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© 1938 Nature Publishing Group