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[ The title illustration gives a pictorial representation of the troposphere, the lowest of the layers of the atmosphere. In it are generated practically all the atmospherics which trouble wireless reception, for it is only in this region that air is dense enough and winds strong enough to cause intense electrical discharges.
A typical summer thundercloud is shown with its accompaniment of lightning and electrified rain. On the ground we must imagine the wireless station as competing with the noises from unsilenced electrical machines of all kinds. This region is filled with wireless waves pouring along the surface of the earth and forming a wireless atmosphere which is densest in the troposphere. ]
[ THE purpose of this article is to explain in a simple way the various factors which affect our reception of wireless signals Chief among these factors is the atmosphere, whose many layers, sensitive as they are to the influence of solar radiation and to invaders such as meteors, cosmic rays, and electrified particles of all kinds, control reception at medium and long distances.
Indeed, our knowledge of the upper atmosphere, in those rarified regions which man can scarcely hope to reach, is chiefly due to wireless waves, which, as they return bent and twisted to earth, reveal the story of their adventures in the high sky a hundred miles above us. ]
It is now well known that the successful transmission of wireless waves over long distances is due to a strange series of events which takes place in the upper reaches of the great ocean of air which is flung around the earth.
The air which we breathe and explore in balloons and aeroplanes has no influence on wireless waves. It is transparent to them just as it is transparent to the light of the sun or to the cosmic rays which stream in from outer space and pass through unhindered. Save for transient disturbances due to lightning and to the effects of electrified drops of rain falling upon aerials, our reception of wireless programmes is unaffected by the atmosphere at these low levels.
But high in the sky, far above the highest level to which a balloon has ever carried a human being, lie the regions which have a profound influence on the path of wireless waves. From fifty to a hundred miles above our heads, where the air is so thin that the most delicate barometer would fail to detect its pressure, and where molecules of gas are so few that one of them may travel for a mile before colliding with another, there are layers of intense electrification which serve to prevent the escape of all except the very shortest waves.
Two of these regions are more permanent than the others. The lower one is the Heaviside layer, and fifty miles farther up lies the Appleton layer. Though permanent in the sense that they never fade away completely, we must not think of them as rigid and burnished mirrors set in the sky. We may with greater truth regard them as tides in the high atmosphere, waxing and waning as the earth turns between day and night, varying with the phases of the moon, sensitive to those gigantic solar convulsions which appear as sunspots, and appreciably disturbed by the showers of meteors which from time to time invade the upper air.
The behaviour of these layers is, as we shall show, largely dependent on events which take place far below in the lower regions of the atmosphere, and since the lower regions are also the seat of disturbances which are of great importance to radio workers it seems worth while to begin at the surface of the earth and try to explain, as we travel farther and farther upwards, the somewhat surprising discoveries which have been made of late years in the domain of meteorology regarded from a radio point of view.
The lowest layer, the troposphere (the word means a region of movement), is the domain of wind and rain, and of clouds and thunderstorms. It extends upwards to a height of about six miles and so surmounts the tops of the highest mountain peaks. In this region the air becomes steadily colder with increasing altitude, the fall in temperature amounting to about 9 deg. .C. per mile, so that at the upper boundary of the troposphere the thermometer may read - 55 deg. C., a degree of cold as intense as is ever reached in Polar regions on the earth's surface. At this temperature all known liquids are frozen, mercury included. This coldness is at first thought surprising, for at an altitude of six miles the sun's rays have an intensity greater than at the earth's surface, since they have not yet suffered absorption by the clouds which lie below, and, indeed, Piccard in his balloon ascent to a height of ten miles found that the sealed metal car in which he was enclosed became uncomfortablv hot.
But air differs from metal in being practically transparent to solar radiations except for the small fraction which extends into the far intra-red spectrum, and since a body can only grow hot by absorbing energy the intense radiation which pours down fails, even on the equator, to warm the air through which it passes. Hence it is the surface of the earth and not the air which is heated by the direct rays of the sun.
It is from this heated surface that the layer of air just above receives warmth, both by direct contact and from the long-waved heat rays emitted by the earth, which, unlike the shorter waves comprised in sunlight, are readily absorbed by air.
The Invisible Man
While on this subject we might make a scientific prediction concerning the Invisible Man described by Mr. H. G Wells. Since this unfortunate creature was perfectly transparent he could never enjoy the warmth of sunlight but must have relied on transmission of heat by contact to his body from the surrounding air and the ground on which he trod.
The warmed air expands and rises owing to its increased buoyancy. As it ascends into more rarified regions it cools both on account of its continuing expansion and because it radiates away its heat; this loss cannot be made good by absorption, at least, not until its temperature falls to about -55 deg. C., for, as we have already pointed out, the absorptive power is small. And so within the limit of the shell of gas called the troposphere there is a steady fall of temperature with increase in elevation.
The radio worker has good reason to consider the troposphere as the home of unwanted radio signals, which are due largely to lightning and to a smaller extent to electrified rain. Rain, fortunately, is strictly local in its effects, and is only electrified if it has fallen through a turbulent atmosphere, but the atmospherics which plague us nightly reach our aerials from thunderstorms up to distances of a thousand miles away. Let us consider the cracklings and muttering; which at times disturb our enjoyment of the gentle music of Schubert as faint echoes from gigantic and brutal Wagnerian compositions roaring over mid-Europe or far out in the Atlantic, and we may then regard them more tolerantly.
Thunderstorms and Wireless
Few people realise how enormous is the power emitted in a thunderstorm or the reasons for the production of the considerable charge of electricity accumulated prior to its discharge as a flash of lightning.
It is not easy to electrify raindrops. Dr. Simpson, in his attempts to produce a thundercloud in the laboratory, found that no electrification was produced by freezing drops of water or by thawing the resulting hailstones, or by friction of falling drops against still air. But when air was blown upwards against the drops so that the relative speed exceeded twenty miles an hour, spray was torn off and the upward-carried spray was charged negatively, while the drops were left behind with a positive charge. Thus, an upward blast of air of sufficient intensity acts as a kind of Wimshurst machine, generating electricity as long as rain is falling.
Now, upward air currents of this kind obviously occur in cumulus clouds, those white, massive, towering structures which are thunderclouds par excellence. For the summit of the cloud is often observed to be in violent motion and to be blown out like the head of a cauliflower. Moreover, the hailstones which frequently occur in thunderstorms must originate as raindrops which, after their formation, are carried up to higher and colder altitudes where they freeze, and to carry an average hailstone upwards an air blast at twenty miles an hour is required.
So far, then, the explanation of the working of the great electric generator is acceptable. The next question is, whence comes the vertical draught?
Imagine a hot, still afternoon when the warmed surface air is drifting gently upwards. Over a moist region the air is more heavily charged with water vapour than over the surrounding drier surface, but in any case the air as it rises enters colder and colder altitudes. At a height of, say, a mile, the moist air becomes cooled to a point where its water vapour reaches saturation and begins to condense as a fine rain. Now, as steam engineers know well, when vapour condenses heat is given out - the latent heat of steam- - to such an amount that for every pound of water formed enough heat is produced to raise five pounds of water to boiling point.
This heat, produced by condensation, has a profound effect on the behaviour of the column of moist air - the cumulus cloud. For the heated air becomes more buoyant than air at the same level outl side the cloud, and wind begins to rush into the cloud from the regions outside and from below. Here, then, is the origin of the updraught which, like air ascending in a heated chimney, blows up along the core of the cumulus, and attains enough speed to disrupt and electrify the falling rain.
The Thundercloud Dynamo
The numerical figures relating to thunderclouds are of startling magnitude. In five seconds a charge of 20 coulombs (equivalent to 4 amperes flowing for five seconds) may be carried up by the ascending spray, thus forming an electric condenser with an upper negatively charged plate and a lower positive plate formed by the positively charged raindrops or by the earth below. In thirty seconds the potential difference may amount to five thousand million volts, whereupon a flash occurs.
The thundercloud machine keeps on generating electricity at this high pressure, and if we reckon on a flash every thirty seconds we arrive at a figure of the average horse power of the stupendous value of four million horse power. Compare with this the performance of the great Battersea power station which (if its third steam turbine has been installed at the date of writing) is capable of 300,000 horse power. One thundercloud equals thirteen Battersea stations!
So, if on a night in which atmospherics tend to drown the wireless programme any reader should feel resentment mounting within him, I would recommend him to read again this article and ponder over the remarkable events and gigantic forces of which he has the privilege to be an auditor. Tout comprendre, c'est tout pardonner.
The Stratosphere, the world of fair weather and intense sunshine.
[ Introducing the stratosphere, a calm, cold, dead world of fair weather and intense sunshine, roofed by the ozone layer where for the first time we encounter an electrified region fore-shadowing those which in still higher regions have a profound efffect in deflecting and spreading wireless rays over the earth's surface. ]
UP to a height of six miles or so we picture a scene of rising columns of warm air and descending columns of cold air, so that there is enough circulation to keep the air thoroughly mixed. As mentioned in the previous article, the direct rays of the sun have but little power to warm the air through which they pass. It is by direct contact with the warm surface of the earth, and to a smaller extent by absorbing heat radiation long wavelength from the ground, that the rising columns of gas are heated. As the air drifts upwards it expands and so steadily becomes colder, and we might at first sight imagine that this process would go on till, on the outskirts of the atmosphere, the temperature would approach the cold of interstellar space.
Absolute zero: 273 degrees below zero Centigrade.
But the temperature does not in reality drop to a level approaching this value, for the simple reason that at a height of about six miles, where the temperature has fallen to - 55 deg C., the heat radiated away per second by the gas is now so small that it is just balanced by the heat absorbed from the heat waves radiating upwards from the ground. This ground radiation is just as strong six miles up as it is at the earth's surface; an unexpected result, perhaps.
Suppose that you are at a certain height and surrounded by a sphere opaque to heat radiation except for a small window which is perfectly transparent. Then you will see a plot of ground from which, and from which only, heat will reach you. Now imagine that you have risen to twice this height. The visible plot increases to four times the area, but since by the inverse square law the radiation received from each square foot of ground is only a fourth of what it was before, the heat pouring in through the window and reaching the centre of the sphere is 4 times one quarter times, i.e. exactly the same as at the lower level.
Now, as we open one window after another the same thing will happen, so that finally, when the whole sphere is removed we have the result that the heat received (and absorbed) is the same at any height, so long as the height is so small (say, 100 miles) that the earth below can still be re garded as a flat surface.
Well, then, the position is that at or above a height of six miles air adjusts itself to a temperature (-55 deg, C.) at which it receives just as much heat as it loses, and accordingly we should find a calm, cold region devoid of wind and cloud, [ This is true most of the time, but occasionally wind and thin cloud have been detected in the lower layers of the stratosphere ] for if the temperature is the same everywhere there is no reason why wind should blow. This is the stratosphere; it reaches up to a height of eighteen miles, and then, owing to the presence of a layer of ozone which we shall describe presently, the temperature once more begins to change.
This constancy of temperature in the stratosphere has frequently been verified by inspection of the readings of recording thermometers carried by pilot balloons which have ascended. as high as eighteen miles.
The Ascent of Man
Till 1931 no human presence had ever invaded the Stratosphere, but meanwhile Auguste Piccard had been constructing a balloon to carry an airtight aluminium sphere capable of holding two men, and in this he hoped to ascend into that unbreathable Arctic air. His first attempt was a failure, but on May 27th, 1931, in the still hour which ushers in the dawn, the anchor ropes were released and Piccard and his companion set off at twenty miles an hour-upward bound for the stratosphere.
He reached a height of ten miles. Nine-tenths of the atmosphere lay below him; the mercury had dropped to three inches. Overhead, away from the sun, the sky was dark, though not dark enough to reveal the stars, and of an intense violet purple ; we do not see this colour from the ground, for it is absorbed by the column of air above us. Europe lay below, visible for 280 miles in every direction, and the Rhine, looking like a rivulet was clearly seen. But it was the silence, the perfect and uncanny silence, that they feared. Silence is so hard to come by in these days that man is uneasy in its presence, and a strange thing happened: the interior of the gondola rose to a tropical temperature, though outside its thin wall was the cold of tbe stratosphere. All day they hung, a speck in the sky, till the chill of sunset checked the lift of the balloon and they drifted down to make a safe landing on a mountain glacier.
[2014 note- today students can send a balloon up to 35 km high, with a web cam, and return pictures from that height, tracking the balloon using GPS satellite signals, monitoring any conditions they wish eg air temperatures....]
Wireless Waves at Low Levels
Let us now look down from the ceiling of the stratosphere and regard the stream of wireless messages which pass in the region below. First there are the waves which follow the curvature of the earth, spreading like a thin film over the surface as if their feet were forced to tread on the ground. It is not on account of any mysterious affinity for the earth's surface that they follow this curving path, but simply because long waves of any kind can turn corners and follow curves more easily than short waves can.
Even light waves, less than a thousandth of a millimetre long, bend slightly round corners, the red waves more than the shorter blue ones. On looking at a distant street lamp through a handkerchief we see a brilliantly coloured pattern due to the unequal bending of different colours round eachthread of the fabric.
Waves of sound several feet in length spread extensively round walls and buildings and curve outwards from the cone of a loudspeaker, while the high notes of short wavelength tend to keep along the axis.
And wireless waves measuring hundreds of metres from crest to crest bend still more easily.
This departure of waves from the straight path puzzles many students. But the difficulty disappears as soon as we begin to think about what a wave really is.
Take the case of a pure musical tone reaching us from a source a mile away. Pulses of compressed air arrive at regular intervals with pulses of rarified air sandwiched in between. Any sheet of air which is all in the same state, say, of compression, is called a wave front, and though each wave front is a spherical surface diverging from the source, we may regard any small portion of it as flat by the time it has travelled a mile. This is a plane wave front, and the first question is : Why does it remain plane as it travels on?
Well, suppose the whole front removed except for a tiny patch. This patch of compressed air in expanding will send out a spherical pulse of compressed air in all directions. Now every part of the wave front sends out a wavelet of this kind, and the result is that after, say, 1/1,000th second the wavelets all touch a new plane 13 inches ahead of the original plane. This is the new wave front ; it has been formed by innumerable diverging wavelets, but the result is just as if the wave consisted of a solid sheet of compressed air travelling broadside on at the speed of sound.
But now suppose an obstacle in the way. At any point P in the region of shadow, wavelets will arrive from the points 1, 2, 3, 4 of the wave-front. If we make the line 2P half a wavelength longer than 1P, and 3P half a wavelength longer than 2P, and so on, then 2 will arrive at P just out of phase with 1, and so these two disturbances will nearly cancel; similarly, 4 nearly cancels 3, and so on, and the intensity at P is much less than it would be if the obstacle were removed, as P sinks deeper into the shadow the cancellation becomes more complete.
Thus there is no sudden transition from sound to silence, but a gradual decrease in intensity as we plunge deeper into the shadow. In other words, sound bends or diffracts round corners. This is true for radiation of any wavelength. Thus, for a note of 1,000 c.p.s., whose wavelength is about one foot, P is ten feet inside the shadow and a hundred feet away from the obstacle.
For wireless waves 100 kilometres long the same proportion of energy is diffracted to reach P, if the distances are 1,000 and 10,000 km. While for given light, wavelength 1/2,000th of a millirnetre the dis tances become tiny, for P is now 1/200th mm. inside the shadow and 1/20th mm. away from the obstacle.
For wireless waves, then, the bending is on a grand scale. For a given angle of diffraction, as the wavelength increases, the point at which the intensity falls to any specified fraction recedes farther and farther away from the obstacle.
The Ground Wave
When the single obstacle is replaced by a continuously curving obstacle, such as the earth's surface, the same kind of bending into the region of shadow goes on. Though the mathematical investigation is formidable the final result is a very simple one and its physical meaning is easily grasped. It turns out that as the wave from the trans mitting aerial pours over the horizon the signal intensity, expressed in (millivolts per metre), falls off in such a way that it can be expressed as the product of two terms.
At the source the rays diverge in all directions so that the intensity varies inversely as the square of the distance, falling to quarter value every time the distance is doubled. But the diffracted rays pour round in a thin film and accordingly the disturbance spreads only in two dimensions, like the diverging ripple from a stone dropped into water; and so the intensity varies inversely as the distance instead of the square of the distance.
The first term gives this law of inverse distance. If the wavelength were infinitely great this term would be sufficient, so that, for example, the intensity 1,000 km away from the transmitter would be one-tenth of the intensity at 100 km. But for waves used in broadcasting the second term is important. It expresses an attenuation which depends on wavelength and is greater the shorter the wavelength.
If we had a receiver with a perfectly linear detector then the loud-speaker intensity would always be proportional to the incoming high-frequency intensity at the aerial, i.e., to (millivolts per metre) squared, and so, we could plot the intensity level in decibels calculated from the diffraction formula at various distances.
The results take no account of losses due to eddy currents induced in the ground; they are strictly true only for a perfectly conducting earth, but it will presently be shown that even for the actual earth they are not far from the truth at great wavelengths.
Ground Wave from Different Stations
Now place our ideal receiver 100 kilometres from a station and turn up the volume control till the loudspeaker output is 130 decibels above the threshold of silence, i.e., an intensity at the limit of the ear's endurance (a marvellous receiver, this). Let us also arrange with the station operator to broadcast on different wavelengths with the proviso that he must keep the station strength so adjusted that the output of our loud speaker (100 km away) remains constant in loudness. Then whisk our set away 1,000 km (620 miles) from the station and note how the intensity falls off.
If the station begins on an infinitely long wave (an impossible state of affairs, but let it pass) the sound intensity level falls by only 10 decibels, a hardly noticeable amount. An 18-kilometre wave, like that from Rugby, is attenuated by 21 decibels - the sound output is now that of two very loud motor horns 20ft. away.
Perhaps it would be better to show -the results in the form of the following list:-
Wavelength: Decrease at 1000km Loudness
Infinite......10 decibels........Absolutely unbearable
18 kilometres.21 decibels........2 loud horns at 20 ft
1400 metres...36 decibels........Riveter at 40 feet
300 metres....54 decibels........Man shouting at 4 feet
50 metres.....90 decibels........Soft radio music
10 metres....146 decibels........Not a whisper
The attenuation of the ground wave increases enormously at short wavelengths.
We are not far wrong down to 1.4 km waves in attributing the observed intensity at distances of 1,000 km to diffraction (i.e., bending) alone.
May 2401, 1935
The Ozone Layer, and Its Protective Action
[THE Wireless waves which run along the earth are attenuated by their efforts to follow the curving surface and by the eddy currents set up in the badly conducting soil. At higher levels we meet the ozone layer which bends down waves of sound just as the Heavyside and Appleton layers bring down the wireless sky waves to earth. ]
We previously showed how the ground wave, the daylight wave which receives no help from the upper atmosphere, travels round the curving surface of the earth. So far we have regarded the earth as a perfect conductor, so that the attenuation is due only to progressive weakening of the ground wave by portions of it continually leaving the surface film and flying off into space, as raindrops are flung off a rapidly revolving wheel. Short waves are more attenuated than long waves because they cannot bend so easily.
We may note that, with such a copper- plated earth, waves could not penetrate into the ground, and an underground wireless set would receive no signals whatever. Actually signals penetrate to a depth of about one wavelength, as is known from recent researches carried out with a receiver in a mine 300 metres deep. Long wave signals were received down to a wavelength of about 400 metres, but waves shorter than this were unable to penetrate.
Absorption by the Ground
But the earth is not a perfect conductor, and the currents induced in it by the ground wave fritter away their energy by warming the soil. So we have here a further degree of attenuation which, while negligible for wavelengths down to twenty kilometres, becomes increasingly important as wavelength diminishes and is pronounced in the medium and short wave broadcast regions. T. L. Eckersley has worked out a theoretical formula, which takes resistance into account and gives the signal strength at any distance from a station of known power and wave- length. His results agree excellently with signal strengths noted from Warsaw 1.4 km. experimental station, and also from the old Daventry 5XX, and with observations in the band 200-500 metres.
[The earth’s resistivity is taken as 10e13 e.m.u., i.e., 10,000 ohms per cubic centimetre.]
It is evident that attenuation is greater with short waves. Thus the signal strength at a distance of 100 km. is reduced to a tenth of its value when we recede to 150, 260, 420 km. for wavelengths of 10 m., 300 m., 1.4 km. respectively.
So much for the ground wave. During the noon hours of daylight in summer the high reflecting layers are out of action, and we must rely on the ground wave alone. It is reliable, does not suffer from distortion or fading, and has the sole disadvantage that its range is limited to a few hundred miles except in the long wave region. I really believe that we should all be much happier if we received ground waves only, unaccompanied by their rather temperamental companions which behave so oddly as they drop down from the night sky.
Let us now resume our position at the top of the stratosphere- 18 miles up (30 kilometres) --and before continuing our upward climb take a last glance at the scene below. The gulf beneath is full of wireless waves of all sorts of wavelengths, but we may be sure that shortwaves, say, 100 metres in length, which may reach us at this altitude will never reach earth again except by reflection from some high ionised layer. For we are 300 wavelengths up, and any wavelets sent down to earth will be completely cancelled by wavelets just out of phase sent down by neighbouring parts of the wave.
For long waves, however, we are still bathed in the ground wave, for we are, for example, barely two wave-lengths up, with respect to the 18 km. wave from Rugby.
The Ozone Layer
Now, as we continue our climb a surprise awaits us. The temperature is rising rapidly! The rise amounts to 10 deg. C. per mile, so that six miles above the ceiling of the stratosphere the temperature has risen from - 55 deg. C. to 5 deg. above freezing point, and four miles farther up it is 45 deg. C. -a tropical heat.
The first suspicion as to this strange state of affairs came from the study of meteors. Meteors usually become visible at 60 miles up and disappear at 50 miles; they glow for a second or two with a brightness equal to that of a star of the first magnitude, and it is calculated that these little spheres of nickel-iron, only as big as small shot, radiate a power of 2 kilowatts during their brief incandescence, like a much overloaded pointolite lamp.
This heat is due to friction in the air, and Professor Lindemann, of Oxford , calculated that if the air temperature above the stratosphere remained at -55 deg. C. the air would be too thin to give the frictional heat produced. But the required density of air could be obtained if the air were warmer, and so he predicted a summer heat above the stratosphere. It may seem wrong to talk of hot air being denser than cold air, for air expands with heat. But if the atmosphere were very cold the air would shrink to a thin layer round the earth, so that there would be very little at a height of sixty miles. Any rise of temperature would cause this layer to expand upwards, so that the density, though smaller at ground level, would be greater at sixty miles.
Now, it is known that a layer of ozone forms the ceiling of the Stratosphere, extending from eighteen to twenty-four miles; there is not much of it, for if it were all collected and brought to ground level the layer would be only three millimetres thick. But its absorption. of ultra-violet light is so strong that it cuts a complete gap in the U.V. solar spectrum, and, indeed, absorbs 6 per cent of the whole solar radiation. Here, then, we have the reason for high temperature in this region of air.
Wireless Waves in the Ozone Layer
This layer is probably ionised, judging by the ionisation which is produced when we make ozone in the laboratory by exposing a flask of oxygen to ultra-violet light. It is the first of the ionised layers which we meet on our upward journey, and we naturally enquire whether it acts as a reflector of wireless signals.
The reply is that no reflection has yet been detected from the ozone layer. No doubt the ionisation is so slight that only very long waves are affected -longer than are used to-day in signalling.
But it reflects waves of sound
Reflection of Sound Waves
It has been established without doubt in recent years that sound may travel twenty-five miles up into the air and come down again. Sound travels more quickly in warm than in cold air (a rise of 100 deg. C, increases the speed by 16 per cent.), so that on entering a warm layer the wave bends downwards like a beam of light entering a glass prism. For a long time it had been known that sound behaved strangely in the atmosphere. The noise of a great explosion had been noticed to die away to zero at a distance of sixty miles and then to reappear at about 120 miles. But it was only when Lindemann deduced from his work on meteors that high temperatures exist above the stratosphere that meteorologists guessed the reason for this zone of abnormal audibility.
The sound at first curves upwards, since, in the troposphere the temperature is steadily falling, In the stratosphere the temperature is constant and the curve becomes straight. In the ozone layer the rising temperature refracts the waves arid they finally come down 120 miles away.
A heavy gun fired at Shoeburyness was heard' at Birmingham, I32 miles away. The time for travel along the surface should have been 620 seconds, but the actual time was 726 seconds. This delay of 106 seconds means an extra path of twenty miles, due to the long curving flight above the stratosphere. Similar results were obtained when a great ammunition dump was exploded at Oldebroek in Holland. Though in the long flight to England the waves of audio frequency were attenuated below the human audible threshold, yet the longer waves which carried most of the energy undoubtedly reached the eastern counties. In Cambridge at the calculated moment windows rattled, dogs barked, and birds flew screaming from the trees.
A Heaviside Layer for Sound
In the reflection and refraction of sound by the ozone layer we have a remarkable analogy with the behaviour of wireless waves in the Heaviside and Appleton layers, which exist at much greater heights. But the mechanism is quite different: in the former case we have sound going faster when warm air is encountered, in the latter wireless waves go faster when the air becomes impregnated with freely moving electrons.
There is, however, a general similarity in the effects produced in the two cases. Round the source a circular region extends in which the ground wave is audible : this is fringed by a zone in which nothing is heard, and beyond this silent zone or skip distance comes a terrain of abnormal audibility, due to waves dropping down to earth from mirrors set high in the sky.
Before we say farewell to the ozone layer let us realise that life on earth is shielded by it from harmful solar radiation.
What, then, if the layer were destroyed by an increase in the amount of hydrogen in the lower atmosphere, which would combine with ozone to form water vapour.
We should then be exposed to rays of the kind which a mercury lamp in a quartz tube emits. Eyes would be inflamed and blindness would rapidly attack us as the lens of of the eye became opaque; painful sores would break out on exposed skin, and probably all life which had not the wit to hide from the hostile sun would miserably perish.
The Ionosphere, the Home of the Heaviside and the Appleton Layers
[Above the ozone layer lies the vast territory of the ionosphere where such visitors from the sun as corpuscles and alpha-particles are captured. It is lit by the streamers of the polar aurora, whose spectrum tells us of the constitution of the air at these heights. Here are the Heaviside and Appleton layers, and possibly others as well, which reflect, wireless waves and make long-distance communication possible.]
THE region which lies above the ozone layer is one which as yet has been imperfectly charted, so that it is impossible to draw accurate maps of the pressure, temperature and composition of the upper atmosphere.
Some information comes from the aurora, whose height has been measured over many years by the use of theodolites at stations spaced far apart. The heights vary from 80 to 600 kilometres, or higher, and at all heights the auroral spectrum shows lines due to atomic oxygen and molecular nitrogen. No lines of hydrogen and helium have been seen, and it is believed that these light gases, which we might expect to accumulate in high regions, have long ago disappeared into space. If this is so the high regions must have a temperature of about 600 deg. C. to give these gases enough energy of motion to escape against the gravitational pull of the earth.
But these things, though of interest to the meteorologist, are not of such great importance to the radio worker as is the state of electrification at high levels, and it is fitting that it is by the behaviour of wireless waves that this information has been obtained.
We are apt to take for granted the existence of the Heaviside and Appleton layers, but the enquiring mind must ask why electrification should exist, and why it should be concentrated in layers instead of being spread all over the upper sky, why these layers rise and fall, and why they reflect wireless waves down to earth.
Causes of Electrification
We know, from experiments in the laboratory that when ultra-violet light falls on oxygen or nitrogen it ejects electrons from the atoms or molecules, thus leaving positively electrified ions while the negative electrons wander about till they are captured by ions. It is only very short waves of light which can ionise air, waves which never reach earth since they are absorbed at high levels. Here, then, we have a reason for expecting ionisation in the upper air, and probably other ionising agents are also at work, such as cosmic rays, meteors, and corpuscles of high velocity ejected from the sun.
We next ask why this electrification is not continuous throughout the atmosphere instead of being found in a series of stratified layers. To this question a simple answer can be given: Take, for example, the waveband which ionises the oxygen molecule, a band which begins at 100uu. Now, when this radiation first enters the atmosphere it suffers little absorption because the air is very thin. But, the air pressure increases rapidly, and consequently the rays are more and more quickly absorbed till at 40 miles up absorption is complete and ionisation ceases.
Hence, ionisation is negligible at 120 miles and at 40 miles, and so it must rise to a maximum somewhere in between. In fact, the ionisation is proportional both to the intensity of the rays and to the density of the gas.
On any reasonable assumptions the ionised layer will always show fairly well-marked boundaries, and any constituent of air which is ionised by solar radiation will give rise to such a layer.
What are these constituents at heights above 50 miles? We have direct evidence from the spectral lines of the polar auroras, which tell us that oxygen and nitrogen are present at all heights. Hydrogen and helium do not appear, and, as we have said, there are reasons for believing that they have escaped into space. Hence O, O2, N, N2 probably form the chief constituents at high levels. Each of these may form a distinct ionised layer.
Actually, there are two permanent layers, the Heaviside at 60 miles and the Appleton at 120 miles, but it is of great interest that Appleton has found in addi- tion two intermediate layers which appear and disappear capriciously, and it may be that these four layers correspond to ionisa tion of O, O2, N, N2. But this is merely surmise, and more information is needed before the layers can be definitely labelled. Meanwhile, let us get down to the question which has probably been in the reader’s mind for some time, a question which writers seldom answer except in formidable mathematical symbols-why are waves reflected from ionised regions?
Bending of Waves in an Ionised Layer
When light falls on a glass plate, it bends upwards by an amount which depends on the refractive index of the glass: the greater the refractive index the greater the bending. From a different point of view the bending occurs because light travels more slowly in glass, and so the wave front swings round as it crosses the surface, In fact, the speed of travel in glass is got by dividing the speed in air by n, the refractive index. Thus, if n=1.5, an average value for glass, the speed in glass is reduced to 1/ 1.5 of the speed in air.
If a glass were discovered with n smaller than unity, then light would travel more quickly in it than in air, and the rays would be bent downwards.
All this is true also for wireless waves which differ from light only in having a greater wavelength. But for wireless waves we can measure n by a totally different method, for Clark Maxwell showed that the refractive index of any material is connected with the quantity K, called the dielectric constant by the relation n= Square Root of K.
The dielectric constant K is defined as follows:
Take an air condenser, and measure its capacity; then fill the air space with, say, a block of ebonite, and measure the capacity again.
Then the ratio of the new capacity to the old is K, the dielectric constant of ebonite. In an insulator whose power losses are negligible the current leads the e.m.f. by 90 degrees, and on entering such a medium wireless waves would bend upwards, as in and would travel on without any attenuation.
But what we require is a medium which bends the waves down, as in the Heaviside layer. How are we to obtain this effect?
Think of a single atom in the insulator of the capacitor, where the atom is a massive block with an electron attached by a spring. As the horizontal electric force applied from the condenser plates oscillates, the electron will be set into forced vibration along the horizontal. As we know, the electrical equivalent of a mass and a spring is an inductance in series with a capacity, the inductance representing the mass of the electron and the capacity the compliance (lack of stiffness) of the spring, and the current due to an applied e.m.f. represents correctly the current due to the electron vibrating under the same e.m.f. applied to the condenser plates.
NOW, if the compliance is small (a stiff spring), the equivalent capacity C is small, and the capacity reactance predominates. Hence, the current due to the electron leads the e.m.f. by 90 deg.
Now, every atom with its stiffly connected electron contributes a tiny capacity of this kind and we see that ebonite introduces a lot of capacities in parallel with the air condenser, and so the total capacity goes up.
Thus we have now a physical picture of what the dielectric constant of ebonite means.
The next step is to imagine the spring in getting weaker and weaker, and finally disappearing, so that the electron is free. As this goes on the capacity C gets greater, and finally is so great that the reactance becomes purely inductive.
Now, the inductive currents lag by 90 deg. behind the e.m.f., and so are opposite in direction to the current due to the maincondenser plates. The total current is reduced by the presence of free electrons between the plates. In other words, the dielectric constant of the condenser is now less than unity.
The plates are only required as a convenient means of applying electric force. But in wireless waves the electric force is present anyhow, so that we can remove the plates, and we are left with ionised air in which the current, though still leading by 90 deg., is smaller than it would be in un-ionised air. That is, the dielectric constant K, and hence the refractive index is less than unity.
So the waves will bend down and we have made an important step towards understanding the behaviour of radio waves in the Heaviside layer.
Reflection of Waves
Now let us see how a ray may be brought down to earth again. If the angle between the upgoing ray and the surface is made smaller, the refracted ray becomes more nearly horizontal, and at a certain stage just fails to enter the layer, but instead runs horizontally along the lower surface.
Just beyond this stage there occurs what is known as total reflection, where there is no refracted ray and reflection occurs as from a perfect mirror, the sort of reflection which in the case of light takes place in the prisms of pris- matic field-glasses .
Again, if the ionisation increases in the layer in an upward direction, as it is sure to do, the bending increases continuously .
Lastly, if the ionisation is anywhere so great that the dielectric constant becomes zero, we get total reflection at that point, even if the ray has been travelling vertically upwards .
To sum up, then, we get reflection when the current due to oscillating electrons just balances the capacity current, and this must happen if the density of free electrons in a layer is great enough.
In fact, this electron density can be calculated by noting the highest frequency at which reflection takes place. It can be shown that at this critical frequency N=0.0125 times f squared, where N is in millions of electrons per cubic centirnetre and f is the frequency in megacycles (millions of cycles per second).
Thus, in a particular case it was observed that wavelengths down to 53 metres were reflected by the Appleton layer, while for shorter waves no reflection took place. The corresponding frequency is 5.7 megacycles, and hence, from the formula N= 0.4, i.e., at that moment the maximum number of free electrons was 400,000 per cubic centimetre.
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