592 



NATURE 



{Oct. 1 8, 1888 



having its wheels thoroughly geared up both with them- 

 selves and with those of the initial medium, so that there 

 is no slip or dissipation of energy at the surface. In this 

 case none of the radiation will be lost : some will be re- 

 flected and some transmitted according to ordinary and 

 well-known mechanical laws. The part transmitted will 

 suddenly begin to travel at a slower pace, and hence if the 

 incidence were oblique would pursue a somewhat different 

 path. Also, at the edges of the obstacle, or at the boundary 

 of any artificially limited portion of the wave, there will 

 be certain effects due to spreading out and encroaching 

 on parts of the medium not lying in the direct path. 

 These refraction and diffraction effects are common to all 

 possible kinds of wave propagation, and there is nothing 

 specially necessary to be said concerning electrical radia- 

 tion on these heads which is not to be found in any work 

 on the corresponding parts of optics. 



Concerning the amount and direction of the reflected 

 vibrations there is something to be said however, and 

 that something very important ; but it is no easy subject 

 to tackle, and I fear must be left, so far as I am concerned, 

 as a distinct, but perhaps subsequently-to-be filled-up, 

 gap. 



If the gearing between the new medium and the old 

 is imperfect, if, for instance, there were a layer of slip- 

 pery wheels between them, representing a more or less 

 conducting film, then some of the radiation would be 

 dissipated at the surface, not all would be reflected and 

 transmitted, and the film would get to a certain extent 

 heated. By such a film the precise laws of reflection 

 might be profoundly modified, as they would be also if the 

 transition from one medium to another were gradual in- 

 stead of abrupt. But all these things must remain for the 

 present part of the unfilled gap. 



Electric Radiation encounteri?ig a Conductor. 



We will proceed now to the case of a conducting 

 obstacle — that is, of waves encountering a medium whose 

 electrical parts are connected, not by elasticity, but by 

 friction. It is plain here that not only at the outer layer 

 of such a medium, but at every subsequent layer, a cer- 

 tain amount of slip will occur during every era of accelera- 

 tion, and hence that in penetrating a sufficient thickness 

 of a medium endowed with any metallic conductivity 

 the whole of the incident radiation must be either reflected 

 or destroyed : none can be transmitted. 



Refer back to Fig. 43 (vol. xxxvii. p. 347), and think of 

 the rack in that figure as oscillating. Through the cog- 

 wheels the disturbance spreads without loss, but at the 

 outer layer of the conducting region A B c D a finite slip 

 occurs, and a less amount of radiation penetrates to the 

 next layer, efgh, and so on. Some thickness or other, 

 therefore, of a conducting substance must necessarily 

 be impervious to electric radiation : that is, it must be 

 opaque. 



Conductivity is not the sole cause of opacity. It would 

 not do to say that all opaque bodies must be conductors. 

 But conductivity is a very efficient cause of opacity, and 

 it is true to say that all conductors of electricity are 

 necessarily opaque to light ; understanding, of course, 

 that the particular thickness of any homogeneous sub- 

 stance which can be considered as perfectly opaque must 

 depend on its conductivity. It is a question of dissipation, 

 and a minute but specifiable fraction of an original dis- 

 turbance may be said to get through any obstacle. 

 Practically, however, it is well known that a thin, though 

 not the thinnest, film of metal is quite impervious to light. 



When one says that conductivity is not the sole 

 cause of opacity, one is thinking of opacity caused by 

 heterogeneity. A confused mass of perfectly trans- 

 parent substance may be quite opaque ; witness foam, 

 powdered glass, chalk, &c. 



Hence, though a transparent body must indeed be an 

 insulator, the converse is not necessarily true. An insulator 



need not necessarily be transparent. A homogeneous 

 flawless insulator must, however, be transparent, just as 

 a homogeneous and flawless opaque body must be a 

 conductor. 



These, then, are the simple connections between two 

 such apparently distinct things as conducting power for 

 electricity and opacity to light which Maxwell's theory 

 points out ; and it is possible to calculate the theoretical 

 opacity of any given simply- constructed substance by 

 knowing its specific electric conductivity. 



Fate of the Radiation. 



To understand what happens to radiation impinging 

 on a conducting body it is most simple to proceed to the 

 limiting case at once and consider a perfect conductor. 

 In the case of a perfect conductor the wheels are 

 connected not even by friction ; they are not connected at 

 all. Consequently the slip at the boundary of such a 

 conductor is perfect, and there is no dissipation of 

 energy accompanying it. The blank space in Fig. 38 

 (vol. xxxvii. p. 345), represented a perfectly conduct- 

 ing layer. Ethereal vibrations impinging on a perfect 

 conductor practically arrive at an outer confine of their 

 medium : beyond there is nothing capable of trans- 

 mitting them ; the outer wheels receive an impetus 

 which they cannot get rid of in front, and which they 

 therefore return back the way it came to those behind 

 them with a reversal of phase : the radiation is totally 

 reflected. It is like what happens when a sound-pulse 

 reaches the open end of an organ-pipe ; like what happens 

 when sound tries to go from water to air ; like the last of 

 a row of connected balls along which a knock has been 

 transmitted ; and our massive elastic wheels are able to 

 represent the reversal of phase and reflection quite 

 properly. 



The reflected pulses will be superposed upon and 

 interfere with the direct pulses, and accordingly if the 

 distances are properly adjusted we can have the familiar 

 formation of fixed nodes and stationary waves. 



The point of main interest, however, is to notice that a 

 perfect conductor of electricity, if there were such a thing, 

 would be utterly impervious to light : no light could 

 penetrate its outer skin, it would all be reflected back : the 

 substance would be a perfect reflector for ethereal waves of 

 every size. 



Thus with a perfect conductor, as with a perfect non- 

 conductor, there is no dissipation. Radiation impinging on 

 them is either all refracted or some reflected and some 

 transmitted. It is the cases of intermediate conductivity 

 which destroy some of the radiation and convert its 

 ethereal vibrations into atomic vibrations, i.e. which 

 convert it into heat. 



The mode in which radiation or any other electrical 

 disturbance diffuses with continual loss through an im- 

 perfect conductor can easily be appreciated by referring 

 to Fig. 43 again. The successive lines of slip, A B c D, 

 efgh, &c, are successive layers of induced currents. 

 An electromotive impulse loses itself in the production of 

 these currents, which are successively formed deeper and 

 deeper in the material according to laws of diffusion. 



If the waves had impinged on one face of a slab, a 

 certain fraction of them would emerge from the other face — 

 a fraction depending on the thickness of the slab accord- 

 ing to a logarithmic or geometrical-progression law of 

 decrease. Oliver J. Lodge. 



{To be continued?) 



PRESENT POSITION OF THE MANUFAC- 

 TURE OF ALUMINIUM. 

 THE recent opening of new works for the manufacture 

 of aluminium at Oldbury, near Birmingham, is 

 distinctly an epoch in the history of this interesting 

 metal. 



