Hertzian wave

“ Fig. 7.—Successive Stages in the Production of a Semi-loop of Electric Strain by a Marconi Aerial Radiator.
The matter may be viewed, however, from another standpoint. Good conductors are opaque to Hertzian waves; in other words, are [Pg 13] non-absorptive. The energy of the electric wave is not so rapidly absorbed when it glides over a sea surface as when it is passing over a surface which is an indifferent conductor, like dry land. In fact, it is possible by the improvement of the signals to detect a heavy fall of rain in the space between two stations separated only by dry land. ”

“ Since the earth is a conductor, we must suppose that there is a continual migration of the electrons forming the atoms of the earth, and that when one electron enters an atom, another leaves it. Hence, corresponding to the electric wave in the space above, there are electrical changes in the ground beneath. This view is confirmed by the well-known fact that the achievement of Hertzian wave telegraphy is much dependent on the nature of the surface over which it is conducted, and can be carried on more easily over good conducting material, like sea water, than over badly conducting dry land. ”

“ The distance, therefore, between Poldhu and Cape Cod is only at most about twelve thousand wave-lengths, and stands in the same relation to the length of the Hertzian wave used as does a body the diameter of a pea to the wave-length of yellow light. There is unquestionably a large amount of diffraction or bending of the electric wave round the earth, and, proportionately speaking, it is larger than in the case of light waves incident on objects of the same relative size. ”

“ It is impossible yet to state precisely what is the efficiency, in the ordinary sense of the word, of a Hertzian wave radiator; how much of the energy imparted to the aerial falls back upon it and contributes to the production of the spark, and how much is discharged into the ether in the form of a wave.
Nothing is more remarkable, however, than the small amount of energy which, if properly utilised in electric wave making, will suffice to influence a sensitive receiver at a distance of even one or two hundred miles. ”

“ In subsequently dealing with the details of transmitting arrangements, attention will be directed to the necessity for what telegraphists call a "good earth" in connection with Hertzian wave telegraphy. This only means that there must be a perfectly free egress and ingress for the electrons leaving or entering the aerial, so that nothing hinders their access to the conducting surface over which the wave travels. There must be nothing to stop or throttle the rush of electrons into or out of the aerial wire, or else the lines of strain cannot be detached and and travel away. ”

“ Hence, all active Hertzian wave telegraph stations, like all active volcanoes, are generally found near the sea. In those cases in which a multiple aerial has to be put up consisting of many wires, one mast may be insufficient to support the structure, and several masts arranged in the form of a square or a circle have to be employed. The illustrated papers have reproduced numerous pictures of the Marconi power stations at Poldhu in Cornwall, Glace Bay in Nova Scotia, and Cape Cod in the United States. ”

“ It may be observed at the outset that the difficulty of dealing with the subject as freely as many desire is that Hertzian wave telegraphy is no longer merely a subject of scientific investigation, but has developed into a business and involves, therefore, other interests than the simple advancement of scientific knowledge. ”

“ The amount of bending of light waves round material objects is exceedingly small, because the average length of light waves is about one fifty-thousandth part of an inch. In the case of Hertzian wave telegraphy, we are, however, dealing with ether waves many hundreds of feet in length, and the waves sent out from Poldhu have a wave-length of a thousand feet or more, say one fifth to one quarter of a mile. ”

“ The conditions when the discharges of large condensers are passed between spark balls are entirely different from those when the quantity of the spark, or to put it in more exact language, the current passing, is very small. In the case of Hertzian experiments it is necessary, as shown by Hertz, to maintain a high state of polish on the spark balls when they are employed for the production of short waves of small energy, but when we are dealing with large quantities of energy at each discharge, those methods which succeed for laboratory experiments are perfectly impracticable. ”

“ We have no space to refer to the whole of the steps of discovery which led up to the invention of all the various forms of the modern electric kumascope or wave detector. Suffice it to say that the researches of Hertz in 1887 threw a flood of light upon many previously obscure phenomena, and enabled us to see that an electric spark, [Pg 50] and especially an oscillatory spark, creates a disturbance in the ether, which has a resemblance in Nature to the expanding ripples produced by a stone hurled into water. ”

“ It is the electron alone which has a grip of the ether. To create an ether wave, we have to start or stop crowds of electrons very suddenly. If in motion, their motion implies energy, but it is not only their energy which is concerned in the wave making, but the acceleration, positive or negative—i.e., the quickness with which they are started or stopped. ”

“ We can not define more clearly the nature of this ether strain or displacement until we know much more about the structure of the ether than we do at present. We can picture to ourselves the operation of compressing air as an approximation of the air molecules, but the difficulty of comprehending the nature of an electric wave arises from the fact that we cannot yet definitely resolve the notion of electric strain into any simpler or more familiar ideas. ”

“ Even if we cannot, it proves nothing as regards the reality of such space. During ages no one conceived such a thing as the Hertzian waves, and even to-day we have no direct sense-impression of them. They exist none the less. As a matter of fact, we find it difficult to conceive space of three dimensions. ”

“ When it is a case of transverse waves in an elastic medium, reflected by an advancing obstacle, the origin of the working pressure is not so obvious, because we cannot easily formulate a mechanism for the advancing reflector like that of the lamina above employed. In the case of light-waves we can, however, imagine an ideal material body, constituted of very small molecules, that would sweep them in front of .it with the same perfection as a metallic mirror actually reflects the longer Hertzian waves. ”

“ From the fact that all Hertz waves travel with the same speed as light, and from our ability to imitate, as you have seen, the well-known optical phenomena with Hertz radiation of short wave-length, the great induction has been made that all æther waves have the same essential nature, and that invisible actinic rays, light rays, dark-heat rays, and Hertz rays are all of them æther waves of various wave-lengths and amplitudes. ”

“ Hertz, in generating electric waves intermediate in amplitude between those of sound and light, discovered that opaqueness is a relative term—arm a wave with appropriate dimensions and it has a passport through any substance whatever; a stone wall or a wooden door becomes as permeable as plate glass to sunshine. ”

“ But we can extend our investigations in the other direction; we find that the ether carries many waves longer than light-waves. Special photographic emulsions can reveal the existence of waves five times longer than violet-light waves. Extending below the limits of visibility are waves we detect as heat-waves. Radiant heat, like the heat from a fire, is also a form of wave-motion in the ether, but the waves our senses recognise as heat are longer than light-waves. There are longer waves still, but our senses do not recognise them. ”

“ The coherer, at any rate in a room, does not seem well adapted to interference experiments; it is probably too sensitive, and responds even at the nodes, unless they are made more perfect than is easily practicable. But really there is nothing of much interest now in diffraction effects, except the demonstration of the waves and the measure of their length. There was immense interest in Hertz’s time, because then the wave character of the radiation had to be proved; but every possible kind of wave must give interference and diffraction effects, and their theory is, so to say, worked out. ”

“ The whole theory of the trail of waves made by a canal-boat is only comprehensible if it is clearly seen that a water-surface wave has a certain velocity determined by its wave-length. If the wave-speed is small, the waves are short. As the speed increases the waves get longer. Or the matter may be put in another way. We may say that just as a pendulum has a certain rate of vibration depending on its length, so a water wave has a certain frequency, and therefore, a certain speed of propagation dependent upon the wave-length, or shortest distance from one wave-crest to the next. ”

“ In the case of a simple harmonic wave-motion, which alone we need consider, the wave-velocity () is the quotient of the wave-length () by the period of vibration () . These quantities can be determined with extreme accuracy. In media which are sensibly homogeneous but not isotropic the wave-velocity for any constant value of the period, is a quadratic function of the direction cosines of a certain line, viz., the normal to the so-called "plane of polarization." The physical characteristics of this line have been a matter of dispute. Fresnel considered it to be the direction of displacement. ”

“ Translated into ordinary English the phrase signifies, with accuracy and comprehensive completeness, the full details of "a disturbance periodic both in space and time." Anything thus doubly periodic is a wave; and all waves—whether in air as sound waves, or in ether as light waves, or on the surface of water as ocean waves—can be comprehended in the definition.
What properties are essential to a medium capable of transmitting wave-motion? Roughly we may say two: elasticity and inertia. Elasticity in some form, or some equivalent of it,—in order to be able to store up energy and effect recoil ”

“ In the theory of double refraction the difficulties are no less serious. As a general rule there are in an anisotropic elastic solid three possible directions of vibration (§ 6) , at right angles to each other, for a given direction of the waves, but none of these lies in the wave-front. In order to make two of them do so and to find Fresnel’s form for the wave-surface, new hypotheses are required. On Fresnel’s assumption it is even necessary, as was observed by Green, to suppose that in the absence of all vibrations there is already a certain state of pressure in the medium. ”

“ As a wave progresses over the surface, wave-energy is continually being imparted to portions of the water in front, and it is transferred away from others in the rear. In the very act of setting a fresh particle of water in oscillation, the portions already vibrating must diminish their own motion. They may hand on the whole of their energy or only a part of it to their neighbours. This distinction is a very important one, and it determines whether a single act of disturbance shall create a solitary wave or wave-train in a medium. ”

“ This experiment was the first to show the propagation of electric current in the form of waves, and Hertz calculated the time of such a wave running back and forth in the wire as the one-hundred-millionth of a second. The possibility of making such rapid oscillations opened up a whole new field of research, which has been greatly exploited in the last twenty-five years. Not only did Hertz show that the current in a wire was propagated in waves, but he also showed that the electromagnetic effects which are able to induce currents in other wires are also propagated across free space in waves. ”

“ The transmission of a disturbance or vibratory motion by means of waves, as in the case of sound, was a well-recognized principle, and Young and Fresnel applied it most successfully to the phenomena of light. Wave motion, in a general way, is only possible in a sensibly continuous medium, such as water, air, etc., and the theory that light was a vibratory disturbance transmitted by means of waves necessitated the assumption of the existence of such a medium throughout all space in which light travelled. ”

“ Furthermore, ever since Hertz’s first discovery, whenever waves had to travel through a metal grid or alongside a plane conductor, it was natural to arrange the electric oscillations so as to be normal to the conducting lines or plane, for if they were tangential they excited electric currents therein, and their energy became wasted in the production of heat. So, in so far as earth and water are conductors, it is desirable to use radiation polarised in a horizontal plane, i.e., with the electric oscillations vertical, if considerable distances are to be traversed by it. ”

“ His voice was cut off by the familiar spit and flash of the huge blue spark, and a thin ozonic odour filled the closed room, strangely like the smell of summer air after a thunder-storm. The rapt and wistful eyes of the woman watched him as he worked, touched into wonder before the inscrutable, humbled into momentary amazement by the unfathomable mystery of Hertzian waves. ”

“ It has already been stated that a wave can only exist, or be created, in or on a medium which resists in an elastic manner some displacement. The ordinary water-surface wave is termed a gravitation wave, and it exists because the water-surface resists being made unlevel. There is, however, another thing which a water-surface resists. It offers an opposition to small stretching, in virtue of what is called its surface tension. ”