple way of doing so was to consider a forced eleetro- magnetic disturbance created by moving a magnet Why do we have Winter Heating? with wedge-shaped gap with velocity V in a direction THE layman will answer "To make the room transverse to its magnetie field B and in a plane con- The student of thermodynamics will per taining the normals to its plane faces and it was easy warmer. 8E haps so express it:"To import the lacking (inner, to see that in this case curl E= as regards magni- thermal)energy."If so,then the layman's answer is right,the scientist's wrong. dB 8B 8B da .8B dy tude and that（=流=活+za证+8ya证 We suppose,to correspond to the actual state of affairs,that the pressure of the air in a room always 8B dz Be 8d-5 so that Maxwell's third 8B da 8B equals that of the external air.In the usual notation, the (inner,thermal)energy is,per unit mass, 8B equation-curl E=B,becomes u■coT. or E=-VB,if E=0 when V =0.Similar treat- (An additive constant may be neglected.)Then the ment of Maxwell's fourth equation,curl =4D energy content is,per unit of volume 8H 8D ul CopT, of course gave sa=4xV5a,or H-4r:VD-VJ or,taking into account the equation of state, if H=0 when D=0,and J=4D.As a dis. Ple RT. appears in the integration,the equations E=-VB and H-VJ are independent of the direction of we have propagation. ul cup/R. Up to this point,I had not considered the flux- For air at atmospheric pressure, cutting principle,but the correspondence of the equationE=-VB with the dynamo designer's or w2=0-0604cal.cm-3=604Cal.m.3 flux-cutting formula of course struck me,and as I The energy content of the room is thus independent of had long ago come to the conclusion that Faraday's the temperature,solely determined by the state of the rotating disk experiment showed that the induced barometer.The whole of the energy imported by E.M.F.was due to the cutting of the magnetic fux,it the heating escapes through the pores of the walls was apparent immediately that the equation E of the room to the outside air. -VB was not a mere transformation of Maxwell's I fetch a bottle of claret from the cold cellar and third equation but an expression of an independent put it to be tempered in the warm room.It becomes experimental fact.Similarly Rowland's demonstra. warmer,but the increased energy content is not tion that a magnetie field was induced by the rotation borrowed from the air of the room but is brought in of two oppositely charged disks or the cutting of an from outside. 1938 Nature Publishing Group
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Supplement to NA T URE of May 21,1938 909 Then why do we have heating For the same same statistical results as the hard plus soft radiation reason that life on the earth needs the radiation of The following result was obtained Out of every the gun.But this does not exist on the incident 100 particles of the hard radiation,41 give no visible energy,for the latter apart from a negligible amount effect.18 tracks were found in section I,21 in II, is re-radiated,just as a man,in spite of continual 22 in III and 23 in IV.Thus the hard radiation absorption of nourishment,maintains a constant gives considerably less secondaries than the hard body-weight. Our conditions of existence require a determinate 2 degree of temperature,and for the maintenance of this there is needed not addition of energy but addition of entropy. As a student, read with advantage a small book by F. Wald entitled "The Mistress of the World and her Shadow" These meant energy and entropy. In the course of advancing knowledge the two seem to me to have exchanged places. In the huge manufactory of natural processes,the principle of en. tropy occupies the position of manager,for it dictates the manner and method of the whole business,whilst the principle of energy merely does the book- keeping, balancing credits and debits. R.EMDEN. Kempterstrasse 5, Zurich. plus soft radiation. Assuming now that the 30 Secondary Effects of the Hard and Soft Components particles without visible secondary effect in the first of Cosmic Rays experiment all belong to the hard component,we shall find,in agreement with experiments,that the Ir has been shown in a previous paperi that the soft component amounts to about 27 per cent of the major part of the electron showers produced in a lead total radiation at the earth's surface. plate are formed by a'cascade process'in accordance In order by these calculations to get a correct with the shower theory proposed by Bhabha and value of this distribution factor,we are furthermore Heitler:and Carlson and Oppenheimer3.Our experi- forced to assume that the quantities of hard particles ments have been made with an automatic cloud being transformed into soft particles in the filter chamber.Four lead plates of 3 cm.thickness are are negligible. On this assumption,we can now placed inside the chamber in a distance of 5 cm.A caleulate the statistical distribution of secondaries cloud chamber divided up in this way offers good for soft rays alone.We will find that practically all opportunity of studying the way in which the the soft rays give visible secondary effects,and as showers are generally produced. distinet from the hard rays the number of secondaries Fig.I shows a typical 'cascade process'. In later increases rapidly from one section to the other in the experiments two counters were used to detect the downwards direction.The number of secondaries passage of the particles through the cloud chamber. stated in this way seems to be in good agreement One was placed above and the other below the with the electron absorption theory of Bhabha and chamber in such a position that any ray passing Heitler. straight through both must also go through the The theory has been proposed that the hard rays chamber and the four lead plates.The intention was are electrons having an energy higher than some to study the production of secondaries when the critical energy above which the quantum absorption primaries pass the lead plates and to make statistical theory is not valid.If this be correct,we shall have investigations of their frequency in the various to assume that the iron filter will decrease the energy sections.Figs.2 and 3 show some track photographs of some of the hard particles and in this way produce made in this way.Secondary,not ionizing,light a new soft radiation component.Our experiments quanta are here seen to have produced ionizing seem,however,to show that this cannot be the case. particles near the main ray. and consequently we shall have to conclude that the A statistical examination of results,obtained with. hard particles cannot be electrons. out screening the chamber,gives the following result As to the secondary effect of the hard rays,it Out of every 100 particles (hard plus soft radiation) might be mentioned that the secondaries most fre- passing the chamber,30 were without visible quently form a branch of the main rays,as shown secondary effect.26 tracks were found in section I, in Fig.4.Secondaries-electron pairs and single 35 in II,44 in III,and 45 in IV,the sections being rays-among which are found strongly ionizing numbered from above.The cloud chamber was then nuclear particles,may,however,also appear at some screened with an iron filter of 33 em.in order to distance from the main ray according to the effects absorb the soft radiation.In this way it was possible shown in Figs.5 and 6.Such cases show that the to ascertain whether the hard radiation gave the hard radiation also may be accompanied occasionally 1938 Nature Publishing Group
© 1938 Nature Publishing Group