CHAPTER VI.
WAVES AND RIPPLES IN THE ÆTHER.
HAVING in the last chapter explained the nature and mode of production of electric oscillations, and shown that when these take place in an open electric circuit or long straight rod they give rise to certain actions at a distance, rendered evident by the changes taking place in the conductivity of metallic powders, we have now to present the outlines of a proof that these actions are really due to a wave-motion of some description set up in the æther, which in nature is essentially the same as that which constitutes the agency we call light.
We shall begin by studying a few of the epoch-making discoveries we owe to the celebrated Heinrich Hertz, announced in a series of famous researches with which he surprised and delighted the scientific world in the years 1887 and 1888. These investigations opened a new and remarkable field of experimental work.
The precise form of apparatus used by Hertz in these researches is, however, unsuited for lecture demonstration, and I shall use on this occasion some arrangements of my own, which are only convenient modifications of appliances previously employed by other experimentalists. The devices here shown are, however, very convenient for public demonstrations.
This apparatus consists of two parts, a part for generating electric waves, which we shall call the radiator, and a part for detecting them, which is called the receiver.
The radiator consists of a zinc box, A ([see Fig. 73]), which is provided with hollow trunnions, and can be fixed to a suitable stand and turned in any direction. The box has an open end to it, and in its interior there are two brass rods about 4 inches long, each terminating in brass balls, S, 1 inch in diameter. These rods are thrust through corks fixed in the end of two ebonite tubes, which pass through the hollow trunnions of the box. The rods have their ends attached to very closely wound spirals of gutta-percha-covered wire contained in the ebonite tubes. These spirals are called choking coils. When the balls are arranged in the interior of the box in their proper position, they are about ¹⁄₁₆ inch apart, and the rods to which they are attached are in line with each other.
Fig. 73.—Electric wave radiator (A) and receiver (B).
The outer ends of the choking coils are connected to an induction coil or electrical machine, say a small Wimshurst machine, suitable for producing electric sparks about 2 or 3 inches in length. If then sparks are taken between the balls, we have an arrangement which is, in fact, a small Hertz oscillator or radiator. It has been fully explained in the last chapter that the action of the induction coil or electrical machine is first to create a difference in the electric condition of the balls, such that one is positively electrified and the other negatively. The balls and rods and the surrounding air, as already explained, then form a sort of Leyden jar or condenser, and in virtue of the electromotive force the air is electrically strained around the balls. When this strain reaches a particular value, the air between the balls passes at once into a conductive condition, and we have a discharge which is oscillatory in nature produced between the conductors. We may consider that the electrical charges on the two rods rush backwards and forwards, setting up on the rods an oscillatory surface electric current, and that this is accompanied by a very rapid reversal of the strain in the surrounding non-conductor or dielectric. This state of affairs results in sending out into space an effect called an electric wave.
Turning, then, to the receiver B ([Fig. 73]), we notice that this consists of a similarly shaped metal box, having in it a board to which are fixed two short nickel wires. These are crossed without touching in the interior of a small ebonite box ([see Fig. 74]). The wires are just covered inside the box with a very small quantity of fine nickel filings. To the end of the zinc receiver-box is fixed a long lead pipe, in the interior of which are two insulated wires, c, d.
Fig. 74.—Electric radiation detector (Miller).
These wires are joined to the extremities of the nickel wires in the receiver-box and then, passing through the lead pipe, they enter another metal box which contains a battery and electric bell. The pinch of nickel filings in the small ebonite box is not an electric conductor in its ordinary condition, and hence the electric circuit, including the battery and bell, is not complete. If, however, an electric oscillation is set up in the nickel receiver-wires, the mass of metal particles connecting them at once becomes a conductor, because little metallic granules stick or cohere together. The battery is thus able to send an electric current through the circuit, which includes the coherer, and the electric bell is caused to ring. It may be mentioned that in the actual apparatus employed the arrangement is not quite so simple. The coherer would be permanently injured if we were to attempt to send through it an electric current strong enough to ring an electric bell. Hence we associate with the coherer a contrivance called a relay. A single voltaic cell, E (a dry cell) ([see Fig. 75]), is joined up in series with the coherer C and this relay R. The latter is a sort of switch or circuit-closer of such kind that when a very feeble current passes through it it closes a second circuit through which a much stronger current can pass. The transition of the nickel filings from a non-conductive to a conductive condition is, therefore, only the means by which a very small current of electricity is allowed to pass through the circuit of an electro-magnet which forms the circuit of the relay. This action causes a piece of iron to be attracted, and this again in turn closes another circuit, and so enables the current from a second battery, F, of five or six cells to actuate the electric bell G. The arrangement of the two batteries, the relay coherer, and bell will be understood by studying the diagram of connections in [Fig. 75].
The really important condition in securing success in the performance of the experiments made with this apparatus is that the long wires which connect the receiver-box with the metal box containing the bell, battery, and relay shall be entirely enclosed in a lead pipe without joint, which is soldered at one end into the receiver-box and at the other into the battery-box. Another practical point is that these wires, where they enter the battery-box, must have included in their circuit two little coils of insulated wire of a good many turns, which are called “choking coils.” A third element of success is that the coherer or sensitive conductor shall be sensitive enough, but not too sensitive. This condition can only be obtained by a process of trial and failure. Being provided with these two pieces of apparatus, we can now proceed to exhibit a series of experiments of great interest.
Fig. 75.
In the first place, let the radiator-box and receiver-box be placed a few feet apart with their open mouths facing each other, like two guns arranged to fire down each other’s throats. Then, if all is in order when we make an electric spark between the two balls of the radiator, the electric bell in connection with the receiver will begin to ring, showing that the coherer in the receiver-box has been affected and made conductive by the electric wave sent out from the radiator-box. If a smart rap is then given to the receiver-box the clinging metallic filings in the ebonite box will be separated again and, the circuit being interrupted, the bell will stop ringing.
This being done, the radiator-box is then turned a little on one side by rotating it round its hollow trunnions like a gun until the open mouths of the two boxes no longer face each other. It will then be found, on repeating the former experiment, that the bell will not ring when a spark is made between the balls. A little experimenting will show that the action which affects the coherer is propagated out from the radiator-box in straight lines like the light from a lamp, and that we are here dealing with something which has all the character of radiation. In the next place, let the receiver- and radiator-boxes be again arranged with their open mouths facing each other. We make a spark and again secure the responsive action of the bell. We shall now proceed to prove that this effect, which is called electric radiation, passes quite freely through certain substances, but is more or less completely stopped by others. For instance, if we hold a sheet of iron, tinfoil, or even paper covered with silver leaf between the open mouths of the radiator and receiver, we find that the bell of the receiver will not ring even when a rapid series of oscillatory sparks are made in the radiator. These sheets of metal, thick or thin, are quite opaque to the electric radiation proceeding from the spark-balls. On the other hand, we find a sheet of paper or card, a wooden board, a sheet of glass, a slab of wax or bitumen, sulphur, marble, or slate, are all quite pervious or transparent, and when held between the radiator and receiver do not hinder at all perceptibly the action of the former on the latter. We conclude, therefore, that some bodies are opaque and some transparent to the electric radiation. But the classification does not agree with the classification as regards opacity or transparency for light. Wood, marble, and pitch are optically opaque, but electrically transparent. The general law, however, which decides the question of opacity or transparency for electric radiation, is as follows: All good electrical conductors are opaque to electric radiation, and all good insulators or non-conductors are transparent.
Hence we see at once why metal sheets are opaque, and wood, wax, or glass transparent, to the electric radiation from the spark-balls.
We may go one step further. If we take some sheets of perforated zinc or wire gauze, or even a large packet of pins, or paper bag full of iron filings, we shall find that all these bodies are practically opaque to the electric rays. Moreover, we can show that not only is the above law true for solids, but it holds good for liquids as well. I have provided here a number of flat glass bottles which are filled with various liquids, salt water, fresh water, solution of soda, paraffin oil, olive oil, turpentine and methylated spirits.
If we test an empty glass bottle between the radiator and receiver, we can assure ourselves that the bottle itself is transparent to the electric radiation.
If, then, we take the bottles containing the various liquids and test them one by one between the radiator and the receiver, we find that the bottles containing the paraffin oil, the olive oil, and the turpentine are transparent to the electric radiation, but that the bottles containing the salt water, the fresh water, the solution of soda, and the methylated spirits are all opaque. The oils and liquids similar to them are all good non-conductors, whereas water and various aqueous solutions are fairly good conductors of electricity, and hence these liquids, although they are all about equally transparent to light, behave very differently to electric radiation. As regards the electric ray, a bottle full of pure water is as opaque to the electric radiation we are here using as it would be to light if it were filled with black ink.
Experiment shows that every object containing water, or which is wet, is exceedingly opaque to the electric radiation we are employing. Thus, for instance, if I take a dry duster folded in four, and hold it in the path of the electric ray, you see that it is quite transparent, and that the bell attached to the receiver rings as easily as if there were no duster there at all. If, however, we dip the duster in water, and then hold it between the radiator and receiver, we find that the wet duster is perfectly opaque.
The human body consists largely of water which exists in the tissues, and hence it is not surprising to find that the hand or any part of the body placed between the radiator and receiver intercepts the electric ray. You see, if I hold my hand in front of the radiator, that nothing is able to escape from it, when sparks are made between the balls, which can affect the receiver. In the same way it can be shown by experiment that the human head is perfectly opaque—in fact, much more opaque than an equally thick block of wood; and this opacity to the electric ray is due in a veritable sense to the water in the brain. All dry animal tissues, such as leather, bone, gelatine, and flesh, if dry, are very transparent to electric radiation of the kind we are now using, but if these objects are made thoroughly wet, then they become intensely opaque.
Fig. 76.—The reflection of an electric ray.
We can, then, proceed to show that this electric radiation can be reflected, just like light or sound, by metal or other conducting surfaces, and that the law of reflection of the electric ray is the same as the law of reflection for rays of light or sound. If we place the radiator A with its mouth upwards, still preserving the receiver B in a horizontal position, it is possible to adjust the two very near to one another, but yet so that the radiation from the radiator does not affect the receiver. If I now hold a metal plate, P, at an angle of 45° above the mouth of the radiator, you will notice that the bell at once rings, thus showing that the electric radiation has been reflected into the receiver-box ([see Fig. 76]). Also we find that a very small deviation from the angle of 45° is sufficient to prevent the effect. Careful experiments in the laboratory show that the electric ray is reflected according to the optical law, viz. that the angle of reflection is equal to the angle of incidence. We find that any good conducting surface will, in this manner, affect the electric radiation. Thus I can reflect it from a sheet of tinfoil or even from my hand, and the fact that I can, so to speak, take hold of this electric radiation, and deflect it in different directions by the palm of my hand, produces in the mind a very strong conviction that we are dealing with something of a very real nature in experimenting with this electric radiation.
It will be in your remembrance that, in the chapter in which we were dealing with waves in the air, I showed you a very interesting experiment illustrating the refraction of rays of sound by means of a carbonic acid prism, and I have now to bring before you an exactly analogous experiment performed with electric radiation. Here, for instance, is a prism made of paraffin wax, a substance which you have already seen is transparent to the electric ray. If we arrange the radiator- and receiver-boxes at an angle to one another, it is possible so to adjust them that the electric radiation projected from the radiator-box A just escapes the receiver-box B, and does not therefore cause the bell to ring ([see Fig. 77]). When this adjustment has been made we introduce the paraffin prism P into the path of the electric ray, and if the adjustments are properly made, we find that the electric ray is bent round or refracted, and that it then enters the receiver-box and causes the bell to ring. This experiment was first performed by Hertz with a very large pitch prism, but his apparatus was too cumbersome for lecture purposes, and the smaller and more compact arrangement you see before you is therefore preferable for present purposes.
Fig. 77.—The refraction of an electric ray.
I have it in my power to show you a still more remarkable experiment in electric refraction. It is found that dry ice is very transparent to these electric rays, but if the ice is wetted on the surface, then, as you have already learnt, the film of moisture is opaque. We have had constructed for the purposes of this lecture a prism of ice by freezing water in a properly shaped zinc box. This prism is now being arranged between the radiator and the receiver, and its surfaces must next be dried carefully with dusters and white blotting paper to remove every trace of moisture. When this is done we find we can repeat with the ice prism the same experiment performed just now with the paraffin prism, and we can refract the electric ray. If you will recall to your memory the statements which were made in connection with the refraction of rays of sound and waves of water, you will remember that it was pointed out that the refraction of a ray of sound and the bending of a train of water waves was due to the passage of the waves in the air or in the water from a region where they were moving quickly to a region in which they were compelled to move more slowly; and it was furthermore shown that this bending must take place whenever a plain wave of any kind passes in an oblique direction from one region to another region where it undergoes an alteration in velocity. In other words, it was shown that the bending or refraction of the direction of motion of a wave, whether in air or water, is a proof that there is a difference in its velocity in the two places bounded by the surface at which the refraction takes place. If this bending takes place in such fashion that the ray is bent towards the perpendicular line drawn to the bounding surface, which is the same thing as saying if the line of the wave is bent so as to make a less angle with the bounding surface after it has passed from one region to the other, then it shows that the wave-motion travels more slowly after it has passed the bounding surface than before.
If we now return to the consideration of the electric experiment with the prism of paraffin or ice, we shall find that this, properly interpreted, gives us a proof that the electric radiation travels more slowly in paraffin wax or ice than it does in air, and the ratio between its velocity in air or in empty space and its velocity in any non-conductor is called the electric index of refraction for that non-conductor. This index can be determined by making two measurements. First, that of the refracting angle of the prism; and secondly, that of the deviation of the ray.[26] I have made these two experiments for the prisms of paraffin and ice in my laboratory, and I find the electric refractive index of paraffin to be 1·64, and the electric refractive index of ice to be 1·83.
In connection with the refraction of rays of sound, it was pointed out that a curved surface has the power to diverge or converge rays of sound, and you will remember that we employed a sound-lens for converging the rays of sound diverging from a whistle, just as an ordinary burning-glass, or double convex lens, can be employed to bring the rays of sunlight to a focus. We shall now attempt a similar experiment with the electric ray. A block of paraffin is fashioned into the shape of a semi-cylinder, flat on one side and convex on the other, and this plano-convex paraffin lens has a convex surface having a radius of 6 inches. If I place the radiator A and receiver B about 4 feet apart, then by making a few little adjustments it is possible to so arrange matters that the radiation which proceeds from the radiator is not powerful enough at a distance of 4 feet to sensibly affect the coherer and make the bell ring ([see Fig. 78]). If, however, I adjust the paraffin lens L halfway between, I shall converge this electric radiation to a focus just about the place where the coherer is situated, and the consequence is that on making sparks between the balls of the radiator, we find that the bell attached to the receiver at once rings.
Fig. 78.—Converging a beam of electric radiation.
We have, therefore, here brought to a focus, by means of a paraffin lens, the electrical radiation just in the same manner that an ordinary burning-glass focuses the rays of light and heat of the sun, and enables us to light with it some paper or a cigar. We have, therefore, indubitable proof in all these experiments that we have something proceeding from the radiator which is capable of being reflected or refracted just like the rays of sound or ripples on the surface of water; and, moreover, we find that this electric radiation passes through some substances but not through others. There is, therefore, a strong presumption that we are here dealing with something which is similar in nature to light, although it cannot affect the eye. In order that we may complete the proof we must show that this radiation is susceptible of interference. This proof may be partly obtained from the consideration of the following facts connected with the opacity or transparency of wire grating to the electric radiation:—
Fig. 79.
I have here a wooden frame across which are strained some wires about a quarter of an inch apart ([see Fig. 79]). If we hold this frame or grid in front of the radiator so that the direction of these wires is at right angles to the direction of the radiator rods which carry the balls, we find that the grid is quite transparent to the electric radiation, but if we turn the grid round so that the wires of the grid are parallel to the radiator rods, we find at once that the grid becomes perfectly opaque. The same experiment can be prettily shown by means of a paper of pins. Here are some large carpet pins arranged in rows in paper, and if I hold this paper of pins in between the radiator and receiver with the pins parallel to the radiator, it is perfectly opaque to the electric ray, but if I turn it so that the pins are at right angles, it is quite transparent. The same experiment succeeds with a paper of ordinary pins, but not so well with a paper of midget pins.
The explanation of this action of a grid is as follows: You have already seen that an alternating current in one electric circuit can produce another alternating current in a secondary circuit placed parallel with the first. It is not difficult to show, either experimentally or from theory, that when the primary current is an electrical oscillation—that is, a very rapid alternating current—the current in the secondary circuit is also an electrical oscillation of the same frequency or rapidity, but that the currents in the two circuits, primary and secondary, are always moving in opposite directions at the same moment. Accordingly, if we hold a grid in front of the radiator, the wires of the grid have what are called induced oscillations set up in them, and these induced oscillations themselves create electric radiation. Accordingly, it is clear that if a grid of this kind is held near to a radiator with the wires of the grid parallel to the radiator rods, we have two sets of radiations produced which, at any point on the side of the grid furthest from the radiator rods, must neutralize one another, and therefore destroy each other’s effect. Hence it is possible to cause the electric radiations proceeding from two electric circuits parallel with each other to destroy one another at a distant point; and we may, therefore, make use of the same arguments as in the case of a similar experiment with light to prove that this electric radiation must be a wave-motion.
It would occupy too much of our time, and it would involve the discussion of matters which are rather beyond the scope of elementary lectures, if we were to enter into a complete analysis of all the arguments proving that this electric radiation, which proceeds from an electric oscillator, is really a wave-motion. I may, however, mention one fact, which has been the outcome of an enormous amount of experimental research, and that is, that the velocity of this electric radiation through space is identical with that of light. It has already been mentioned that a ray of light flits through space at the rate of 1,000,000,000 feet, or nearly 186,500 miles a second. By suitable and very ingenious arrangements, physicists have been able to measure the velocity of electric radiation, and have found in every case that its velocity of propagation is precisely the same as a ray of light.
Let us, then, summarize briefly what we have learnt. We find that when we set up an electrical oscillation in an open circuit consisting of two metallic rods placed in one straight line, we have proceeding from this circuit an electrical radiation which is capable of being propagated through space, which moves in straight lines, can be reflected and refracted, can exhibit the phenomena of interference, and moreover which is propagated with exactly the same velocity as light. Is it possible to resist the conclusion that this effect which we call electric radiation, and the similarly behaving physical agency which we call light, must both be affections of the same medium? It is hardly necessary to occupy time with experiments in showing that a ray of light can be reflected and refracted by mirrors and prisms, and converged or diverged by transparent lenses. These are simple optical facts, and if you are not familiar with them it will be easy for you to make their acquaintance by studying any simple book upon optics; but I should like to draw your attention to the fact that, in addition to rays of light and electric radiation, we are acquainted with another kind of radiation, which is also susceptible of being refracted, and that is commonly called dark heat.
Supposing that we take an iron ball and make it red hot in a furnace, then, in a perfectly dark room, we see the ball glowing brilliantly, and we are conscious by our sensations that it is throwing off heat. Let us imagine that the ball is allowed to cool down to a temperature of about 500° C.; it will then just cease to be visible in a perfectly dark room, but yet if we hold our hand or a thermometer near to it, we can detect its presence by the dark radiant heat it sends out. Experiments show that even when the ball is brilliantly incandescent, nearly 98 or 99 per cent. of all the radiation it sends out is dark heat, and only 1 or 2 per cent. is radiation which can affect the eye as light. It is quite easy to show that this dark heat can be reflected just like light. If I fix this red-hot ball in the focus of a metallic mirror and lift up ball and mirror nearly to the ceiling and then place upon the table another convex, polished, metallic mirror, the top mirror will gather up and project downwards the radiation from the iron ball and the bottom mirror will converge that to a focus. If then we fix a red-hot ball in the focus of the upper mirror and allow it to cool until it is just not visible in the dark, we shall find that we can still ignite a piece of phosphorus or some other inflammable substance by holding it in the focus of the bottom mirror, thus showing that the dark radiation from the iron ball is susceptible of reflection just as are rays of light or electric rays. In fact, if time permitted, it would be possible to show a whole series of experiments with dark radiant heat which would prove that this radiation possesses similar properties of luminous or electric radiation in its behaviour as regards reflection, refraction, and interference.
A vast body of proof has been accumulated that all these forms of radiation are merely varieties of one and the same thing, and that the only thing in which they really differ from one another is in what is called their wave-length. At this point I will remind you once more of that general law which connects together the velocity of propagation of a wave-motion, the wave-length and the frequency. It is expressed in the formula: wave-velocity (V) equals frequency (n) multiplied by wave-length (λ), or in symbolical language—
V = nλ
Accordingly, if the velocity of propagation can be determined, and if the frequency or periodicity of the wave-motion is known, then the wave-length can be found from the above simple rule; or conversely, if the velocity of propagation and the wave-length are known, the frequency is determined.
The wave-length of various kinds of monochromatic (one-colour) light can be easily determined by means of Young’s experiment on interference. If the distance between the two small holes from which the two streams of light emerge is measured, and if the distance from them to the screen and also the distance of the first dark band from the central line is determined, it is then very easy to calculate the difference in the distances from the two holes to the dark band. This difference, however, must, as already explained, be equal to one-half wave-length of the light employed. Experiments made in various ways have shown that the wave-length of yellow light is not far from the fifty thousandth part of an inch.
Hence as the velocity of visible light is 186,500 miles per second, or 1000 million feet, or 12,000 million inches per second, whilst the wave-length is something like ¹⁄₅₀₀₀₀ inch, it is clear that the frequency, or number of light waves which enter the eye per second, must be reckoned in millions of millions. In fact, it ranges from 400 to 700 billions. There is a certain difference of opinion as to what is meant by a billion. We here use the word to signify a million times a million, a million being a thousand times a thousand.
The following table shows us the frequency or number of waves per second, corresponding to light rays producing colour-sensations of various kinds:—
| Vibration Rates of Æther Waves affecting the Eye as Light. | |||||
| Colour sensation. | Vibrations per second. | ||||
| Deep red | 400 | billions. | |||
| Red-orange | 437 | ” | |||
| Yellow-orange | 457 | ” | |||
| Yellow | 509 | ” | |||
| Green | 570 | ” | |||
| Blue-green | 617 | ” | |||
| Blue-violet | 696 | ” | |||
| Violet | 750 | ” | |||
Investigation has shown that the quality in a light ray which causes it to affect our eye with a particular colour-sensation is its wave-length, whereas the quality which affects our eyes as brightness or brilliancy is due to the amplitude of the waves. It is somewhat difficult to realize at first that, outside of ourselves, there is no such thing as colour. Colour is a sensation produced when æther waves of a certain wave-length enter the eye and fall on the retina. If the retina is stimulated 400 billions of times per second, we experience a sensation of redness, and if it is stimulated 700 billion times per second, we experience a sensation of blueness; but externally, there is no such thing as red and blue, there is only a difference in wave-frequency. It is astonishing when we learn for the first time that 400 millions of millions of times per second something in the back of our eyes is moved or stimulated whenever we look at a lady’s red dress, a surgeon’s red lamp, or the red petal of a geranium flower.
You will notice, on referring to the above table of frequencies, that the range of sensibility of the human eye is very much smaller than that of the ear. Our eyes are wonderful instruments for detecting wave-motion in the æther, and our ears are appliances for detecting wave-motion in the air. The ear, however, is, as explained in a previous chapter, sensitive to air-vibration forming musical tones which lie between 30 and 30,000 per second, and these numbers are in the ratio of 1000 to 1, and cover a range of about ten octaves. The eye, however, is only sensitive to æther-vibrations which lie in frequency between 400 and 700 billions per second, and these numbers are in the ratio of nearly 2 to 1, or comprise only one octave.
The question, of course, immediately arises—What are the properties of æther waves the frequency of which lies outside the above limits?
Scientific investigation has made us acquainted with a vast range or gamut of æther-vibrations, and we are able to summarize our present knowledge as follows:—
The physical effect we call light, and that which we have up to the present moment merely called electric radiation, are really identical in nature, and both consist in waves propagated through the space-filling æther, the only difference between them is in wave-length and wave-amplitude. In between these two classes of radiation comes a third, which is called the dark-heat radiation, and beyond the limits of visible radiation we are acquainted with another group of æther waves which cannot affect the eye as light, but which from their power to affect a photographic plate, is called actinic radiation. Hence, briefly speaking, four great groups of æther waves are known to us, called respectively—
1. Actinic, or photographic rays.
2. Luminous, or light rays.
3. Ultra-red, or dark-heat rays.
4. Electric, or Hertz rays.
Convincing proof has been afforded that these various rays are essentially the same in nature, and that they consist in periodic disturbances or waves propagated through the æther in every case with the velocity of 186,500 miles, or 1000 million feet, or 30,000 million centimetres per second.
We may, therefore, say that these classes of æther waves differ from each other only in the same sense in which a bass note in music differs from a treble one; that is, the difference is a difference in frequency.
Just, therefore, as we have a gamut, or scale of musical tones, or air-vibrations of increasing frequency, so we may arrange a gamut or scale of æther waves progressively placed according to their vibration-rates. Our present knowledge concerning æther waves can best be exhibited by arranging in a chart a series of numbers showing the wave-lengths of the waves with which we are so far acquainted. As a limit of length we shall take the one-thousandth part of a millimetre. Most persons know that a millimetre is a thousandth part of a metre, and is a short length nearly equal to one twenty-fifth of an inch. The thousandth part of a millimetre is called a micron, and is denoted by the symbol 1μ. This last is therefore an exceedingly short length, nearly equal to one twenty-five thousandth part of an inch.
Following, also, the musical nomenclature, we shall speak of all those waves included between two wave-lengths, one of which is double or half the other, as an octave. Thus all the various waves whose wave-lengths lie between 1μ and 2μ in length are said to be an octave of radiation. As a preliminary to further discussion let us consider, in the first place, the simple facts about the radiation which affects our eyes as light.
The light which comes to us from the sun is not a simple thing. It consists of æther waves of many different wave-lengths mingled together. Sir Isaac Newton first revealed to us the compound nature of white light by his celebrated experiment with a glass prism, and his optical discoveries were the starting-point for our information on this subject. If a beam of sunlight is allowed to fall on a glass prism, the rays of light of different wave-lengths which compose it are each bent or refracted to a different degree. In free space æther waves of various wave-lengths all travel, as far as we know, at the same rate. This equality in speed is, however, disturbed the moment the waves enter a transparent material substance such as glass. The velocity of propagation is then reduced in all cases, but it is generally more reduced for the shorter waves than for the longer ones; and as a consequence the rays of shorter-wave lengths are more bent or refracted than the rays of longer wave-length. We have, therefore, a dispersion of the component rays, or a sorting out or analysis of the mixture of rays of various wave-lengths, and if we receive the light on a screen after passing through the prism we have a band of coloured light called a spectrum, which consists of a series of patches of light each of a different wave-length. The component rays of the original beam of light are spread out fan-fashion by the prism. We may note, in passing, that it is not every transparent body when fashioned into a prism which thus analyzes the light into a fan-shaped beam with rays of various wave-lengths arranged in the order of their wave-lengths. The substances which behave as does glass or water when made into prisms are said to exhibit normal dispersive power. There are, however, some bodies, such as iodine or an alcoholic solution of fuschine, which exhibit anomalous dispersion and refract some longer waves of light more than some shorter ones. The arrangements for forming a normal spectrum are as follows: We pass a beam of light from the electric lamp through a lens, and place in front of this lens a metal plate with a narrow vertical slit-shaped opening in it. At a proper distance in front of the slit we place another lens, and project upon the screen a sharp image of the slit in the shape of a bar of white light. Placing a hollow glass prism filled with bisulphide of carbon in front of the last lens, we find that the various rays in the white light are dispersed, and we produce on the screen a band of rainbow-coloured light, called the spectrum. This spectrum is in reality a series of differently coloured images of the slit placed side by side. By making use of the principle of interference as disclosed by Young, it is possible to make a measurement of the wave-length of the rays of light which produce the sensation of various colours when they fall upon the eye. Thus the wave-length of those æther waves which produce the sensation of deep red is 0·75μ, and that of the waves producing the sensation of violet when they fall upon the retina of the eye is 0·43μ. The whole of the visible spectrum is therefore included within a single octave of æther radiation. Within these limits any change in the wave-lengths makes itself felt in our eyes as a change of colour. It is commonly said that there are seven colours in the spectrum—red, orange, yellow, green, blue, indigo, and violet. As a matter of fact, a highly trained eye can discover about a thousand different tints in the spectrum of white light. Time will not allow us to enter into any discussion of what is called colour-vision and the theory of sensations of colour. The fact I wish to impress upon you here is that, outside of ourselves, there is no such thing as colour. The rays of light which produce these sensations of colour when they enter the eye differ from one another only in wave-length and wave-amplitude. Hence there is a complete analogy between light of different colours and sounds of different pitches or tone. Red light differs from blue light only as a bass note in music differs from a treble note. Hence you must distinguish very carefully between a ray of light in itself, and the sensation it produces when it falls upon the retina of the eye. Our eyes are gifted with a marvellous power of detecting slight differences between the wave-length and the amplitude of the rays which may stimulate two adjacent portions of the retina of our eyes.
That range of sensibility is, however, very limited. Supposing we allow a ray having a wave-length greater than 0·75 or less than 0·43 to enter the human eye. It produces no sensation of light at all. Accordingly, if we form a spectrum with sunlight, we find a tolerably sharp limit to the visible spectrum. Supposing, however, we allow the spectrum to fall upon a sensitive photographic plate, we find that the plate will be chemically acted upon far beyond the limits of the visible violet end of the spectrum. Hence we learn that beyond the violet there is radiation of a kind which is invisible to the eye, yet can affect a photographic plate. This is called the ultra-violet, or actinic radiation.
Schumann, in 1893, measured waves in actinic radiation of a wave-length as short as 0·1μ, or one two hundred and fifty thousandth part of an inch, and hence we may say that we are acquainted with at least two octaves of invisible ultra-violet or actinic radiation, or æther waves have wave-lengths lying between the limits 0·1μ and 0·4μ.
In a similar manner very delicate heat-detecting instruments or thermometers called bolometers, or thermopiles, show us that beyond the visible-red end of the normal spectrum there is radiation called the ultra-red radiation, or dark-heat, which cannot affect the eye.
The wave-length of dark-heat radiation has been measured up to a limit of 67μ by Professor Rubens and Professor Nichols in 1897 and 1898. Accordingly, we can assert that beyond the red end of the spectrum we are acquainted with six octaves or more of ultra-red radiation, viz. that lying in wave-length between 0·75μ and 67μ.
We may represent the above facts in another way as follows: In most pianos the keyboard extends over a range of seven or eight octaves. Imagine a piano having a keyboard with nine octaves, and that each key was labelled to correspond with a light wave of a particular length. At the extreme treble end let the first key be labelled 0·1, and at the extreme base end let the last key be labelled 51·2. Then the various octaves will be comprised between the keys marked 0·1, 0·2, 0·4, 0·8, 1·6, 3·2, 6·4, 12·8, 25·6, and 51·2 ([see Fig. 77]).
Suppose that each key when struck caused some kind of electric radiator to emit an æther wave whose wave-length reckoned in microns or thousandths of a millimetre, is indicated by the number on the key. Of all this great gamut of æther waves only the notes of one octave, viz. the third from the treble end, the wave-lengths of which lie between 0·4μ and 0·8μ, affect the retina of the human eye as light.
Those waves in the two octaves higher up, that is, of wave-length less than 0·4μ, are able powerfully to affect a photographic plate, and so, indeed, do some of the waves in the visible octave. We may, in fact, say that all the æther waves with which we are acquainted, the wave-length of which is less than about 0·5μ, are able to make an impression upon a photographic plate. These rays, whatever their wave-lengths, are called the actinic rays.
On the other hand, all the æther waves with wave-length greater than about 0·8μ, and for six octaves further down, can only be recognized by their ability to heat a delicate thermopile or other heat-measuring instrument. They cannot affect the eye, and they have little or no effect in decomposing silver salts and impressing a sensitive photographic surface.
GAMUT OF ÆTHER WAVES.
Fig. 80.
It should be noted, however, that whilst there are more or less definite limits to the wave-lengths of the eye-affecting radiation, and probably also to the actinic, or photographic radiation (radiation of some wave-lengths being both visible and actinic), rays of every wave-length are in some degree thermal, or heat-producing. The term dark-heat radiation is, however, generally restricted to radiation of that wave-length which is non-visible and non-actinic. This mode of presenting the facts will call your attention again to the narrow limits of sensibility of the human eye as compared with those of the ear.
The above-mentioned range of wave-lengths does not, however, exhaust our powers of æther-wave production. If we skip over six octaves lying below the limits of the longest dark-heat wave with which we are acquainted, we should arrive at a wave whose wave-length is about 4000μ, or 4 millimetres. At this point we encounter the shortest æther waves which have yet been made by means of electrical oscillations in the fashion first discovered by Hertz.
It is not possible to define exactly the wave-length limits of radiation as yet made by means of electrical oscillations. Lampa has experimented with æther waves made by the Hertz method, the wave-length of which was not more than 4 millimetres. Professors Lodge, Rhigi, Bose, Trouton, the author, and many others, have carried out quasi-optical experiments with electrically made æther waves, the wave-length of which ranged from a few millimetres to several inches. Hertz’s own work was chiefly done with æther waves from 1 or 2 feet to 30 or 40 feet in wave-length. More recently, æther waves of 800 to 1000 feet in wave-length have been employed in wireless telegraphy. Perhaps we shall not be wrong in saying that we are acquainted with sixteen or seventeen octaves of æther-wave radiation which is made electrically, and is usually called the Hertz radiation.
Between the radiation of greatest wave-length which proceeds from hot or incandescent bodies such as the sun, the electric arc, or a hot ball, and that of the shortest wave-length which has been created by means of electrical oscillations set up in some form of Hertz oscillator, there is a range of six octaves of æther waves which, so far as we know, have not yet been manufactured or detected. Herein lies an opportunity for much future scientific work. We have to discover how to create and recognize these interconnecting wave-lengths. 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. Thus we see, as Maxwell long ago predicted, that light in all probability is an electro-magnetic phenomenon, and therefore all optical effects must be capable of receiving an electro-magnetic explanation. The inclusion thus made of the whole science of Optics within the domain of Electricity and Magnetism is one of the grandest achievements of Physical Science. It stands second only to Newton’s great discovery of universal gravitation, which reduced all Physical Astronomy to pure Dynamics, and showed that the force concerned in the falling of a stone is identical with that which holds the planets in their orbits, and controls the motions of galaxies of suns.
At the end of the last chapter it was explained that these Hertz radiations are created in the æther by the suddenly starting, stopping, or reversing the motion of crowds of electrons, which are, as it were, instantly released from a state of pressure or tension, and set moving inside a straight insulated conductor, which forms an open electric circuit. The radiations we call light and dark heat are probably, therefore, started in a similar manner by vibrations of the electrons which form parts of, or which build up, atoms. There are many physical phenomena which seem to show that the electrons which we can detach from atoms in a high vacuum tube are capable of vibrating freely in definite periods when in connection with their atom. If the atoms are able to move freely, and if each is practically independent, as is the case in a gas, and if they are then caused to radiate by any means, the radiation emitted by the vibration of these electrons consists of certain definite wave-lengths. Hence, when we form the spectrum of an incandescent gas, we find it to consist of several detached bright lines, each corresponding to one particular wave-length, and we do not obtain a uniformly graduated band of coloured light. If an atom is struck by colliding with another, and then left to itself, it appears as if the electrons which compose it and form part of it are set in vibration, and each executes its oscillation in some definite period of time. An atom has, therefore, been compared to a “collection of small tuning-forks,” which, if rudely struck, would result in the emission of a set of air-wave trains, each one corresponding in wave-length to one particular tuning-fork which emitted it. Hence, if we could administer a blow to such a congeries of tuning-forks, and then analyze the compound sound, we should obtain a sound spectrum consisting of separated tones—in other words, a bright line spectrum of the complex sound. Supposing, however, that we have a mass of atoms much more closely in contact, as in the case of a solid body, the continual collisions between the atoms and the closer contact between them cause the vibrations of the electrons to be “forced,” and not “free.” Hence the electrons are compelled to execute all varieties of irregular motion, and these predominate over their regular free natural vibrations. Accordingly, the waves emitted are of a large variety of wave-length, and when the radiation is analyzed by a prism, we obtain a continuous spectrum, or band of many-coloured light, as the result of the separation of the rays of different wave-lengths present in it.
It is this fact which renders our present method of creating artificial light so excessively uneconomical.
All our practical methods for making light consist in heating a solid body in one way or another. In the case of the electric light we heat electrically a carbon rod or filament, or else, as in the Nernst lamp, a rod composed of magnesia and the rare earths. In the case of the lime-light we heat a cylinder of lime. In an ordinary gas or candle flame we heat small particles of carbon, and the same is the case even in the sun itself.
But this process manufactures not only the single octave of radiation which can affect our eyes as light, but a dozen other octaves of radiation to which they are insensible. Hence it follows that of the whole radiation from a gas flame, only about 3 per cent. is eye-affecting light, the remainder is dark heat. In the case of an incandescent electric lamp, this luminous efficiency may amount to 5 per cent., and in the electric arc to 10 or 15 per cent. There is, however, always a great dilution of the useful light by useless dark heat.
The proportion of the light or eye-affecting radiation to the dark heat in the total radiation from any source of light increases with the temperature, but it is not always merely a question of temperature. Thus the electric arc is hotter than a candle flame, and the sun is hotter than the electric arc. Hence, whilst the luminous rays only form 3 parts out of 100, or 3 per cent. of the radiation of a candle, they constitute 10 to 15 per cent. of those of the electric arc, and more than 30 per cent. of those of the sun. On the other hand, the glow-worm and the fire-fly seem to have possession of a knowledge and an art which is as yet denied to man. It has been shown by Professor Langley and Mr. Very that nearly the whole of the radiation from the natural torch of the fire-fly is useful light, and none of it is useless dark heat. Hence these photogenic (light-producing) insects have the art, which we have not, of creating cold light, or unadulterated luminous radiation.
At the present moment in ordinary incandescent or glow-lamp electric lighting we require to expend an amount of power, called one horse-power, to produce illumination equal to that of 600 candles. Supposing, however, that all our power could be utilized in generating merely the rays useful for vision, or which can impress our eyes, we might be able to create by the expenditure of one horse-power more than twenty times as much illumination, that is, a light equal to 12,000 candles.
These figures show us what rewards await the inventor who can discover a means of generating æther waves having wave-lengths strictly limited to the range lying between the limits 0·4μ and 0·7μ without, at the same time, being obliged to create radiation comprising longer waves which are not useful for the purpose of rendering objects visible to us. For the purposes of artificial illumination we require only the æther waves in this one particular octave, and nothing else.
This increase in the efficiency of our sources of artificial illumination is only likely to be brought about when we abandon the process of heating a solid substance to make it give out light, and adopt some other means of setting the electrons in vibration.
It is almost impossible to discuss the subject of æther waves without some reference to the most modern utilization of them in the so-called wireless telegraphy. Without entering upon the vexed questions of priority, or on the historical development of the art, we shall simply confine our attention here to a consideration of the methods employed by Mr. Marconi, who has accomplished such wonderful feats in this department of invention.
We have already seen that when two insulated conductors are placed with their ends very near together, and are then electrified, one positively and the other negatively, and then allowed to be suddenly connected by an electric spark, they constitute an arrangement called an electrical oscillator. If the conductors take the form of two long rods placed in one line, and if their contiguous ends are provided with spark-balls separated by a small gap, we have seen that we have shown that, under the above-mentioned conditions, electric currents of very high frequency are set up in these rods. For creating these oscillations, an instrument called an induction coil or spark-coil is generally employed. You will understand the arrangements better if a brief description is given first of the spark-coil itself as used in wireless telegraphy.
Fig. 81.—A 10-inch induction coil for wireless telegraphy (Newton).
The appliance consists of a large bundle of fine iron wires, which are wound over with a long coil of insulated wire. This forms the primary coil. It is enclosed entirely in a tube of ebonite. One end of this coil is a contact-breaker, which automatically interrupts an electric current flowing from a battery through the primary coil ([see Fig. 81]). A hand-key is also placed in the circuit to stop and start the primary current as desired. Over the primary coil is a very long coil of much finer silk-covered copper wire, called the secondary coil. The length of this coil is very considerable, and may amount to many miles. The secondary coil is divided into sections all carefully insulated from each other. Another important part is the condenser. This consists of sheets of tinfoil laid between sheets of waxed paper, alternate tinfoil sheets being connected. The arrangement forms virtually a Leyden jar, and one set of tinfoils is connected to one side of the automatic break, and the other to the adjacent side. When, therefore, the primary circuit is interrupted by the break, the condenser is at that moment thrown into series with the primary coil. The rapid interruption of the primary current causes a secondary current in the fine-wire coil. The automatic contact-breaker makes from ten to fifty such interruptions per second. At every “break” of the primary a very high electromotive force is generated in the secondary circuit, which may amount to many hundreds of thousands of volts. This very high secondary electromotive force is able to cause an electric discharge in the form of a spark between brass balls connected to the secondary circuit terminals. Coils are generally rated by the length (in inches) of the spark they can produce between brass balls about ¹⁄₂ inch in diameter. The coil most commonly used in wireless telegraphy is thus technically termed a “10-inch induction coil,” from the length of the spark this particular type of coil can produce.
If the insulated brass balls, called the spark-balls, connected to the secondary terminals, are placed an inch or so apart, and the hand-key in the primary circuit is closed, a battery connected to the primary circuit will send a rapidly interrupted current through the primary coil, and a torrent of sparks will pass between the spark-balls. The primary current of the 10-inch coil is usually a current of 10 ampères, supplied at a pressure of 10 volts.
If the hand-key is raised or pressed, it is possible to make long or short torrents of secondary sparks.
Suppose, then, that we connect to the secondary spark-balls two long insulated rods, and place the spark-balls about ¹⁄₄ inch apart. On pressing the hand-key, we obtain a peculiarly bright crackling spark between the balls, which is an oscillatory spark, and at the same time, as already described, electrical oscillations are set up in the rods and electric waves given off. We may represent to ourselves these electrical oscillations in the rods as similar to the mechanical vibrations which would be set up in a long elastic wooden rod, clamped at the middle and set in vibration at the ends. Or we may consider them similar to the fundamental vibrations of an open organ-pipe, the middle of the pipe corresponding with the middle of the rod. In comparing the mechanical vibrations of the rod or the acoustic vibration of the air in the organ-pipe with the electrical oscillations in the long rods, we must bear in mind that the displacement of the rod or the air in the organ-pipe at any point corresponds with electrical pressure, or potential, as it is called, at any point in the long oscillator. Hence, bearing in mind the remarks in the fourth lecture, it will be evident to you that just as the length of the air wave emitted by the open organ-pipe is double the length of the pipe, so the length of the electric wave thrown off from the pair of long rods is double their total length.
Instead of using a pair of rods for the electrical oscillator, it was found by Mr. Marconi to be an improvement to employ only one insulated rod, held vertically, and to connect it to one spark-ball of the coil, and to connect the opposite spark-ball to a metal plate buried in the earth. Then, when the spark-balls are placed a little apart and the hand-key pressed, we have a torrent of oscillatory sparks between the “earthed ball” and the insulated rod ball. This sets up in the rod electrical oscillations, which run up and down the rod. It is easy to show that there is a strong electric current passing into and out of the rod by connecting it to the spark-ball by means of a piece of fine wire. When the sparks are taken, we find this wire will become hot, it may be red hot, or sometimes it may be melted.
By applying the principles already explained, it is not difficult to demonstrate that in the case of an oscillator consisting of a single rod connected to one spark-ball the electric waves thrown off are in wave-length four times the length of the rod.
The electrical actions taking place, therefore, are as follows: At each interruption of the primary current of the spark-coil there is an electromotive force created in the secondary circuit, which gradually charges up the insulated rod until it attains a state in which it is said to be at a potential or electrical pressure of several thousand volts. The spark then happens between the balls, and the rod begins to discharge.
This process consists, so to speak, in draining the electric charge out of the rod, and it takes the form of an electric current in the rod, which has a zero value at the top insulated end, and has its maximum value at the spark-ball end.
Also, when the oscillations take place, we have variations of electric pressure, or potential, which are at a maximum at the upper or insulated end, and have a zero value at the spark-ball end. From the rod we have a hemispherical electric wave radiated. In the language of wireless telegraphists, such a simple insulated rod is called an insulated aerial, or an insulated antenna.
A simple insulated aerial has, however, a very small electrical capacity, and it can store up so little electric energy that the whole of it is radiated in the first oscillation. Hence, strictly speaking, we have no train of electric waves radiated, but merely a solitary wave or electric impulse. The effect on the æther thus produced corresponds to the effect on the air caused by the crack of a whip or an explosion, and not to a musical note or tone as produced by an organ-pipe.
We can, however, make an arrangement which is superior in electric wave-making power, as follows:—
Fig. 82.—Transmitter for wireless telegraphy.
The vertical rod, or antenna, A, is not insulated, but is connected by its lower end with one end of a coil of insulated wire, S, wound on a wooden frame ([see Fig. 82]). The other end of this last coil is connected to a metal plate, e, buried in the earth. Around the wooden frame is wound a second insulated wire, P, one end of which is connected to one spark-ball of the induction coil, and the other end to the outside of a Leyden jar, L, or collection of jars. This double coil on a frame is called an oscillation transformer. The inside of this condenser is connected to the second spark-ball of the induction coil I. When these spark-balls S are placed a short distance apart, and the coil set in action, we have a torrent of oscillatory electric sparks between these balls, and powerful oscillations set up in one circuit of the oscillation-transformer. These oscillations induce other oscillations in the second circuit of the oscillation-transformer, viz. in the one connected to the aerial. The oscillations produced in the air-wire, or aerial, are therefore induced, or secondary oscillations. The aerial wire, or antenna, has therefore a much larger store of electric energy to draw upon, viz. that stored up in the Leyden jars, than if it was itself directly charged by the coil.
In order, however, to obtain the best results certain adjustments have to be made. It has already been explained that every open electrical circuit has a certain natural time-period for the electrical oscillations which can be set up in it. This is technically called its tune.
If we administer a blow to a suspended pendulum we have seen that, if left to itself, it vibrates in a definite period of time, called its natural period. In the same manner, if we have a condenser or Leyden jar having electrical capacity which is joined in series with a coil of wire having electrical inertia or inductance, and apply to the circuit so formed a sudden electromotive force or impulse, and then leave the circuit to itself, the electric charge in it vibrates in a certain definite period, called its natural electrical periodic time.
The aerial, or antenna, is simply a rod connected to the earth, but it has a certain inductance, and also a certain electrical capacity, and hence any metal rod merely stuck at one end in the earth has a perfectly definite periodic time for the electrical oscillations which can be produced in it. We may compare the rod in this respect with a piece of steel spring held at one end in a vice. If we pull the spring on one side, and let it vibrate, it does so in accordance with its natural time-period for mechanical vibrations. The sound waves given out by it have a wave-length equal to four times the length of the spring. In the same manner the fundamental wave-length of the electric waves emitted by an “earthed aerial,” or rod stuck in the earth, when an electric impulse is applied to its lower end, and electrical oscillations are set up in it, have a wave-length equal to four times that of the rod. Hence to obtain the best result the circuit, including the aerial A, must be “tuned” electrically to the circuit including the Leyden jar L.[27]
A consideration of these arrangements will show you that if the hand-key in the primary circuit of the induction coil is pressed for a long or short time, we have long or short torrents of sparks produced between the secondary balls, and long or short trains of electric waves emitted from the aerial, or earthed vertical wire.
Whenever we have any two different signals, we can always make an alphabet with them by suitable combinations of the two. In the well-known Morse alphabet, with which every telegraphist is as familiar as we all are with the printed alphabet, the sign for each of the letters of the alphabet is composed of groups of long and short symbols, called dots and dashes, as follows: Each letter is made by selecting some arrangements of dots or dashes, these being the technical names for the two signs. The Morse code, as used all over the world, is given in the table below—
| The Morse Alphabet. | |||||
| A | – ––– | J | – ––– ––– ––– | S | – – – |
| B | ––– – – – | K | ––– – ––– | T | ––– |
| C | ––– – ––– – | L | – ––– – – | U | – – ––– |
| D | ––– – – | M | ––– ––– | V | – – – ––– |
| E | – | N | ––– – | W | – ––– ––– |
| F | – – ––– – | O | ––– ––– ––– | X | ––– – – ––– |
| G | ––– ––– – | P | – ––– ––– – | Y | ––– – ––– ––– |
| H | – – – – | Q | ––– ––– – ––– | Z | ––– ––– – – |
| I | – – | R | – ––– – | ||
| The Morse Numerals. | |
| 1 – ––– ––– ––– ––– | 6 ––– – – – – |
| 2 – – ––– ––– ––– | 7 ––– ––– – – – |
| 3 – – – ––– ––– | 8 ––– ––– ––– – – |
| 4 – – – – ––– | 9 ––– ––– ––– ––– – |
| 5 – – – – – | 0 ––– ––– ––– ––– ––– |
| Full Stop – ––– – ––– – ––– | |
| Signal for calling up – – – ––– – – – ––– | |
The process of sending a wireless message consists in so manipulating the key in the primary circuit of the induction coils that a rapid stream of sparks passes between the secondary balls for a shorter or for a longer time. This gives rise to a corresponding series of electric waves, radiated from the aerial. The dash is equal in duration to about three dots, and a space equal to three dots is left between each letter, and one equal to five dots between each word. Thus, in Morse alphabet the sentence “How are you?” is written—
| – – – – | ––– ––– ––– | – ––– ––– | ||
| H | O | W |
| – ––– | – ––– – | – | ||
| A | R | E |
| ––– – ––– ––– | ––– ––– ––– | – – ––– | ||
| Y | O | U |
We have, in the next place, to explain how the signals sent out are recorded.
Fig. 83.—Marconi receiving arrangement for wireless telegraphy.
At the receiving station is erected a second insulated aerial, antenna, or long vertical rod, A ([see Fig. 83]), and the lower end is connected to the earth through a coil of fine insulated wire, P, which forms one circuit of an oscillation-transformer. The secondary circuit, S, of this oscillation-transformer, which is called a jigger, is cut in the middle and has a small condenser, C1, inserted, consisting of two sheets of tinfoil separated by waxed paper ([see Fig. 83]), and to the ends of this circuit is connected the coherer, or metallic filings tube, T, which acts as a sensitive receiver. The Marconi sensitive tube ([see Fig. 84]) is made as follows. A glass tube about ¹⁄₄ inch in diameter and 2 inches long has two silver plugs put in it, and these are soldered to two platinum wires which are sealed into the closed ends of the tube. The ends of the plugs are cut in a slanting fashion and made very smooth. These ends very nearly touch each other. A very small quantity of very fine metallic powder consisting of nineteen parts nickel and one part silver is then placed between the plugs. The quantity of this powder is scarcely more than could be taken up on the head of a large pin. The glass tube is then exhausted of its air and sealed. The tube is attached to a bone rod by means of which it is held in a clip.
Fig. 84.—Marconi coherer.
To the two sides of the above-mentioned condenser are connected two wires which lead to a circuit including a single voltaic cell, V, and a relay, E. The relay is connected to another circuit which includes a battery, B, and a piece of apparatus called a Morse printer, M, for marking dots and dashes on a strip of paper.
The working details of the above rather complicated system of apparatus devised by Mr. Marconi would require for its full elucidation a large amount of explanation of a technical character. The general reader may, however, form a sufficiently clear idea of its performance as follows:—
When the electrical waves from the distant transmitting station reach the aerial at the receiving station, they set up in it sympathetic electrical oscillations. The most favourable conditions are when the two aerials at the distant stations are exactly similar. These electrical oscillations, or rapid electric currents, set up an electromotive force in the secondary circuit of the oscillation-transformer, and this acts, as already explained, upon the metallic filings in the coherer-tube and causes it to become an electrical conductor. The cell attached to the relay then sends a current through the conductive circuit so formed and operates the relay. This last contrivance is merely a very delicate switch or circuit-closer which is set in action by a small current sent through one of its circuits, and it then closes a second circuit and so enables another much larger battery to send a current through the Morse printer. The printer then prints a dot upon a moving strip of paper and records a signal. One other element in this rather complicated arrangement remains to be noticed, and that is the tapper. Underneath the coherer-tube is a little hammer worked by an electro-magnet like an electric bell. This tapper is set vibrating by the same current which passes through the Morse printer, and hence almost as soon as the latter has begun to print, the sensitive tube receives a little tap which causes the metallic filings to become again a non-conductor, and so arrests the whole of the electric currency. If it were not for this tapper, the arrival of the electric wave would cause the printer to begin printing a line which would continue. The dot is, so to speak, an arrested line. If, however, trains of electric waves continue to arrive, then dots continue to be printed in close order, and form a dash on the paper strip. It will thus be seen that the whole arrangements constitute an exceedingly ingenious device of such a nature that a single touch on the hand-key at one station causing a spark or two to take place between the spark-balls makes a dot appear upon a band of paper at the distant station; whilst, if the hand-key is held down so that a stream of sparks takes place at the transmitting station, a dash is recorded at the receiving station. The means by which this distant effect is produced is the train of electric waves moving over the earth’s surface setting out from one aerial and arriving at the other.
Fig. 85.
The reader who has difficulty in following the above explanations may perhaps gather a sufficiently clear notion of the processes at work by considering a reduced, or simplified, arrangement. Imagine two long insulated rods, A, A′ ([see Fig. 85]), like lightning-conductors set up at distant places. Suppose each rod cut near the bottom, and let a pair of spark-balls, S, be inserted in one gap and a coherer or sensitive tube, C, in the other. At one station let an electrical machine have its positive and negative terminals connected to the two spark-balls, and at the other let a battery and electric bell be connected to the ends of the coherer. Then, as long as the coherer remains in a non-conductive condition, the electric bell does not ring. If, however, a spark is made between the balls, in virtue of all that has been explained, the reader will understand that the coherer-tube becomes at once conductive by the action of the electric wave sent out from the transmitter-rod. The battery at the receiver-rod then sends a current through the coherer, and rings the bell.
All the other complicated details of the receiver are for making the process of stopping the bell and beginning over again self-acting, and also for the production of two kinds of signals, a long and a short, by means of which an alphabet is made. In order that we may have telegraphy in any proper sense of the word, we must be able to transmit any intelligence at pleasure, and not merely one single arbitrary signal. This transmission of intelligence involves the command of an alphabet, and that in turn requires the power of production of two kinds of signals.
It remains to notice a few of the special details which characterize Mr. Marconi’s system of wireless telegraphy. In establishing wireless communication between two places, the first thing to be done is to equip them both with aerials. If one station is on land, it is usual to erect a strong mast about 150 feet high, and to the top of this is attached a sprit. From this sprit a stranded copper wire is suspended by means of an insulator of ebonite, so that the upper end of the wire is insulated. The lower end of the wire is led into a little hut or into some room near the foot of the mast in which is the receiving and transmitting apparatus.
If the apparatus is to be installed on board ship, then a similar insulated wire is suspended from a yardarm or from a sprit attached to a mast. Each station is provided with the transmitting apparatus and the receiving apparatus, and the attendant changes over the aerial from one connection to the other so as to receive or send at pleasure.
In the case of long-distance wireless telegraphy, the aerial is not a single wire, but a collection of wires, suspended so as to space them a little from each other. Thus in the case of the first experiments made by M. Marconi across the Atlantic, the aerial erected on the coast of Cornwall consisted of fifty stranded copper wires each 150 feet in length suspended in a fan-shaped fashion from a long transverse stay upheld between two masts. The wires were spaced out at the top and gathered in together at the bottom.
The question which almost immediately occurs to most people to ask is how far it is possible to prevent the electric waves emanating from one station affecting all receiving instruments alike within a certain radius. The answer to this is that considerable progress has been made in effecting what is called “tuning” the various stations. In speaking of acoustic resonance it has been pointed out that a train of air waves can set up vibration in other bodies which have the same natural period of vibration. Thus, if we open a piano so as to expose the strings, and if a singer with a strong voice sings a loud true note and then stops suddenly, it will be found that one particular string of the piano is vibrating, viz. that which would give out if struck the note which was sung, but all the rest of the strings are silent. It has been pointed out that every open electric circuit has a natural electrical time-period of vibration in which its electric charge oscillates if it is disturbed by a sudden electromotive force and then left to itself. If the two aerials at two stations are exactly alike, and if the various circuits constituting the oscillation-transformers in the transmitting and receiving appliances are all adjusted to have the same electrical period, then it is found that the stations so tuned are sympathetic at distances vastly greater than they would be if not so tuned. Hence it is possible to arrange wireless telegraph apparatus so that it is not affected by any electric waves arriving from a distance which have not a particular time-period.
Mr. Marconi has also proved that it is possible to receive on the same aerial, at the same time, two different messages on separate receiving instruments from two distant but properly tuned transmitting stations.
Since the date of these pioneer inventions many different forms of wave detector have been discovered, and wireless telegraphy has shown itself to be of the greatest utility in effecting communication between ship and ship, and ship and shore. Its value in enabling intelligence to be transmitted from lightships or lighthouses to coast stations cannot be over estimated. One very remarkable feature of the apparatus as arranged by Mr. Marconi is the small space it occupies. It is in this respect most admirably adapted for use on board ship. It only requires a long, insulated, vertical wire which can easily be suspended from a mast, and the whole receiving and transmitting apparatus can be placed on board ship in a small cabin. Employing the sensitive tube and Marconi receiving arrangements, messages can easily be sent 150 miles over the sea-surface by means of an aerial 150 feet high and a 10-inch induction coil.
It is a curious fact that better results are obtained over a water-surface than over land. Two similar stations with the same appliances can communicate at two or three times greater distance if they are separated by sea than if they are on land and have no water between. This is connected with the fact that electric waves are not able to pass through sea-water, but can diffuse through dry earth. The sea-surface acts somewhat like an optical reflector or mirror, and the electric waves glide along its surface. The rotundity of the earth within certain limits hardly makes any perceptible effect upon the ease of communication. The waves sent out by the transmitter of a long-distance wireless station are from 3000 to 20,000 feet in length, and there is, therefore, a considerable amount of bending or diffraction. It is a familiar fact, as already explained, that a wave-motion, whether on water or in air, spreads round an obstacle to a certain extent. Thus an interposing rock or wall does not form a sharply marked sound-shadow, but there is some deflection of the air waves by the edge of the obstacle. The amount of bending which takes place depends on the length of the wave.
If we take two places on the sea-surface 200 miles apart, the surface of the sea at the halfway distance is just 1¹⁄₄ miles above the straight line joining the places. In other words, the rotundity of the earth interposes a mountain of water 1¹⁄₄ miles high between the places. The electric waves used in wireless telegraphy have a wave-length of about 600 to 1000 feet, or say five or six to the mile. Hence the interposition of an object, the height of which is one-fortieth of the distance, is not sufficient to make a complete electric shadow. If we were, for instance, blowing a trumpet creating air waves 5 feet long, the interposition of a cliff between two places a mile apart, but so situated that the cliff protruded to the extent of 40 yards across the line joining them, would not cut off all sound. There would be diffraction or diffusion enough of the air waves to enable the sound to be heard round the corner. In the same manner the electric waves are, so to speak, propagated round the corner of the earth. More remarkable still, they have been detected, when sufficiently powerful, at a distance of 6000 miles from the generating station, and in this case they must have travelled a quarter of the way round the earth.
A good conception of the relative speeds of water waves, air waves, and æther waves can be gained by considering the time each of these would take to cross the Atlantic Ocean, travelling in its own medium. Suppose we could, at the same moment, create a splash in the sea near England sufficiently great to cause a wave which would travel over the surface of the Atlantic at the speed of many ocean waves, say at 30 miles an hour. To cover a distance of 3000 miles this water wave would then require 100 hours. Imagine that we could, at the same moment, make a sound loud enough to be heard across the same ocean, travelling at the rate of 1100 feet a second, or about 700 miles an hour, the sound wave would cross from England to the coast of the United States in about four hours. If, however, we were to make an æther wave it would flit across the same distance in about the sixtieth part of a second.
If you have been able to follow me in these descriptions, you will see that the progress of scientific investigation has led us from simple beginnings to a wonderful conclusion. It is that all space is filled with what we may call an ocean of æther, which can be tossed into waves and ripples just as the air we breathe is traversed in all directions by aerial vibrations, and the restless sea by waves and ripples on the water-surface. We cannot feel or handle this imponderable æther, but we have indubitable proof that we can create waves in it by suddenly applying or reversing something we call electric force, just as we are able to produce air or water waves by the very sudden application of mechanical force or pressure. These æther waves, when started, not only travel through the ocean of æther with astonishing speed, but they are the means by which enormous quantities of energy are transferred through space.
From every square yard of the sun’s surface energy is cast forth at a rate equal to that produced by the combustion of eleven tons of best Welsh coal per hour, and conveyed away into surrounding space by æther ripples, to warm and light the sun’s family of planets. Every plant that grows upon the earth’s surface is nourished into maturity by the energy delivered to it in this way. Every animal that basks in the sunlight is kept warm by the impact of these æther waves upon the earth. All the coal we possess buried in the earth’s crust, and in this age of steam forming the life-blood of the world, has been manufactured originally by æther ripples beating in their millions, in long-past ages, upon the vegetation of the primeval world.
But in another way the æther serves as a vehicle of energy—in the form of an electric current. Every electric lamp that is lighted, every electric tram-car that glides along, is drawing its supply of energy through the æther. The wire or conductor, as we call it, serves to guide and direct the path of the energy transferred; but the energy is not in but around the wire. We have lately learnt to make what we may best describe as billows in the æther, and these are the long waves we employ in wireless telegraphy. But in telegraphy, whether with wires or without, we are merely manipulating the æther as a medium of communication, just as in speech or hearing we use the air.
We therefore find our physical investigations lead us to three great final inquiries, when we ask—What is the nature of electricity, æther, and energy? Already, it seems possible, we may obtain some clue to an answer to the first question, and find it in a study of the electrons, or tiny corpuscles which build up atoms. Concerning the structure of æther, physical investigation, which has revealed its existence, may be able to analyze a little more deeply its operations. But the question, What is Energy? seems to take us to the very confines of physical inquiry, where problems concerning the structure of the material universe seem to merge into questions concerning its origin and mystery. In its ultimate essence, energy may be incomprehensible by us, except as an exhibition of the direct operation of that which we call Mind and Will. In these final inquiries into the nature of things, the wisest of us can merely speculate, and the majority but dimly apprehend.
We must not, however, travel beyond the limits of thought proper for these elementary lectures. Their chief object has been to show you that the swiftly moving ocean waves, which dash and roll unceasingly against the coast-lines of our island home, are only instances of one form of wave-motion, of which we find other varieties in other media, giving rise to all the entrancing effects of sound and light. In these expositions we have been able to do no more than touch the fringe of a great subject. Their object will have been fulfilled if they have stimulated in you a desire to know more about these interesting things. Every star and flower, every wave or bird that hovers over it, can tell us a marvellous story, if only we have eyes to see, and ears to hear. We may find in the commonest of surrounding things a limitless opportunity for intelligent study and delight. When, therefore, you next sail your boat upon a pond, or watch ducks or swans swimming, or throw stones into a pool, or visit the seaside, may I hope that some of the matters here discussed will recur to your minds, and that you will find a fresh meaning and new interest in these everyday objects. Yon may thus, perhaps, receive an impulse attracting you to the study of some chapters in the “Fairy Tale of Science,” more wonderful than any romance woven by the imaginations of men, and open to yourselves a source of elevating pleasure, which time will neither diminish nor destroy.