The idea of attributing the phenomena of electricity to perturbations produced in the medium which transmits the light is already of old standing; and the physicists who witnessed the triumph of Fresnel"s theories could not fail to conceive that this fluid, which fills the whole of s.p.a.ce and penetrates into all bodies, might also play a preponderant part in electrical actions. Some even formed too hasty hypotheses on this point; for the hour had not arrived when it was possible to place them on a sufficiently sound basis, and the known facts were not numerous enough to give the necessary precision.
The founders of modern electricity also thought it wiser to adopt, with regard to this science, the att.i.tude taken by Newton in connection with gravitation: "In the first place to observe facts, to vary the circ.u.mstances of these as much as possible, to accompany this first work by precise measurements in order to deduce from them general laws founded solely on experiment, and to deduce from these laws, independently of all hypotheses on the nature of the forces producing the phenomena, the mathematical value of these forces--that is to say, the formula representing them. Such was the system pursued by Newton. It has, in general, been adopted in France by the scholars to whom physics owe the great progress made of late years, and it has served as my guide in all my researches on electrodynamic phenomena.... It is for this reason that I have avoided speaking of the ideas I may have on the nature of the cause of the force emanating from voltaic conductors."
Thus did Ampere express himself. The ill.u.s.trious physicist rightly considered the results obtained by him through following this wise method as worthy of comparison with the laws of attraction; but he knew that when this first halting-place was reached there was still further to go, and that the evolution of ideas must necessarily continue.
"With whatever physical cause," he adds, "we may wish to connect the phenomena produced by electro-dynamic action, the formula I have obtained will always remain the expression of the facts," and he explicitly indicated that if one could succeed in deducing his formula from the consideration of the vibrations of a fluid distributed through s.p.a.ce, an enormous step would have been taken in this department of physics. He added, however, that this research appeared to him premature, and would change nothing in the results of his work, since, to accord with facts, the hypothesis adopted would always have to agree with the formula which exactly represents them.
It is not devoid of interest to observe that Ampere himself, notwithstanding his caution, really formed some hypotheses, and recognized that electrical phenomena were governed by the laws of mechanics. Yet the principles of Newton then appeared to be unshakable.
Faraday was the first to demonstrate, by clear experiment, the influence of the media in electricity and magnetic phenomena, and he attributed this influence to certain modifications in the ether which these media enclose. His fundamental conception was to reject action at a distance, and to localize in the ether the energy whose evolution is the cause of the actions manifested, as, for example, in the discharge of a condenser.
Consider the barrel of a pump placed in a vacuum and closed by a piston at each end, and let us introduce between these a certain ma.s.s of air. The two pistons, through the elastic force of the gas, repel each other with a force which, according to the law of Mariotte, varies in inverse ratio to the distance. The method favoured by Ampere would first of all allow this law of repulsion between the two pistons to be discovered, even if the existence of a gas enclosed in the barrel of the pump were unsuspected; and it would then be natural to localize the potential energy of the system on the surface of the two pistons. But if the phenomenon is more carefully examined, we shall discover the presence of the air, and we shall understand that every part of the volume of this air could, if it were drawn off into a recipient of equal volume, carry away with it a fraction of the energy of the system, and that consequently this energy belongs really to the air and not to the pistons, which are there solely for the purpose of enabling this energy to manifest its existence.
Faraday made, in some sort, an equivalent discovery when he perceived that the electrical energy belongs, not to the coatings of the condenser, but to the dielectric which separates them. His audacious views revealed to him a new world, but to explore this world a surer and more patient method was needed.
Maxwell succeeded in stating with precision certain points of Faraday"s ideas, and he gave them the mathematical form which, often wrongly, impresses physicists, but which when it exactly encloses a theory, is a certain proof that this theory is at least coherent and logical.[23]
[Footnote 23: It will no doubt be a shock to those whom Professor Henry Armstrong has lately called the "mathematically-minded" to find a member of the Poincare family speaking disrespectfully of the science they have done so much to ill.u.s.trate. One may perhaps compare the expression in the text with M. Henri Poincare"s remark in his last allocution to the Academie des Sciences, that "Mathematics are sometimes a nuisance, and even a danger, when they induce us to affirm more than we know" (_Comptes-rendus_, 17th December 1906).]
The work of Maxwell is over-elaborated, complex, difficult to read, and often ill-understood, even at the present day. Maxwell is more concerned in discovering whether it is possible to give an explanation of electrical and magnetic phenomena which shall be founded on the mechanical properties of a single medium, than in stating this explanation in precise terms. He is aware that if we could succeed in constructing such an interpretation, it would be easy to propose an infinity of others, entirely equivalent from the point of view of the experimentally verifiable consequences; and his especial ambition is therefore to extract from the premises a general view, and to place in evidence something which would remain the common property of all the theories.
He succeeded in showing that if the electrostatic energy of an electromagnetic field be considered to represent potential energy, and its electrodynamic the kinetic energy, it becomes possible to satisfy both the principle of least action and that of the conservation of energy; from that moment--if we eliminate a few difficulties which exist regarding the stability of the solutions--the possibility of finding mechanical explanations of electromagnetic phenomena must be considered as demonstrated. He thus succeeded, moreover, in stating precisely the notion of two electric and magnetic fields which are produced in all points of s.p.a.ce, and which are strictly inter-connected, since the variation of the one immediately and compulsorily gives birth to the other.
From this hypothesis he deduced that, in the medium where this energy is localized, an electromagnetic wave is propagated with a velocity equal to the relation of the units of electric ma.s.s in the electromagnetic and electrostatic systems. Now, experiments made known since his time have proved that this relation is numerically equal to the speed of light, and the more precise experiments made in consequence--among which should be cited the particularly careful ones of M. Max Abraham--have only rendered the coincidence still more complete.
It is natural henceforth to suppose that this medium is identical with the luminous ether, and that a luminous wave is an electromagnetic wave--that is to say, a succession of alternating currents, which exist in the dielectric and even in the void, and possess an enormous frequency, inasmuch as they change their direction thousands of billions of times per second, and by reason of this frequency produce considerable induction effects. Maxwell did not admit the existence of open currents. To his mind, therefore, an electrical vibration could not produce condensations of electricity. It was, in consequence, necessarily transverse, and thus coincided with the vibration of Fresnel; while the corresponding magnetic vibration was perpendicular to it, and would coincide with the luminous vibration of Neumann.
Maxwell"s theory thus establishes a close correlation between the phenomena of the luminous and those of the electromagnetic waves, or, we might even say, the complete ident.i.ty of the two. But it does not follow from this that we ought to regard the variation of an electric field produced at some one point as necessarily consisting of a real displacement of the ether round that point. The idea of thus bringing electrical phenomena back to the mechanics of the ether is not, then, forced upon us, and the contrary idea even seems more probable. It is not the optics of Fresnel which absorbs the science of electricity, it is rather the optics which is swallowed up by a more general theory.
The attempts of popularizers who endeavour to represent, in all their details, the mechanism of the electric phenomena, thus appear vain enough, and even puerile. It is useless to find out to what material body the ether may be compared, if we content ourselves with seeing in it a medium of which, at every point, two vectors define the properties.
For a long time, therefore, we could remark that the theory of Fresnel simply supposed a medium in which something periodical was propagated, without its being necessary to admit this something to be a movement; but we had to wait not only for Maxwell, but also for Hertz, before this idea a.s.sumed a really scientific shape. Hertz insisted on the fact that the six equations of the electric field permit all the phenomena to be antic.i.p.ated without its being necessary to construct one hypothesis or another, and he put these equations into a very symmetrical form, which brings completely in evidence the perfect reciprocity between electrical and magnetic actions. He did yet more, for he brought to the ideas of Maxwell the most striking confirmation by his memorable researches on electric oscillations.
-- 4. ELECTRICAL OSCILLATIONS
The experiments of Hertz are well known. We know how the Bonn physicist developed, by means of oscillating electric discharges, displacement currents and induction effects in the whole of the s.p.a.ce round the spark-gap; and how he excited by induction at some point in a wire a perturbation which afterwards is propagated along the wire, and how a resonator enabled him to detect the effect produced.
The most important point made evident by the observation of interference phenomena and subsequently verified directly by M.
Blondlot, is that the electromagnetic perturbation is propagated with the speed of light, and this result condemns for ever all the hypotheses which fail to attribute any part to the intervening media in the propagation of an induction phenomenon.
If the inducing action were, in fact, to operate directly between the inducing and the induced circuits, the propagation should be instantaneous; for if an interval were to occur between the moment when the cause acted and the one when the effect was produced, during this interval there would no longer be anything anywhere, since the intervening medium does not come into play, and the phenomenon would then disappear.
Leaving on one side the manifold but purely electrical consequences of this and the numerous researches relating to the production or to the properties of the waves--some of which, those of MM. Sarrazin and de la Rive, Righi, Turpain, Lebedeff, Decombe, Barbillon, Drude, Gutton, Lamotte, Lecher, etc., are, however, of the highest order--I shall only mention here the studies more particularly directed to the establishment of the ident.i.ty of the electromagnetic and the luminous waves.
The only differences which subsist are necessarily those due to the considerable discrepancy which exists between the durations of the periods of these two categories of waves. The length of wave corresponding to the first spark-gap of Hertz was about 6 metres, and the longest waves perceptible by the retina are 7/10 of a micron.[24]
[Footnote 24: See footnote 3.]
These radiations are so far apart that it is not astonishing that their properties have not a perfect similitude. Thus phenomena like those of diffraction, which are negligible in the ordinary conditions under which light is observed, may here a.s.sume a preponderating importance. To play the part, for example, with the Hertzian waves, which a mirror 1 millimetre square plays with regard to light, would require a colossal mirror which would attain the size of a myriametre[25] square.
[Footnote 25: I.e., 10,000 metres.--ED.]
The efforts of physicists have to-day, however, filled up, in great part, this interval, and from both banks at once they have laboured to build a bridge between the two domains. We have seen how Rubens showed us calorific rays 60 metres long; on the other hand, MM. Lecher, Bose, and Lampa have succeeded, one after the other, in gradually obtaining oscillations with shorter and shorter periods. There have been produced, and are now being studied, electromagnetic waves of four millimetres; and the gap subsisting in the spectrum between the rays left undetected by sylvine and the radiations of M. Lampa now hardly comprise more than five octaves--that is to say, an interval perceptibly equal to that which separates the rays observed by M.
Rubens from the last which are evident to the eye.
The a.n.a.logy then becomes quite close, and in the remaining rays the properties, so to speak, characteristic of the Hertzian waves, begin to appear. For these waves, as we have seen, the most transparent bodies are the most perfect electrical insulators; while bodies still slightly conducting are entirely opaque. The index of refraction of these substances tends in the case of great wave-lengths to become, as the theory antic.i.p.ates, nearly the square root of the dielectric constant.
MM. Rubens and Nichols have even produced with the waves which remain phenomena of electric resonance quite similar to those which an Italian scholar, M. Garba.s.so, obtained with electric waves. This physicist showed that, if the electric waves are made to impinge on a flat wooden stand, on which are a series of resonators parallel to each other and uniformly arranged, these waves are hardly reflected save in the case where the resonators have the same period as the spark-gap. If the remaining rays are allowed to fall on a gla.s.s plate silvered and divided by a diamond fixed on a dividing machine into small rectangles of equal dimensions, there will be observed variations in the reflecting power according to the orientation of the rectangles, under conditions entirely comparable with the experiment of Garba.s.so.
In order that the phenomenon be produced it is necessary that the remaining waves should be previously polarized. This is because, in fact, the mechanism employed to produce the electric oscillations evidently gives out vibrations which occur on a single plane and are subsequently polarized.
We cannot therefore entirely a.s.similate a radiation proceeding from a spark-gap to a ray of natural light. For the synthesis of light to be realized, still other conditions must be complied with. During a luminous impression, the direction and the phase change millions of times in the vibration sensible to the retina, yet the damping of this vibration is very slow. With the Hertzian oscillations all these conditions are changed--the damping is very rapid but the direction remains invariable.
Every time, however, that we deal with general phenomena which are independent of these special conditions, the parallelism is perfect; and with the waves, we have put in evidence the reflexion, refraction, total reflexion, double reflexion, rotatory polarization, dispersion, and the ordinary interferences produced by rays travelling in the same direction and crossing each other at a very acute angle, or the interferences a.n.a.logous to those which Wiener observed with rays of the contrary direction.
A very important consequence of the electromagnetic theory foreseen by Maxwell is that the luminous waves which fall on a surface must exercise on this surface a pressure equal to the radiant energy which exists in the unit of volume of the surrounding s.p.a.ce. M. Lebedeff a few years ago allowed a sheaf of rays from an arc lamp to fall on a deflection radiometer,[26] and thus succeeded in revealing the existence of this pressure. Its value is sufficient, in the case of matter of little density and finely divided, to reduce and even change into repulsion the attractive action exercised on bodies by the sun.
This is a fact formerly conjectured by Faye, and must certainly play a great part in the deformation of the heads of comets.
[Footnote 26: By this M. Poincare appears to mean a radiometer in which the vanes are not entirely free to move as in the radiometer of Crookes but are suspended by one or two threads as in the instrument devised by Professor Poynting.--ED.]
More recently, MM. Nichols and Hull have undertaken experiments on this point. They have measured not only the pressure, but also the energy of the radiation by means of a special bolometer. They have thus arrived at numerical verifications which are entirely in conformity with the calculations of Maxwell.
The existence of these pressures may be otherwise foreseen even apart from the electromagnetic theory, by adding to the theory of undulations the principles of thermodynamics. Bartoli, and more recently Dr Larmor, have shown, in fact, that if these pressures did not exist, it would be possible, without any other phenomenon, to pa.s.s heat from a cold into a warm body, and thus transgress the principle of Carnot.
-- 5. THE X RAYS
It appears to-day quite probable that the X rays should be cla.s.sed among the phenomena which have their seat in the luminous ether.
Doubtless it is not necessary to recall here how, in December 1895, Rontgen, having wrapped in black paper a Crookes tube in action, observed that a fluorescent platinocyanide of barium screen placed in the neighbourhood, had become visible in the dark, and that a photographic plate had received an impress. The rays which come from the tube, in conditions now well known, are not deviated by a magnet, and, as M. Curie and M. Sagnac have conclusively shown, they carry no electric charge. They are subject to neither reflection nor refraction, and very precise and very ingenious measurements by M.
Gouy have shown that, in their case, the refraction index of the various bodies cannot be more than a millionth removed from unity.
We knew from the outset that there existed various X rays differing from each other as, for instance, the colours of the spectrum, and these are distinguished from each other by their unequal power of pa.s.sing through substances. M. Sagnac, particularly, has shown that there can be obtained a gradually decreasing scale of more or less absorbable rays, so that the greater part of their photographic action is stopped by a simple sheet of black paper. These rays figure among the secondary rays discovered, as is known, by this ingenious physicist. The X rays falling on matter are thus subjected to transformations which may be compared to those which the phenomena of luminescence produce on the ultra-violet rays.
M. Benoist has founded on the transparency of matter to the rays a sure and practical method of allowing them to be distinguished, and has thus been enabled to define a specific character a.n.a.logous to the colour of the rays of light. It is probable also that the different rays do not transport individually the same quant.i.ty of energy. We have not yet obtained on this point precise results, but it is roughly known, since the experiments of MM. Rutherford and M"Clung, what quant.i.ty of energy corresponds to a pencil of X rays. These physicists have found that this quant.i.ty would be, on an average, five hundred times larger than that brought by an a.n.a.logous pencil of solar light to the surface of the earth. What is the nature of this energy? The question does not appear to have been yet solved.
It certainly appears, according to Professors Haga and Wind and to Professor Sommerfeld, that with the X rays curious experiments of diffraction may be produced. Dr Barkla has shown also that they can manifest true polarization. The secondary rays emitted by a metallic surface when struck by X rays vary, in fact, in intensity when the position of the plane of incidence round the primary pencil is changed. Various physicists have endeavoured to measure the speed of propagation, but it seems more and more probable that it is very nearly that of light.[27]
[Footnote 27: See especially the experiments of Professor E. Marx (Vienna), _Annalen der Physik_, vol. xx. (No. 9 of 1906), pp. 677 _et seq._, which seem conclusive on this point.--ED.]
I must here leave out the description of a crowd of other experiments.
Some very interesting researches by M. Brunhes, M. Broca, M.
Colardeau, M. Villard, in France, and by many others abroad, have permitted the elucidation of several interesting problems relative to the duration of the emission or to the best disposition to be adopted for the production of the rays. The only point which will detain us is the important question as to the nature of the X rays themselves; the properties which have just been brought to mind are those which appear essential and which every theory must reckon with.
The most natural hypothesis would be to consider the rays as ultra-violet radiations of very short wave-length, or radiations which are in a manner ultra-ultra-violet. This interpretation can still, at this present moment, be maintained, and the researches of MM. Buisson, Righi, Lenard, and Merrit Stewart have even established that rays of very short wave-lengths produce on metallic conductors, from the point of view of electrical phenomena, effects quite a.n.a.logous to those of the X rays. Another resemblance results also from the experiments by which M. Perreau established that these rays act on the electric resistance of selenium. New and valuable arguments have thus added force to those who incline towards a theory which has the merit of bringing a new phenomenon within the pale of phenomena previously known.
Nevertheless the shortest ultra-violet radiations, such as those of M.
Schumann, are still capable of refraction by quartz, and this difference const.i.tutes, in the minds of many physicists, a serious enough reason to decide them to reject the more simple hypothesis.
Moreover, the rays of Schumann are, as we have seen, extraordinarily absorbable,--so much so that they have to be observed in a vacuum. The most striking property of the X rays is, on the contrary, the facility with which they pa.s.s through obstacles, and it is impossible not to attach considerable importance to such a difference.