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from ships at sea to stations on land, at Poldhu each in his own way genand "wireless telegraphy" has become erates waves in that "ether” which, as a matter of such general importance we believe, permeates every speck of that as I sit down to write this article matter and fills every nook and cranny representatives of the nations are as- of the universe, and the success of the sembled in Berlin to consider and, if signal in the one case as in the other possible, to regulate the various politi- depends upon those waves falling upon cal and commercial interests involved a suitable receiver, the human eye or in the new art. But in 1902 the thing some substitute for the eye, at the end was unique. How was it done? Who of their journey through space. And made it possible? And what is the yet there is this difference between the physical basis of this newest invention light waves produced by the savage and built up with such mushroom-like ra- the electric waves generated at Poldhu. pidity by modern physicists? One The latter, to put it very broadly, for thing seems clear. The man who there is a big gap, may be said to begin "pressed the button" in 1902 was Mr. where the former cease. For, while Marconi. But who set him to work? light waves are so small that many Who started the idea ? And what thousands of them can be packed within equipment of data did the pathfinders the compass of a single inch, electric provide for their successors? Was it waves are so big that they may be feet, Faraday, working, for the sake of quiet, miles, or even thousands of miles in first in a cellar at the Royal Institution, length. In all essential qualities, howand later at the Shot Tower by Water- ever, except in size, light waves and loo Bridge? Was it James Clerk Max- electrical waves, so far as we know at well, the originator of the famous sys- present, are identical. The human ese tem of equations known as “Maxwell's is responsive to the small waves, but Theory," or was it Hertz, with his phil- not to the big waves. That is why the osopbical experiments" and their epoch- big waves were not recognized until a naking results, who gave the impulse? special instrument had been constructed Was it to one or all of these great pio for the purpose. neers that we owe the marvels of wire. The first electrician to construct an less telegraphy, and what was the na- instrument which would detect electric ture of their contributions to the sub- waves, and the first to recognize an ject? Here we have problems enough electric wave, was Heinrich Hertz. to demand a whole number of the His account of his experiments was Cornhill Vagazine for their solution. done into English a few years ago by What can we do with them in a single Mr. D. E. Jones, and published under article?
the title of "Electric Waves." It will simplify our task a good deal The questions asked on the first page if I say at once that, looking at the mat. of this article now resolve themselves ter from the physical point of view and into two which are comparatively in the simplest way, there is no essen- straightforward. How were electric tial difference between the flickers of waves discovered and identified with light used as signals by a savage tribes- light waves? How have they been apman when he waves a beacon to warn plied to "Hertzian wave telegraphy" by bis friends a few miles away of the ap- Hertz's successors? Before we can proach of danger, and the invisible sig gain answers to these two questions, nals sent over the ocean from the sta- simple as they seem, it will be necestion at Poldhu. The savage with his sary to go over some old ground, and torch and the highly trained electrician recall for a moment some of the fea
tures of the wave theory of light. If "reflected," "refracted,” and polarized' we do not do this, much that follows like ordinary light. Radiations like the will seem unconvincing and vague, ex- corpuscles of radium, which cannot be cept to those who already are familiar reflected, refracted, and polarized do with the undulatory theory.
not, in this sense constitute light, Light, as we all know, travels through though they may generate light when space in straight lines with a velocity they enter the eye. in air of about 186,000 miles per sec- If we could transport ourselves to the ond. When a ray of light passing days of Newton, and listen to the disthrough the air or any other gas im- cussions of the philosophers of the pinges on a solid object, such as a sheet seventeenth and eighteenth centuries, of polished silver or glass, it may re- we should find one of the burning quesbound, or be "reflected"; or it may pass tions to be this-Can matter act where through the solid partly or wholly, ac- it is not? Is action at a distance cording to circumstances, this being through a perfect void possible or imwhat occurs when the solid is trans possible? To Newton the idea that parent like glass or a diamond. In the gravity might be innate, inherent and latter case, as the ray enters the solid essential to matter, so that one body it is diverted from its original course, might attract another at a distance or "refracted," I at the surface of the through a vacuum without the mediasolid, and again diverted, but in the tion of anything else, was an absurdity opposite sense, when it subsequently into which no man having a competent emerges from the denser and re-enters faculty of thinking in philosophical matthe rarer medium, the air. We all ters could possibly fall. To the thinkknow, also, that ordinary white lighters of the later part of the eighteenth is not homogeneous, but can be resolved century, when the influence of Bosinto several components by means of a covich predominated, on the other hand, triangular glass prism, as Newton the notion that gravity or electric or. taught us in the seventeenth century. magnetic attraction might be propaIt is important to remember, further, gated by a medium seemed as wild and that since Newton's time it has been ridiculous as the idea that matter could discovered that all light is not visible act where it is not appeared to Newton to the human eye; that at our best we a hundred years before. To-day the are but purblind creatures, and that be wheel has turned again, and, guided sides the limited field of light corres- by Thomas Young, Fresnel, Faraday, sponding to the colored band known as Clerk Maxwell, and latest of all by the visible spectrum there are other Hertz, we again seek the aid of an luminous radiations to which the hu- "ether" to account for the propagation man retina does not respond. This in- of light, and to provide a medium visible light has been detected at both through which and by which forces of ends of the spectrum, some beyond the attraction or repulsion seemingly actvisible rays at the violet end of the ing at a distance are transmitted across spectrum, and some beyond the visible space. part at the red end. Thus to the phys. When a beam of light falls perpendicularly icist of the twentieth century the upon a plate of tourmaline cut parallel to the
axis of the crystal, only part of the incident term "light" does not apply only to the
light passes through the tourmaline, and the light we see, but includes other rays properties of the transmitted rays lead us to which, though invisible to us, can be
suppose that in these all the vibrations are
executed in one plane, and transversely to 1 Unless the ray falls perpendicularly upon the direction of the beam. Such light is said the solid.
to be “polarized."
If we abandon the emission theory of Kelvin goes so far as to say that “up Newton, which teaches us that every to the present time we have no light, self-luminous body emits minute ma- even so much as to point a way for interial particles which cause the sensa- vestigation in that direction"; but in the tion of light when they fall upon the case of electric and magnetic phenomretina, and adopt in its place the mod- ena the new physics has been more sueern view that light and radiant heat cessful. consist of waves, it seems to follow that The wave theory of Young and Fresthese waves must be waves of some nel was scarcely established before thing or waves in something. This Faraday observed that a strong magnet something we call “the ether," and what exercises a peculiar action on polarized we know about radiant light and heat light, and proposed, in 1846, as a subject assures us that this ether must not only of speculation, an "electromagnetic fill all space and permeate every speck theory of light.” This theory was deof matter, but must be very different veloped twenty years later by Clerk from anything we are acquainted with Maxwell, who found the "elasticity" of at present. It cannot be solid like a the magnetic medium in air to be so stone, nor liquid like water, nor can nearly identical with that of the lumiit be a gas, for the most perfectly ex- nous ether as to leave little room for hausted vessel can transmit light, and doubt that "these two co-existent, cotherefore must be full of ether; and extensive, and equally elastic media are while the ether must be far less dense really one medium, viz. the ether of the than any known gas, and allow things undulatory theory of light"; and before to move freely through it, yet it must many years had elapsed it was held possess some quality closely akin to the generally by the younger English physrigidity of steel. What it is we do not icists that electrical disturbances are know. We assume its existence and transmitted by means of the ether, and deduce its properties from what we that electric vibrations do not differ esknow about radiant light and heat, and sentially from light waves. In 1883, at about the waves generated by the os. a meeting of the British Association, cillating electric charges of the Leyden the late Professor G. F. Fitzgerald carjar and similar electrical contrivances ried the matter a step further by profor producing flashes of artificial light posing a method of producing electroning. Without an ether, a wave theory magnetic disturbances of comparatively of light would seem an absurdity. For short wave-length by utilizing “the alif light consists of waves, and if the in- ternating current produced when an acterstellar space be a mere void, what cumulator or storage battery is disbecomes of a ray of light emitted by charged through a small resistance," the sun on its journey to the earth dur- and that is how matters stood when ing the period of about eight minutes Hertz turned his attention to the subwhen it is neither on the sun nor on ject early in the year 1886. the earth? Is it not evident that the Probably each of us has seen at some wave theory of light imperatively as time the mimic lightning of a Leyden serts the existence of an ether, and re- jar. If so, two things will be rememopens the great question settled in one bered. First, that at the moment of way by Newton, and in the opposite discharge there was a blinding flash way by his successors in the eighteenth between the two discharging spheres of century? Up to to-day nothing has the apparatus and that this was acbeen done to settle this vexed question companied by a sharp crash or crack. as applied to gravity. Indeed, Lord Secondly, that both the flash and the crash were over in a fraction of a sec- disturbances or waves, and thus radiond. If the experimenter was asked ate energy * into the surrounding ether. to explain this mimic lightning, proba- For a long time, however, it was imbly he said it was due to the flowing possible to prove the existence of these together of two charges of electricity electric waves, because, as I have alpreviously communicated to two metal- ready said, they are quite invisible to lic sheets fixed respectively on the in- the human eye. ner and outer surfaces of the jar, and no Dr. Joseph Henry seems to have doubt this explanation was sufficient for come very near to the discovery of elecits immediate purpose. But it was tric waves about sixty years ago, when, very far from telling the whole story. after describing how it had been found For what the observer saw was not, as possible to magnetize steel needles by he may have supposed, the result of a means of a single spark from the consingle rush of electricity, but was the ductor of an electric machine, though outcome of a series of rushes backwards the needles were thirty yards away and and forwards between the two dis- separated from the spark by two floors charging spheres, which followed one and two ceilings, he went on to say: “It another at a rate that may have been may be inferred that the diffusion of as small as ten thousand, or as great as motion in this experiment is almost ten million, or even a hundred million, comparable with that from a spark in a single second of time. The cor- from a fint and steel in the case of rectness of this description of the dis- light.” But he missed the opportunity, charge of a Leyden jar has been estab- and it was left to Hertz to make the lished by examining the reflections of actual discovery forty years later. similar but less rapidly oscillating The Leyden jar in its ordinary form sparks in revolving mirrors, when was not very suitable for the experithere is seen in the reflection of each ments designed by Hertz, and to obspark, not a continuous band of light, tain his oscillating discharges he enbut definite fluctuations such as would ployed a somewhat different arrangecorrespond to a succession of separate ment of apparatus. You could condischarges.
struct a model of this apparatus We all know that waves are gen- from very simple materials. All erated in still water when it is dis- you would have to do would be to turbed by the shock of a falling stone; obtain a few yards of stout copper that sound-waves can be started in the wire well coated on its surface with air by the vibrations of a tuning-fork; paraffin wax or some other insulatwe believe that luminous waves, or ing material, to wind this wire round a light waves, are started in the ether by large wooden reel, keeping its ends the shock offlint hitting upon hard free, and then to dip the whole in steel and, similarly, electricians, guided melted paraffin to complete the insulaby Faraday, Clerk Maxwell and Fitz- tion of the copper wire. Next you gerald, for some time have believed would have to obtain a much greater that the electric displacements which length of much finer copper wire, also take place during the violent oscilla- well insulated, to wind this round the tions that constitute the discharges of a reel outside the inner coil of thick wire, Leyden jar generate electromagnetic and attach its two ends to an arrangement like that drawn below, passing in the same direction, but of in which A A' represent two metallic series of violent oscillations in wbich plates about sixteen inches square and electricity would jump to and fro B B two stout metallic wires each car across the spark gap, as already derying a well-polished sphere cc. scribed, at a rate corresponding to milFinally, you would have to join the lions of jumps per second, the exact two ends of the stout inner wire of rate depending on the details of the your coil to a small galvanic battery construction of the coil and oscillator provided with an arrangement by employed. Also this discharge, like which the current from the battery the Leyden jar discharge, would, accould be sent through the wire or becording to modern theory, generate cut off from it smartly at short in- waves in the ether which would tervals. Then you would have a travel with the velocity of light, and rough model of an "induction coil" would be recognized by our eyes it with a Hertz oscillator.
3 Sir Oliver Lodge has shown that by using a "A part of this energy at least never returns, large battery of jars the oscillations may be and this, no doubt, is one reason why the disreduced to 5,000 or even to 500 per second charge dies out after a life which only lasts when it becomes comparatively easy to detect for a fraction of a second. them with a mirror.
only these were sensitive to large waves like electric waves.
The "electric eye" or "resonator" by which Hertz succeeded in showing the existence of electric waves was very
simple. It consisted of a piece of wire I fear that home-made apparatus in bent into a ring and provided at its the simple form described above would ends with two polished metallic not enable you to reproduce the re- spheres, whose distance from each sults obtained by Hertz. But if the other could be adjusted very accurately home-made coil were replaced by a by a fine screw provided for the pursimilar coil constructed by Mr. Apps, pose. The dimensions of this aplet us say, and if this were joined up paratus were varied, of course, acto an oscillator such as that which I cording to those of the generator whose have described, you would be in a posi- waves it was to detect. When this tion to attempt to repeat these wonder- simple instrument was held horizonful experiments. The process would tally in a proper position relatively to be quite simple. You would have to that of the "oscillator," and not too connect a galvanic battery to the coil, far off, the waves radiated by the to connect the terminals of the thin, former, as they reached the detector, secondary wire of the coil to the os- set up secondary oscillations in the cillator, to start the automatic arrange- latter, and these soon made their exment for making and breaking the istence manifest by small sparks which current in the thick, primary wire of jumped across the gap between its the coil, and to adjust the position of knobs. the spheres cc. This would bring But though the sparks thus genabout a rapid discharge of sparks be- erated in the resonator strongly sugtween the spheres, producing an effect gest the existence of waves in the not unlike the discharge of a Leyden neighborhood of the Hertz oscillator, jar, but more continuous, and for that they do not by themselves exactly and other reasons more convenient to prove the existence of these waves; work with. The discharge of this in- and, in fact, something very like them strument, like that of the Leyden jar, had been observed by, among others, would not consist of a single spark, a well-known English physicist several nor even of a succession of sparks all years before Hertz made his experi* This should act automatically.
ments, without its dawning upon the