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WIRELESSREALLY EXPLAINED

BY

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LONDON

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MAUI AND PRINTED IN GREAT BRITAIN T THE MAYFLOWSEPRESS, PLYMOUTH. WILLIAM BRENOON AND SON, LTD.

PREFACE TO THIRD EDITION

FORTUNATELY for mankind, science pro-gresses by leaps and bounds ; at the sametime, this fact involves the re -consideration,from time to time, of scientific theorieswhich have been firmly implanted in the mindof the amateur-in this case I refer moreparticularly to the wireless amateur. Thetheory of relativity has led to considerabledoubt in the minds of a good many scientistsas to whether there is such a thing as theether. On the other hand, there are stillmany who contend that it is impossible toexplain such mysteries as wireless phe-nomena, without the assistance of a hypo-thetical ether, which has served and stillserves (even though it may be on an incorrectassumption) to explain the propagation andreception of wireless waves. \Vhatever maybe the outcome of the new theory, for thepresent, the wireless amateur need notgreatly concern himself as to whether wirelesswaves are dependent upon such a medium ornot, and it would be inappropriate, at thepresent moment, to do more than mention thefact that there is a well-founded doubt aboutthe whole subject.

One of the most striking recent develop

Vi PREFACE

Tx-lents in wireless science is the short wavebeam system, by means of which telephonycan be carried on between England andAustralia. Details of this epoch -markingsystem will be found in a companion volumeof this series.

THE AUTHOR

CONTENTSEMAPTER

I. ELECTRICITY AND MAGNETISM

II. ETHER WAVES

PAGE

I

21

III. HOW A WIRELESS MESSAGE IS DES-

PATCHED .. .. 27

IV. A BRIEF HISTORY OF WIRELESS 32

V. HOW A WIRELESS MESSAGE IS

RECEIVED .. . 38

VI. HOW TO OBTAIN AND ERECT AN

AERIAL .. 44

VII. CRYSTAL RECEIVERS 55

VIII. HOW TO CONSTRUCT A CHEAP

CRYSTAL RECEIVER 64

IX. VALVE RECEIVERS .. 73

X. WIRELESS WAVES AND TELEPHONY 79

XI. GENERAL HINTS AND NOTES .. 83

XII. THE MARVELS OF WIRELESS 88

vii

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WIRELESS REALLYEXPLAINED

CHAPTER IELECTRICITY AND MAGNETISM

IN order to derive enjoyment and satisfactionfrom his wireless set, it is not necessary forthe amateur to acquire a detailed knowledgeof electricity, or to immerse himself in com-plex mathematical formulm and calculationsinvolved in the theory of wireless. But if

would take an intelligent interest in thisfascinating subject, it is necessary for himto understand a few elementary facts aboutit, which are capable of being simply ex-pressed and readily understood. For in-stance, electrical and magnetic phenomenalie at the very root of the science of wireless,and he should therefore understand whatthe connection is.

When an electric current passes througha conductor-say through a wire-it sets upwhat is termed a magnetic field round theconductor. The truth of this statement is

proved every time we ring an electric bell,but it is quite a simple matter for the readerto verify it by experiment. Roll a piece ofstiff paper into the form of a small cylinder

9

10 WIRELESS REALLY EXPLAINED WIRELESS REALLY EXPLAINED 11

a few inches long, and wind a single length ofinsulated copper wire round it from end toend, with the turns close together, but notoverlapping. Attach one end of the wireto one of the terminals of an electric dry cellor of an accumulator, and the other end tothe terminal of a switch, and connect byanother wire the other terminal of the switchto the other terminal of the battery, firstplacing the switch in the open position (Fig. 1).

Then insert a small iron bar or a bundle ofiron wires in the cylinder and switch on,and it will be found that the iron has becomemagnetized, and will attract another piece ofiron, although no current has passed throughit. Upon switching off the current it willbe demagnetized.

Now construct another coil of such a sizethat it will fit outside the first cylinder (Fig. 2),

using very fine insulated wire and a much largernumber of turns, place it outside the first coil(known as the primary coil), hold the two endsin your hands and switch on again. At themoment of switching on, the current in thecoil connected to the battery will cause anothercurrent of higher voltage to flow in the outeror secondary coil, and if you have made asufficient number of turns of wire in thesecondary coil, you will feel a distinct shock,which will be repeated every time you switchon and off. If by some means such as thevibrator of an electric bell you cause rapidmaking and breaking of the primary circuit,

12 WIRELESS REALLY EXPLAINED WIRELESS REALLY EXPLAINED 13

you will receive a succession of shocks follow-ing so rapidly upon each other that you willthink it is continuous. The current set up inthe secondary coil is called an induced current,the act of producing it is called induction,and such a coil is an induction coil.

I cannot too strongly urge the reader tomake this experiment-at a cost of a fewpence-because proper appreciation of thisimportant fact will assist greatly towardsunderstanding certain wireless instruments.

Conversely, if a magnet be pushed intoand out of a coil of wire, it will set up amomentary electric current in the coil everytime it is moved in and out. This can beproved by connecting the ends of the coilwire to a galvanometer instead of to abattery. Exactly the same thing happensif the coil is moved and the magnet is

stationary. But if neither is moved nocurrent flows, showing that it is only whenthe magnetic field is disturbed by the passagethrough it of the coil that current is produced.

Theoretically, the influence of a magneticfield extends to an indefinite distance, butthe useful extent of its influence dependsupon several things: notably the strength ofthe current producing it.

The flow of an electric current is Nature'smethod of restoring equilibrium when naturalforces have been displaced. A dynamo doesnot create electricity and pump it alongwires. What it dues is to create what is

14 WIRELESS REALLY EXPLAINED

known as a difference of potential betweentwo points which are called the poles orthe positive and negative terminals of thedynamo. On electrical instruments thepositive terminal is usually denoted by thesign + and the negative by - .

If we carry a weight up to the top of ahouse, we expend energy in overcoming theforce of gravity attracting the weight to theearth. In other words, we create a differenceof potential. If we drop the weight out ofthe window the force of gravity pulls itdown to the ground. We cannot see gravityand we cannot see electricity, but we knowwhat they can do. A dynamo createsbetween its poles a condition correspondingto that which we create when we lift a weight,but electric current must have a conductorbefore it can flow. If we connect the twopoles or terminals of a dynamo by a wire,electricity flows and restores equilibrium-that is, of course, supposing the dynamo isworking.

If in the wire connecting the positive andnegative terminals of a dynamo or of abattery we interpose a motor or other elec-trical appliance, the current can be made tooperate it.

THE VOLT is the unit of measurement ofthe pressure of an electric current. Thedifference of potential set up by a dynamoor by a battery is not the same in every case,

WIRELESS REALLY EXPLAINED 15

but may vary enormously, and the pressureof the resulting electric current varies accord-ingly, much as water pressure in a pipevaries according to the height of the storagetank or reservoir from which the supply ofwater is derived. But this pressure must notbe mistaken for the amount of current.

THE AMPERE is the unit of measurementof the amount of current passing.

RESISTANCE.-Although an electric currentrequires a conductor to enable it to flow, eventhe best conductors, such as copper, offer acertain amount of resistance to the current,just as, although a pipe is necessary toconvey water from a reservoir to a house,there is friction between the water and thepipe that tends to restrict its flow. The unitof resistance is the ohm. If we allow toomuch current to flow through a wire, thewire offers so much resistance that it becomes.hot: it may become red or white hot and fallto pieces.

Now the three terms volt, ampere, andresistance, although denoting different quali-ties, are interdependent. The amount ofwork that could be done by water flowingthrough a pipe depends upon the waterpressure, the size of the hole in the pipe, andthe frictional resistance of the pipe. Simi-larly, the power value of the current thatflows along a conductor depends upon thepressure (voltage), the amount (number of


amperes), and the size and resistance of theconductor. Knowing these we can calculatethe power value of the current, which isdenoted in watts, 746 watts being equal to

horse -power and 746 watts of current forone hour being equal to i horse -power forone hour. A kilowatt is equal to L000 watts.The following simple equations show how,given any two of the three things-volts,amperes, and ohms-we can find the third,.

Number of amperes

Number of ohms

Number of volts

Also:Number of watts

Number of kilowatts-

number of voltsnumber of ohms

_number of voltsnumber of amperes

=number of amperes x num-ber of ohms

=number of amperes x num-ber of volts

number of amperes x num-ber of volts

1000

To make this quite clear let us apply theequations in the case of a current of 10.amperes and ioo volts.

Then:Number of ohms

Number of watts

100= =10I0

=to X loo= moo

Number of kilowatts =10X-100 -too°


A CONDUCTOR is anything along which anelectric current will flow. All metals areconductors, copper being one of the best.Water is a conductor, but not nearly so goodas metals. The earth also is a conductor.

A NON-CONDUCTOR is any substance whichwill not conduct electricity. Thus stone,glass, mica, dry wood and most non-metallicsubstances are non-conductors. Dry air is anon-conductor.CONDENSERS.-A condenser is an apparatus

comprising very thin sheets of metal, sepa-rated by an insulating or non -conductingmaterial such as glass, mica, etc., and is usedfor storing charges of electricity for a shorttime.

The simplest form of condenser is known asa Leyden jar (Fig. 3). This consists of a glassjar with a metallic coating inside and outside,the inner and outer coverings being separatedby the glass which is called the " dielectric "-which means a non -conducting or insulat-ing substance separating two oppositelycharged portions of a condenser. If one ofthe coverings be connected up by a wire toan electrical machine and charged withpositive electricity, this charge will inducea negative charge in the other coveringthrough the glass. If the first covering bealternately charged and discharged, everytime the change occurs a strain will be setup through the glass-not a strain that will

2


break it, but a kind of impulse or wave ofenergy.

If we have two metal plates separated onlyby air, and charge them in the manner justdescribed, and if, when one plate is fullycharged with positive electricity, a wireattached to it were brought near enough to

MIL; CLASS OR OTHER INSULATING

ELECTRICAL CHARGINGMACHINE

71MATERIAL

METAL.

A SIMPLE FORM OF CONDENSER.'

FIG. 3.

the other sheet, electricity would leapthrough the air across the gap. But air,being a non-conductor, offers so much re-sistance that it becomes hot, and so thecharge of electricity appears as a spark orstreak of light. The greater the differenceof potential or pressure the greater the


distance will the charge leap through theair.

The capacity of a condenser means theextent to which it can be charged with elec-tricity. The larger the area of the metalsheets used the greater the capacity, but thewider apart the sheets are placed the less thecapacity.

DIRECT OR CONTINUOUS CURRENT.-Whena dynamo is at work it does not produce aperfectly continuous difference of potential.It creates differences that follow each othervery rapidly. Consequently the flow ofcurrent is not absolutely continuous, butconsists of a series of impulses, so close thatfor practical purposes we may regard themas continuous. These impulses all travel inthe same direction, and the current is knownas " continuous " or " direct." By insertinga vibrator in the circuit (similar to thevibrator of an electric bell) interruptions canbe effected so that the current travels injerks. The current from primary cells andfrom accumulators is also continuous.

ALTERNATING CURRENT.-This is a typeof electric current in which the impulses donot all travel in the same direction, butalternately in opposite directions due to thedifferences of potential being produced sothat they act first in one direction and thenin the opposite direction, with the conse-quence that the direction of each current


impulse is opposite to that of the one beforeand after it. It is generated by a machinecalled an alternator to distinguish it froma dynamo, which generates direct current.

Both alternating and direct current can begenerated at different pressures. In the caseof alternating current, it can also be generatedat different speeds of alternation called theperiodicity or frequency. For instance, whenused for lighting or power purposes theaverage frequency is 6o changes of direc-tion per second. When used for wirelesstransmission the frequency may be as rapidas 20,000 a second when generated by analternator.

SPEED OF ELECTRICITY. - Electricitytravels with the same speed as that of Lightand of electro-magnetic waves-namely,about 186,000 miles a second-IT milliontimes faster than that of an express train.

CHAPTER IIETHER WAVES

ONCE upon a time it was supposed that,beyond the film of air surrounding the world,space consisted of a boundless vacuumthrough which the sun's light and heatpoured down to the earth in a kind ofshower.

But at last scientists began to examinemore carefully into the reason of things, andcame to the conclusion that in order for thesun's light and heat to reach the worldsome kind of medium is necessary. Amedium simply means something by meansof which something else can take place.Thus water is the medium by means ofwhich sea -waves travel. A medium wastherefore assumed and called " nether " or" ether." The ether is supposed to possesssome very extraordinary properties whichare a little difficult to grasp. Althoughinvisible and transparent, it is supposed tobe more than a million times denser thansteel, and to penetrate and permeate allmatter, and yet not to hinder the passage ofsuns and worlds through it.

All matter, whether metals, rocks, liquidsor gases, consists of minute particles called

21


molecules, far too small to be seen by themost powerful microscope. These moleculesin turn consist of atoms, and atoms ofelectrons. Electricity is also believed toconsist of electrons, so that when we getdown to the electron we are near the basisof everything of the existence of which weare conscious.

We know quite well that sound is a sensa-tion produced by air waves travelling ati,ioo feet a second impinging upon theear drum and producing vibrations whichare conveyed to the brain by auditorynerves.

It is now believed that light and heatproceed from the sun through the ether aswaves travelling at a speed of i86,000 milesa second, which is the proved speed of light.Light waves are reflected by objects to oureyes, on the retina of which images are pro-duced and conveyed to the brain by theoptic nerves, where they produce the sensa-tion of light, colour, and form. Heat waves,which are invisible, produce a different sensa-tion-namely, that of heat. It is not thatthe waves themselves are hot, for they canpass through many feet of ice-cold water,and even then, if focussed by a magnifyingglass, will set fire to an inflammable sub-stance.

But light and heat waves are not the onlykinds of ether waves-there are many others.One of these is electro-magnetic waves which


all travel at the same speed, but vary greatlyin length.

Let us consider now the nature of thesewaves, beginning with the usual illustra-tion.

If we drop a pebble into a pool of stillwater we set up ripples which are waves ofenergy, and which travel in ever-wideningconcentric circles away from the point ofdisturbance. As they extend they becomeweaker and weaker. Whilst they are stillthere, drop another, much larger stone, whichwill make a greater disturbance than thefirst. On the surface of the pond you willthen have two different sizes of waves over-:apping each other, one kind being longerthan the other, from crest to crest. Youmight keep on dropping stones in this manner,and you would see that the waves crosseach other.

It is very important to remember thatthe water does not travel along; it onlymoves up and down. It is only the wavesthat travel.

Of course these are only surface waves.But supposing that at half the depth of theocean we were to explode a charge of dyna-mite: the disturbance would set up wavesthat would travel outwards from the pointof the explosion, like expanding globesinstead of circles. And they would not bequite the same kind of wave. It is more orless in this manner that light waves proceed


from the sun in every direction, and you maycompare, if you choose, waves from the sunwith the skins of soap bubbles one withinanother, almost touching each other andjoined together, and always expanding to anindefinite size. As these vast globular wavesspread outward they strike against anythingin their way. Our little world catches onlythe very minutest fraction of each wave.If the skins were some distance apart butstill joined together, they would correspondto electro-magnetic waves.

Now the electro-magnetic waves whichwe produce in the ether for wireless purposespossess the same essential properties as thosethat reach us from the sun, but there arecertain important differences.

Aerials consist of wires leading up mastsand stretching across to other masts. Intothese wires electric current is allowed to flowintermittently. The wires and the earthcorrespond to the two plates of a condenser(described in the last chapter), and theintervening air or ether constitutes the dielec-tric. Every time the current speeds into theaerial and out again, it produces a strain orpulsation in the ether between the wires andthe earth; in other words, it sets up in it anelectro-magnetic wave. All the waves set offat a speed of 186,000 miles a second, theirstrength depending upon the strength of theelectric current producing them. Theirlength from crest to crest varies according


to other considerations-it would be possibleto set up about L000,000 different wavelengths which could be distinguished fromeach other. But. unlike light waves, theseelectro-magnetic waves do not travel instraight lines. They spread out, it is true,but they follow the curvature of theearth.

The reason for this was a puzzle for a time,until at last a theory was evolved, whichapparently offers a satisfactory explanation,and which such eminent scientists as Dr. J. A.Fleming (the inventor of the thermionicvalve) uphold.

It is well known that tremendous up-heavals take place in the sun's glowingmass. Tongues and jets of flame hundredsof thousands of miles in length shoot out,when eruptions occur, and far beyond theseflames immense quantities of dark gases areprojected into space. Probably the bulk ofthese fall back again to the sun, but it isthought that much finds its way beyond theorbit of the earth, and that large quantitiesof this " dust " become entangled in theearth's atmosphere. Of course we must notthink of this as ordinary dust; it may be ofan atomic character, and too fine to penetratethe lower and heavier part of the air. It issupposed that this " dust " is ionised orelectrified, and that it acts as a screen roundthe world, and prevents our electro-magneticwaves from escaping, and that when they


encounter this screen, they are reflected ordeflected by it, and compelled to travelround the world.

That, briefly, is the principle underlyingthe production of electro-magnetic wavesfor wireless.

CHAPTER IIIHOW A WIRELESS MESSAGE IS DESPATCHED

LET us suppose we want to send a wirelessmessage from London overseas.

The form containing the written words ishanded to an operator before whom is aninstrument that looks like a typewriter,running through which is a long narrow papertape instead of an ordinary sheet of paper.The operator taps the keys, but instead oftyping letters the levers punch small holesin the paper tape. These holes representthe Morse code equivalent of ordinary lettersand figures. You know that words andfigures can be expressed in shorthandcharacters. Well, the Morse code is simplya method of expressing letters and figures,in a manner that enables them to be tele-graphed. One way is by different arrange-ments of dots and dashes, and another bymeans of different spacing and arrangementsof holes in a paper tape.

The tape passes on through another instru-ment known as a Wheatstone transmitter(see Fig. 4 at end of book), through whichan electric current is flowing to a distantwireless transmitting station, such as Car-narvon or Chelmsford. On this instrumentare tiny little projecting rods called peckers,over which the tape passes. When a punched

27


hole comes opposite a rod the rod slipsthrough it. The rods are so connected bylevers with electrical contacts, that theirmovement causes interruptions in the flowof the electric current, and in this way theholes in the tape, which are variouslyspaced, pass on the message to the currentin the form of interruptions correspondingto the Morse code.

Now, the current flowing along the landwires is so feeble that it would be quite uselessto send it up into the transmitting aerial,and therefore, when it reaches the transmit-ting station, it has to hand over the messageto a very much more powerful current. Thisis done by means of big transmitting valves.

As the thermionic receiving valve will bedescribed in a later chapter, and as theprinciple is practically the same, it willsuffice for the present to state that, bymeans of the transmitting valve, the messageis delivered to the current which speedsinto the aerial and produces those electro-magnetic waves already described, which inturn have impressed upon them variationscorresponding to the message.

At some transmitting stations, especiallyin Amercia, high - frequency current of20,000 alternations, produced by alternators,is employed instead of transmitting valves.

It will be convenient at this point to givethe very simple formulm showing the relationbetween wave -length, frequency, and velocity.


The velocity being constant (186,000

miles per second), if we know the frequencywe can ascertain the wave -length, or if weknow the wave -length we can ascertain thefrequency.

Frequency

Wave -length

Velocity in feet persecond

Velocity in feet per secondWave -length in feetVelocity in feet per second

Frequency

=Frequency x Wave -length.

The principal methods of generatingelectro-magnetic waves are the spark, thearc, the valve, and the high -frequency alter-nator. Of these, the valve and high -fre-quency alternator are chiefly employed in bigtransmitting stations. The spark, whichwas the original method, is still used on boardship. In cases where the despatch of mes-sages is almost uninterrupted, the continuouswave system, a long, unbroken train of waveswithout interruption, with the power alwayson, is generally employed; in wireless tele-phony and for the broadcasting of speech andmusic it is essential.

The details of the various systems and theirrelative advantages and disadvantages couldonly be properly explained at considerablelength and in a much bigger volume thanthe present one, but we may now consideranother important feature about wireless


waves-namely, their continuity or other-wise.Supposing that we push a pole into a pool

of still water and draw it out again a limitednumber of times, so as to keep time with thewaves we produce, but that, each time, wepush the stick in a less distance. By so doingwe should produce a train of waves, of whichthe biggest would be the first, graduallydwindling down to nothing. Then, startingagain and again, we should produce oneshort train of waves after another. In muchthe same way, the Hertz oscillator (describedin the next chapter) produced a train ofelectro-magnetic waves which quickly dieddown as the oscillations dwindled away, andthe same thing happened in the case of theearlier types of spark generators used inwireless. The consequence was that therewas a pause after each train of waves, untilthe condenser was charged again sufficientlyto cause another spark and a fresh train ofwaves.

Now such separate trains of dwindlingwaves are not so useful for conveying mes-sages as one long continuous chain of waveswould be. Clearly, if the condenser could beso charged and discharged as to prevent theoscillations of the current from dying down,such a continuous chain could be generated.That meant charging and discharging thecondenser in such a manner as to keep theoscillations of constant strength.

WIRELESS REALLY EXPLAINED 3!It was this problem that led to the inven-

tion of the transmitting valve, which sendselectric current oscillating into and out of theaerial without variation of the oscillations.The high - frequency alternator was alsoinvented and serves the same purpose, butthe alternator is such an expensive machinethat for big transmitting stations it is believednothing but valves will be used in future.


CHAPTER IVA BRIEF HISTORY OF WIRELESS

IT is almost impossible to think of wirelesswithout thinking of Senatore Marconi, for itwas he who first applied electro-magneticwaves to telegraphy on a practical andcommercial scale. But the history of wirelessbegins much further back than that.

In the year 1864, a physicist named JamesClerk Maxwell, as the result of many experi-ments with light and with electricity andmagnetism, predicted that there are suchthings as electro-magnetic waves. He didnot succeed in discovering them, but in 1888another physicist, Professor Hertz, succeededin doing so. The apparatus he employed wasvery simple, consisting of two metal plates,each with a wire and small knob on the endof it (Fig. 5). These he placed so that theknobs were a short distance apart. Then heconnected one plate to the positive terminalof a sparking coil, and the other plate to thenegative terminal, and charged the two plates,one positively and the other negatively, thuscausing a difference of potential betweenthem. When the difference became suffi-ciently great, electricity leaped across fromone knob to the other with such energy thatthe difference of potential was reversed; thecharge over -reached itself, so to speak,

32


and surged backwards and forwards untilequilibrium was restored.

Supposing that we have a long trough withan inch or two of water in it, lift it at one end,and then suddenly lower it to a horizontalposition. The water, in trying to distributeitself equally, will surge to the other end ofthe trough first, then back and forth againand again, until at last it comes to rest.That is what happens when electricity isallowed to discharge suddenly-but elec-tricity oscillates to and fro hundreds ofthousands of times a second.

At a little distance from the apparatus,which we will call the oscillator, Hertz helda loop of wire (Fig. 5) with the two ends justseparated. Every oscillation of the currentset up an electro-magnetic wave causing acurrent to flow in the loop, so that as thecurrent passed the gap there were electricsparks. It must be borne in mind that theloop was in no way connected with theoscillator. But the size of the loop had tobe proportioned to the waves generated bythe oscillator, and the loop had to be heldin a certain position.

This simple apparatus was the foundationof the art of wireless as practised to -day.You may think of the oscillator as thetransmitting station, one of the plates asthe aerial and the other as the earth, thewire loop corresponding to the receivingaerial. And, just as the wire loop had to be


proportioned or " tuned " to the waves itreceived, so our receiving aerials have to beproportioned or " tuned " to the waves ofwireless.

But it was a long, long way from thatfirst apparatus used by Hertz to the wirelesssystems now in use.

Hertz then made further experimentsproving that these invisible electro-magneticwaves could be reflected and focussed bymetal reflectors just as easily as light wavescan be reflected and focussed. He alsofound that they would pass through wood andstone.

Righi, Branley, Sir Oliver Lodge, andothers effected improvements both in appar-atus for generating and for detecting electro-magnetic waves, until at last it becamepossible to generate waves in one room andto ring a bell in another.

Strange to say, it does not seem to haveoccurred to these scientists that such wavescould be applied to wireless telegraphy, untilabout twenty - eight years ago Marconiappeared on the scene. It was not longbefore this brilliant inventor conceived theidea of wireless telegraphy, and with that endin view he effected refinement after refine-ment. Finding that the bigger the oscillatorthe further the waves would travel, in orderto make room for still larger models heelevated one plate and buried the other inthe ground. Then he found that the higher


he raised the elevated plate, the better theresults obtained.

In his first experiment in England hesucceeded in transmitting and receivingmessages at a distance of Ioo yards. Rapidlythe distance was extended, until a year laterit had become io miles. Two months afterthat it was 34 miles, and in December, 1897,a ship was fitted with a wireless set and keptup communications with the shore at adistance of 18 miles. In 1899 messageswere received at a distance of Ioo miles,early in 1901 at 200 miles, and in December,1901, the first Transatlantic wireless signalswere despatched and faintly received.

In due course it was found that a lengthof wire carried up a mast and stretched acrossto another mast served the same purposeas the upper plate of the oscillator, and thatis how the present-day wire aerial came intouse.

Space does not permit of a description of allthe detailed improvements effected, but thespark system of generating oscillating currentmay again be mentioned.

The spark system which was the first, andwhich is still used on shipboard for wirelesstelegraphy, corresponds most nearly to theoriginal Hertz oscillator, in which it willbe remembered, the oscillations of thecurrents of electricity, were effected across anair gap. Of course, there are many morecomplications and refinements, and instead

WIRELESS REALLY EXPLAINED 37of the two plates there is an aerial wire andthe earth, but the principle of generating thewaves is the same.

In the case of high -frequency alternators,one terminal is connected to the aerial andthe other to a plate buried in the earth.There is no need of a spark gap to cause thecurrent to oscillate: the oscillations in thiscase are the alternations of the currentgenerated.

CHAPTER VHOW A WIRELESS MESSAGE IS RECEIVED

Ur to the present, we have been consideringhow electro-magnetic waves are generated,and how a message is virtually impressedupon them. Now let us turn to the otherside of the problem-reception.

In the last chapter was described howHertz first detected electro-magnetic waveswith a loop of wire, in which the waves causeda feeble electric current to flow and to sparkacross the gap, and that the loop had to be acertain size, and the wire of a certain thick-ness, before it would respond to the waves atall-in other words, " tuned " to receive thewaves.

The reader may try the following simpleexperiment for himself : Stretch a piece ofpiano wire, which we will call A, between twofixed points. Then, at a little distance fromit, stretch several other piano wires of varyinglength-some slightly longer, and someslightly shorter than A. Now twang one ofthese other wires smartly. If A does notrespond, tune it by tightening or looseningit by degrees, and when youhave it stretchedto exactly the right extent you will find that,when you twang the other wire, A will beginto vibrate in sympathy. Within reasonable

38

WIRELESS REALLY EXPLAINED 39limits, A can be tuned to respond to any ofthe other wires.

Similarly, before we can detect electro-magnetic waves in the ether, our receivingwire or aerial has to be tuned accordingto the kind, or rather length of wave sentfrom a transmitting station. Obviously, itwould be of no use to tune it for only onelength of wave, because we should then onlybe able to receive messages from one trans-mitting station. For every transmittingstation uses a different wave -length, so as toavoid confusion by keeping the signals fromeach station distinct from all the others.The consequence is that means have to beprovided for tuning a receiving aerial, so that,by turning switches, we may quickly adjustit to suit any of a big range of wave -lengths,and thus " pick up " from any broadcastingstation within range. Such tuning, how-ever, is not done by tightening or looseningthe wire: it is done in a very differentmanner, which will be described in a laterchapter.

It must be remembered that at a trans-mitting telegraph station, what is done is toimpress a message on a train of ether waves.At the receiving end, two things may be done.One is, by means of telegraph receivers(see Fig. 6 at end of book), to punch a tapewith the Morse code, and to run the tapethrough an electrically -operated high-speedprinter (Fig. 6), which types the message


in English letters and figures at a speed offrom 120 to 15o words a minute. This isall done automatically, so that only one manis needed to read the tape as it comes out ofthe printer, and to tear it off as each messagecoming through is completed. It is a verywonderful procedure, but lies outside theimmediate interest of the amateur.

The alternative is to translate the messagefrom ether wave language into that of soundwaves, and to listen to them with a telephonereceiver. In the case of telegraphy, thisrequires a knowledge of the Morse code, whichwill be found at the end of this book.

The wireless transmission of music andspeech comes under wireless telephony,which is more complicated than telegraphy.In this case, at the transmitting or broad-casting station, the sound waves of speechor music cause variations in currents ofelectricity, which in turn set up electro-magnetic waves as already described. Atthe receiving end the procedure is exactlyreversed: the ether waves deliver over theirmessage to an electric current, which in turnreproduces the original sound waves in atelephone receiver.

The receiving aerial corresponds to thetransmitting aerial, but whereas the use ofthe transmitting aerial is for generatingether waves, the function of the receivingaerial is to assist the waves to generate elec-tric current again. At some stations, the


same aerial is used both for transmission andreception.

There are two important points to bear inmind. One is that when the waves encountera receiving aerial they do not travel alongit, as is sometimes supposed. What they dois to set up a difference of potential betweenthe aerial and the earth, thus causingcurrents of electricity to oscillate in thewire.

The other is that, whereas powerful cur-rents of electricity are necessary to generatethe waves, not only do those waves becomeweaker the further they spread out, but anaerial only intercepts each wave at a point,so that it only receives a minute fraction of

the original energy. If you think of eachwave as the skin of an imaginary soap bubble,expanded to many miles in diameter, and ofa pin placed on the surface, you will gain someidea of what a very tiny proportion of theenergy of a wave is intercepted by one aerial.

The consequence is that the currentgenerated in an aerial is so feeble that itneeds a delicate apparatus to detect it.Again, it is an oscillating or alternating cur-rent, and has to be transformed into a directcurrent by cutting out all the pulsations inone direction and only using those in theother direction. And it is so weak that itis only capable of producing a strong enougheffect to actuate telephone receivers withinabout fifteen miles of a powerful broadcasting


station. (It is true that better results mayoccasionally be obtained, but they cannot berelied upon.) At great distances the currentis so weak that it has to be magnified, ormade to impress its variations correspondingto the original sound waves, on a strongercurrent capable of reproducing those soundwaves so as to be perceptible in a telephonereceiver; this is done by means of thermionicvalves, which will be described in anotherchapter.

Every receiving station, then, must com-prise the following equipment:

(1) An aerial, which may consist of a wirestretching up a mast, post, or other suitablesupport, or alternatively may be placedinside a house or building, or may consistof a few turns of wire round a wooden frame(known as a frame aerial).

2. A means of detecting the current pro-duced by the waves in the aerial and con-verting this oscillating or alternating currentinto a direct current : this is called ' rectify-ing " the current.

3. A means of tuning the aerial, so thatit can be made to respond to any wave-length employed by broadcasting stationswithin range.

4. A telephone receiver for listening -in.5. The earth (which is always there).More ambitious amateurs possess valve

receivers, which not only rectify but magnifythe incoming current from the aerial. and


enable messages to be picked up from muchgreater distances, and to employ a loudspeaker, so that any number of persons sit-ting in a room may enjoy broadcast musicwithout the use of telephone receivers.Some amateur stations can pick up signalsfrom America. And in big commercialstations there are many complications whichdo not come within the scope of this book.

CHAPTER VIHOW TO OBTAIN AND ERECT THE AERIAL

STRICTLY speaking, this chapter ought tobe devoted to further explanations of howreception is effected, but if the reader hascarefully followed what has already beenstated, it is possible now to begin a practicaldescription of how to instal his receivingstation at the lowest cost. explaining, as wego along, the reasons for the various require-ments.

First of all, however, it is necessary tosecure a Post Office licence, for which applica-tion must be made at the General Post Office.This licence, which costs ios. and has to berenewed yearly, only permits you to indulgein a receiving set. If and when ambitionprompts you to try your hand at wirelesstransmission, you will have to apply for aspecial licence, which you may have adifficulty in obtaining, but which fortunatelydoes not concern us now.

Let us start with the aerial: first, the out-door, and then the indoor type.

The longer and higher the aerial, the betterthe results we shall secure. Unfortunately,the Post Office regulations limit the totallength of wire used by an amateur to ioo feetfor a single wire, although greater lengths are

44


permissible if more than one wire is used andthe wires are placed parallel with each other.

A single length of ioo feet is cheaper andgives better results than wires side by side,although, when space is limited, shorterlengths have to be employed. We will, there-fore, take the case of a single wire.

The wire should be of copper, and consistsof 7 wires of 22 gauge, stranded or twisted.The cost of zoo feet is about three shillings.It is well to obtain a few spare yards in caseof emergency. Do not cut the wire unlessand until it is absolutely necessary; thefewer joints made in it, the less the chance oftrouble. A single wire may be used, but astranded one is less liable to break.

Now comes the problem of erection. Aseverything depends upon the size of thegarden, and the facilities for supporting theaerial, I cannot do better than give a fewillustrations (Figs. 7 to 12) of how anaerial may be fixed, and describe one ofthem. It may be mentioned, however, thatwhen one's garden is too small, it is some-times possible to obtain a neighbour's per-mission to support one end of the aerialon his house, the other end being supportedon your own; indeed, sometimes neighboursshare an aerial in that way. In such a case,two chimney -stacks are the best positions,providing you canreach them to fix the attach-ments. Again, you may sling one end froma tree or a fence-I have even seen a woodel

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48 WIRELESS REALLY EXPLAINED WIRELESS REALLY EXPLAINED 49clothes -prop used ! If you choose a tree,be careful to make sure that none of the twigsor leaves can possibly touch the aerial wire,even in a high wind. The disadvantage of atree, or any support that sways in the wind,is that it may break the aerial.

One of the supporting wires should betightened up, so as to keep the aerial justtaut without unduly straining it.

Supposing that you are going to use yourreceiving set in a room on the ground -floor,and that it takes 3o feet of wire from thereceiver to where the aerial is supported:that will leave a horizontal length of 70 feetof wire.

You will need two insulators of which thereare different types. varying in price fromthreepence to a shilling or two. The cheaperones are of porcelain, which do not insulatequite so well.

The best insulators have two holes in them(Fig. 13), or a ring at each end. The aerial wireis passed through one of these holes or rings,and twisted securely so that it cannot workloose. Through the other hole thread a pieceof stout galvanized iron wire, and fasten theother end of this wire to the support in anyconvenient manner-there is no magic aboutit. But make sure that it is secure, becausea strong wind causes an aerial to exert aconsiderable pull. Pass the other end of thehorizontal portion of the aerial wire throughthe other insulator, and secure the insulator

4


to its support, so that the end of the aerialhangs down near the window of your room.This portion of the aerial is called the down -lead. Be careful that the wires on which theinsulators are slung do not touch the aerialanywhere. The very object of the insulatorsis to prevent the aerial wire from touchinganything else.

Remember that there should be the greatest

FIG. 13.

possible clear height from the ground to theaerial. If possible, avoid passing it over ahouse or building, because any wire, metal,or water in a house acts as the " earth,"and so reduces the effective height of theaerial.

To bring the downlead into the house, borea hole in the window or door -frame, and insertin this hole a tube made of some insulating


material, such as ebonite, porcelain, or glass,slightly longer than the thickness of the frame.The lower end of the download may then bepassed through this tube into the room: thisis called the lead-in. Care should be takenso that wet cannot get through the hole.A better but more expensive arrangement isa special lead-in insulator, with a screwterminal at each end for attaching the wiresto. The price is about is. 6d.

The best form of aerial is one in which thedownlead is continuous with the horizontalportion thus r: this obviates the necessityfor making a joint in the wire. Sometimes,however, it is more convenient to join thedownlead to another part of the horizontalwire: in that case the joint must be in thecentre, so as to form a T with two equal arms.

When there is not room for a single wireaerial, two parallel wires may be employed,as already mentioned. In that case, if thedownlead is, say, 3o feet long, each of thehorizontal wires may be of any convenientlength-say, 55 feet each. Near each end,they must be distanced by means of a stick,if possible, about 6 feet long, so as to keepthem well apart. The downlead must bejoined to each of them at the centre of thespan or at one end. The two -wire aerial is,of course, more troublesome and expensiveto fit up than a single wire.

A house in a valley surrounded by hillsis unfavourably situated for wireless recep-


tion, because high ground surrounding anaerial in a hollow space tends to screen itfrom the waves. In some places it is impos-sible to detect them at all.

A single wire aerial, erected as abovedescribed, should not cost more than aboutsix shillings all told, whereas a mast alonemight cost several times as much.

In cases where it is impossible to erectan outdoor aerial-for instance, if youhappen to live in a flat-indoor aerials maybe used instead, but it may be stated atonce that if you possess a crystal set recep-tion will only be obtained with an indooraerial from a broadcasting station withinfrom 2 to possibly 4 or 5 miles. But withvalve sets indoor aerials will give satisfactoryresults up to hundreds of miles.

Questions that every beginner naturallyasks are: " What is the range of a crystalset ?" and " What is the range of a valveset ?"

Unfortunately it is quite impossible toanswer those questions without explanation.

The answer depends upon the size of youraerial: whether it is of the outdoor, indoor,or frame type; the power of the waves fromthe broadcasting station (which vary con-siderably); the sensitiveness and efficiencyof the receiving set; atmospheric conditions;and the degree of skill and ingenuity yetiexercise to obtain the best results.

For instance, with a crystal set a powerful

WIRELESS REALLY EXPLAINED 53broadcasting station might be heard at twicethe distance at which a less powerful broad-casting station cannot be heard at all. A valveset might pick them both up and others atmuch greater distances. With a sufficientlypowerful valve set, an American stationmight be picked up, whereas a weak stationat half the distance might be inaudible.Again, the owner of a receiving set mightpick up signals which another person witha similar set could not detect at all. It isclear, then, that the maximum range canonly be determined by experiment and ex-perience. Fortunately, once you have yourset installed, it is a very inexpensive matterto try a large number of experiments withvarious arrangements of aerials both out ofdoors and in the house.

When, therefore, you see a crystal set atf3 los. advertised to receive at 25 miles,don't imagine that means you will heareverything broadcasted at that distance.That claim is pretty sure to mean the limitof the instrument under the most favourableconditions.

It has already been mentioned that theelectro-magnetic waves of wireless pass quiteeasily through bricks and mortar and wood,and it is that fact which makes indooraerials possible. The wire, however, shouldbe insulated.

There are many ways in which an indooraerial may be erected. It may be stretched


across the ceiling of an upper room, or evencarried round the picture rail of the roomin which your receiver is placed. To makeit longer it may be carried round the ceilingof a loft or attic, brought down through thefloor into a lower room, and carried roundthat room also. Ordinary staples shouldnot be used for fastening the wire, but eyescrews with insulator rings. Walls subjectto much moisture from condensation andvery damp rooms should be avoided.

A frame aerial consists of a few turns ofinsulated wire round the outer edge of twopieces of wood fixed together on a stand, sothat the wire forms a square measuringabout 3 feet or more on the side.

The advantage of a frame aerial is thatit is portable and can be used either indoorsor out, or carried about from place to place.But it is not of much use with a crystalreceiver.

In a later chapter the relation betweenwave -length and length of aerial will beexplained.

CHAPTER VIICRYSTAL RECEIVERS

CRYSTAL receiving sets are not only by farthe cheapest type to make or to purchase,but, once installed, there is no further outlayas in the case of valve sets, which requirebatteries and accumulators that need fre-quent charging, and expensive valves whichdo not last indefinitely but have to bereplaced periodically by new ones.

A crystal receiver is so-called because apiece of crystal is used to rectify the currentsset up in an aerial by wireless waves.

Certain metallic crystals, of which there isquite a variety, possess the curious pro-perty of only allowing currents of electricityto flow through them in one direction, so thatif such a crystal be placed in an electriccircuit in which an alternating current flows,only the current impulses in one directioncan flow through the crystal; by this meanswe obtain a direct current-which is neces-sary for use with a telephone receiver.

The lead-in wire from the aerial is there-fore connected to a terminal on the receiverstand or baseboard, and this terminal iswired up to the detector, at one end of whichis a pointed piece of metal or wire which canbe moved about. Opposite the end of thiswire, a little lump of crystal is mounted in

55


a holder, which in turn is connected up withother parts of the apparatus to be presentlydescribed.

Supposing that the length of our aerial wereproportioned to receive only one wave-length, then all we should need would bea wire connected to a plate buried in theearth or to a water -pipe (known as the" earth "), and another pair of wires con-nected through a crystal detector to atelephone receiver; this would enable us tolisten -in to concerts. etc., from one broad-casting station within range. But as thatwould limit the enjoyment of wireless towhat we could receive from only one broad-casting station, certain other apparatusbecomes necessary to enable us to tuneour aerial so that we may adjust it to suitdifferent wave -lengths from other broad-casting stations.

To effect this there are introduced a con-denser and an inductance.

Let us make the reason for this perfectlyclear.

An aerial wire ioo feet long will onlyrespond to one particular wave -length. Ifwe increase its length it will respond to agreater wave -length, and the more we increasest the greater the wave -length it will respond to.

An inductance consists of a coil of wireconnected to the aerial wire in such a manneras virtually to increase the length of the aerial.

Again, it has already been explained that

1


the aerial wire and the earth constitute thetwo plates of a condenser. Owing, however,to their fixed distance apart, the capacity isa fixed one, and that fact also fixes the wave-length to which the aerial will respond, butif we can reduce its capacity it will respondto shorter wave -lengths: the more we reduceits capacity the shorter the wave -length towhich it will respond. To fulfil this conditiona condenser may be introduced into thecircuit in series with the inductance. Itmight be thought that an additional con-denser would increase the total capacity, but,strange to say, that is not so. The effectof the condenser actually is to decrease thecapacity of the aerial.

It is clear, then, that by introducing thesetwo things, a condenser and an inductance,and providing means for adjustment, bymerely turning switches we are enabled tovary or tune the aerial to receive any ofa large number of different wave-lengths, andthus to respond to waves from differentbroadcasting stations. It corresponds totuning the piano wire (referred to in Chap-ter V.), by tightening or loosening it; if weonly possessed a means of loosening it oronly of tightening it, we could not tune it torespond to the other wires.

Now the tuning of an aerial could beeffected within certain limits, either byproviding a fixed condenser with an un-varying capacity and a variable inductance,


or by means of a fixed inductance and avariable condenser. (A variable condenseris one of which we can vary the capacity atwill, and a variable inductance is one bywhich the effective length of the aerial maybe varied.) But, clearly, if we have both avariable condenser and a variable inductance,the two will give us a greater range of wave-lengths than if only one is variable.

As it is very important to understand thismethod of tuning a receiving aerial, let usillustrate it. In Figs. 14 and i5 A is anaerial wire, C a variable condenser, E awire leading to earth, I a variable inductance(a coil of wire connecting the aerial andearth wires), S a switch, and 0 to T arecontact studs of the switch. To the wire ofI leading off wires are connected at intervals,each wire being attached to one of the switchstuds. X is a short-circuiting switch.

With the inductance switch in the positionS -T and the condenser adjusted to its fullcapacity, the aerial is so tuned that it willonly respond to the shortest waves it canreceive-shorter than if the condenser werenot there. If we move the switch to theposition SO, the length of the aerial isvirtually increased, so that it will nowrespond to waves of greater length. If wenow adjust the condenser so that it is cutright out of the circuit, and short circuitthe switch X, the aerial will be tuned tothe maximum wave -length it can receive,

WIRELESS REALLY EXPLAINED

911111101

If)

59

6o WIRELESS REALLY EXPLAINED

but, of course, to no other wave -length. Itwill be seen, then, that according to wherewe place the switch and how we adjust thecondenser, so we tune the aerial to a par-ticular wave -length.

That is the function of the inductance andof a variable condenser. When we movethe inductance switch from one stud toanother, we alter the tuning of the aerial, notgradually but in steps, so that, although wemay tune the aerial nearlyenough to catch thewaves, it may not be perfectly in tune, and bya movement of a variable condenser in serieswith the inductance tuning will be effected.

The popular type of variable condenserconsists of a set of thin metal sheets, arrangedparallel with each other, forming -circleor disc in plan, and slightly separated fromeach other. Arranged on a central rod, witha handle to it, is another similar set of plates.By turning the handle, these plates can berotated, so that those of one set are quiteclear of those of the other set, or so thatthey overlap or partly overlap each other,but without touching. The two sets areinsulated from each other. When they aremoved, so as to overlap completely, thecondenser acts at its full capacity; whenthey are entirely clear, it practically does notact at all. Intermediate positions giveintermediate degrees of capacity, and that ishow adjustment can be effected.

But there is another function of a con -


denser: it serves to store up electrical energyand to part with it again at the right moment,and it is in this sense that we are largelyconcerned with it. To serve this purpose, afixed condenser is inserted in a wire acrossthe telephone.

Instead of tapping the inductance coil(i.e., attaching wires to it at intervals),sometimes a metal rod is provided with alittle block of metal on it, making contactwith the coil, which can be slid along thecoil so as to make contact at any point(Fig. 16). This gives a fine adjustment, butthe wire is apt to become defective as theresult of wear and tear.

Since the aerial and the earth constitutethe two plates of a condenser for generatingelectro-magnetic waves in the interveningether, it follows that the current must flowto and fro between the aerial and the earth.Consequently there must be a connection tothe earth. This may be made by connectingone of the wires to a water -pipe. Or it maybe soldered to a metal plate buried in theearth, but in such case it should be buried ata good depth-not just below the surfacewhere the soil may become dry, as dry earthis a bad conductor.

We have now described the essential partsof a receiving station, and it remains but tocouple up the telephone receiver to be readyfor receiving any concert or speech from anybroadcasting station within range.

62 WIRELESS REALLY EXPLAINED WIRELESS REALLY EXPLAINED 63For the benefit of those who desire anexplanation of how a telephone receiver

works, a later chapter will be devoted to adescription of it, and of certain essentialfeatures of wireless telephony waves.

The foregoing must only be regarded as anoutline of a very simple receiving apparatus.As the amateur progresses in the art of wire-less, he will find that even in crystal setsthere is a wide variation in design and in thearrangement of circuits, which in somecases are a great deal more complicated thanthat just described. Not very long ago itwas thought that the maximum range ofcrystal sets would never exceed about20 miles, but even now it is said that thereare sets capable of receiving signals atranges of from 5o to zoo miles or more, and itis probable that still further improvementswill be effected.

It is worse than useless, however, for thebeginner to overburden his mind with manydiagrams. An ounce of practice is wortha pound of theory. When he has fitted uphis first modest set and has used and under-stands it, then is the time to look round witha view to development and improvement.

CHAPTER VIIIHOW TO CONSTRUCT A CHEAP CRYSTAL

RECEIVER

BEFORE reading the following description, itshould be clearly understood that it relatesto the construction of a very simple formof crystal receiver, which will not possess agreat range of reception-possibly not morethan about 15 miles.

The cost of purchasing the parts is sosmall that I do not advise the beginner totry to make them himself. When he hasfitted together this set and really understandsit, if his appetite be whetted for somethingon a more ambitious scale, he will havelearnt a lot that will help him to make theparts of another set himself. Moreover, inany case, the telephone receiver must bepurchased, and that is the most expensiveitem in such a receiving set. In pricing theitems, the cheapest prices are quoted, but insome cases alternative prices are given, becauseby giving a little more you will obtain aquality which will enable you to use parts ofthe apparatus again in a more advancedreceiving set, whereas the cheapest onesmight have to be discarded. The choice mustdepend upon your purse. There is oneconsolation: any aerial that gives satisfactionwith this simple crystal receiver ought

64

1


certainly to be good enough for a moreambitious set, either of the crystal or valvetype.

The first thing is to provide a stand orbaseboard. This may consist of any pieceof wood you choose; mahogany or oak lookswell. It may be of any convenient size-say about 12 inches square. It should beplaned on one surface-preferably on bothsurfaces-and have the edges smoothed andbevelled. To the underside of two of theedges screw two strips of wood. Thenvarnish, or stain and varnish, the stand, andlet it thoroughly dry.

You will now require the following:I. A CRYSTAL DETECTOR, complete with

stand, terminals, and crystal (see Fig. 17 atend of book). This may be purchased forabout 3s. 6d., but a much better quality,with the crystal enclosed in a glass tube,can be obtained for from 5s. upwards.2. AN INDUCTANCE TUNING Corr...-The

cheapest type consists of a coil of insulatedwire wound on a short length of cardboardtube, with leads (short separate lengths ofwire) taken from it at intervals. This maybe purchased for a few shillings-the pricedepends upon the range of wave -lengthrequired. For a reason given in Chapter XI.it is better to obtain a cheap unlacquered coilfirst, which should not cost more than 4S. 6d.3. A CONDENSER.-A fixed condenser maybe obtained for about Is. 9d.

5


4. Two SWITCH ARMS FOR INDUCTANCE,AND Two SETS OF STUD TERMINALS.-Sllp-p03ing that there are 18 wire leads from theinduction coil, then you will need 18 studterminals (at gd. a dozen), 4 stops at lid. each,and 2 switch arms at is. 6d. each.

5. FOUR SCREW TERMINALS.-These maybe of the type illustrated at a cost of aboutIod.

6. A few feet of No. 22 insulated copperwire, for wiring up your set (or electric bellwire would do).

7. A PAIR OF HEAD 'PHONES.-Theseshould be of not less than 4,000 ohms resist-ance. They vary in price from about 22S. 6d.up to several pounds a pair.

Adding up the foregoing items and including6s. for the aerial, it will be seen that forabout £2 it is possible to purchase the parts(allowing for the lowest prices), and to instala receiving set and aerial that should enablebroadcast to be received from a distance of15 miles, with a wave variation of from 200to 600 metres. And for about the setwould include a better quality of telephonereceivers and crystal detector, and a variablecondenser, so that practically everythingwould come in usefully in building up a moreelaborate set later on.

The probability is that sooner or later youwill want to cover a greater range of wave-length; in that case you would need to obtainur to make another induction coil. or even

WIRELESS REALLY EXPLAINED 67two more so arranged that you could switchany of the three into or out of service.

The manner in which the various parts arefitted together is shown in Fig. 18 (at end ofbook) and Figs. 19 and 20 on pp. 68 and 69,and the order of procedure may be as follows:

i. If the induction coil has open ends, cutcircular holes in two pieces of wood, so thatthe ends of the cardboard cylinder will justfit into them, and glue the cylinder in position.When dry, screw the pieces of wood to thebaseboard from underneath. Before screw-ing down to the baseboard, bore a numberof fine holes, and pass the leads from the coilthrough them in their correct order.

2. Near each end of the baseboard drilla pair of holes, insert terminals,and screw them up tight.

3. Fix the crystal detector in the positionshown by means of wood screws. (Thereshould be holes in the base of the crystaldetector for that purpose).

4. Fix the condenser to the baseboard inthe position shown.

5. Mark a point where each of the switcharm pin -holes will be bored, and describea circular curve from this point with a pairof pencil compasses, so that the curved lineis centrally under the contact point of theswitch arm. Mark off, on this curved line,at equal distances apart, a number of pointscorresponding to the number of leads fromthe coil, and at each point bore a hole just

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large enough for the screwed end of thecontact stud to pass through. Then pushthe screwed ends of the contact studsthrough these holes. Try with the com-passes whether they are on a truly circularline and equidistant from the centre point.If so, bore a hole, and fix the switch armin position.

6. Tilt the baseboard, and connect eachof the wire leads from the inductance coilto the terminal end of a stud, taking carethat they are connected up in the right order,and that they do not overlap. It is advisableto solder each connection.

7. Connect a length of insulated wire toeach terminal of the crystal, and to eachterminal . of the condenser, and pass itthrough a small hole in the baseboard.

8. Below the baseboard make the wireconnections between the various terminalsas shown.

9. Mark with letters two of the terminalson the baseboard " A " and " E " indicatingaerial and earth. Also the other twoterminals " +T " and " -T " indicatingthe telephone connections. Attach a smalllinen label to each of the two ends of thetelephone wires with similar marks.

in. With an insulated length of wireconnect the inlead from the aerial to theterminal A.

ii. To the terminal E attach a length ofinsulated wire of such length as will permit

WIRELESS REALLY EXPLAINED 71of " earthing " it. Earthing may consist ofattaching the other end of this wire to awater -pipe, or it may be necessary to take itout of a window and solder it to a pail orsome large metallic object buried deep inthe ground.

We will now suppose that all this (whichis really very simple, and requires no scientificknowledge-for any boy can do it), has beendone, and that the great event-our firstlistening-in-is about to take place.

It is 7.55 p.m. Glancing at the daily news-paper we see that at 8 p.m. Madame Pretti-screem's latest thrill is to be communicatedfrom Macaroni Buildings, on a wave -lengthof 40o metres.

Alas and alack 1 How are we to tune -into 40o metres ?

Well, that is the first penalty of buildingup your own set. If you buy an expensiveset, probably you will find that your switchesindicate how to tune -in to different wave-lengths merely by turning them until the handpoints to a certain figure. That would bevery nice and easy, but-it won't teach youmuch. It is what the rich radio -enthusiastlikes.

With your first little home-made set youmay have to try and try, again and again.But if you have patience you will learn moreby so trying than the author could teach youin many volumes of theory.

And if you have faithfully followed instruc-


tions simply by tuning your aerial, you willcatch Madame Prettiscreem's wave -length.

It should be clearly understood thatalthough this set is not the best or eventhe cheapest to make, it is the mostinstructive type to indulge in for the firsttime.

1

I

CHAPTER IXVALVE RECEIVERS

WHATEVER kind of wireless receiver isemployed, it is essential to rectify the currentfrom the aerial before it can be used in thetelephone. As already explained, in thecase of crystal receivers, rectification isperformed by the crystal itself, and thecurrent is then employed directly to actuatethe telephones.

In the case of valve receivers, not only canthe current be rectified by the valve, but itcan also be magnified in strength-hundredsor even thousands of times if required.But let us first describe the valve itself-the " thermionic " or " three -electrode "valve, as it is called, for the invention ofwhich the world is indebted, in the first place,to Dr. J. A. Fleming.

Whilst experimenting with ordinary incan-descent electric lamps, Dr. Fleming came tothe conclusion that when the filament is glow-ing an invisible something is shot off from it.That something. we call electrons-" thestuff electricity is made of." Dr. Flemingthen had some lamps made, in each of whichthere was a small metal plate with a separateterminal, and discovered that by connectingup the terminal of the metal plate and one

73


of the filament wires in such a manner as toallow an alternating current to flow throughit, only the impulses in one direction couldpass, those in the other direction being forthe most part stopped, much as the valveof an inner tube of a bicycle or motor carwill only allow air to pass through it in onedirection. His invention, therefore, cameto be known as the " Fleming valve," andone of the first uses it was put to was torectify alternating current from wirelessaerials, and render it suitable for use in thetelephone. In this respect the valve servesthe same purpose as a crystal.

Later on, Mr. Lee de Forest made animprovement, which consisted of introducinga piece of metal gauze or a coil of fine wirebetween the ordinary filament and the plate.This was attached to a separate terminal,and from this the modern three -electrodevalve was developed. The valve, as nowmade, consists of a glass bulb-exhaustedof air-with a straight tungsten filamententering and leaving the bulb, much as inthe case of an ordinary electric lamp.Around the filament is a little cylinder ofmetal gauze, or a coil of fine wire calledthe " grid," and round the grid is a thincircular cylinder of metal known as the" plate." The filament, grid, and plate donot touch each other, the grid being connectedto a separate tenninal, and the plate toanother terminal, so that there are four

WIRELESS REALLY EXPLAINED 75terminals, two for the filament, one for thegrid, and one for the plate.

Each of the terminals is in the form of aspigot or little round rod of brass, andeach of the four spigots fits into a corre-sponding hole or socket on a wireless re-ceiver stand. The sockets are connectedto wires leading to batteries and other partsof the apparatus. When pushed into theseholes, current from an accumulator lightsup the filament, and as it does so the filamentprojects a stream of negative electrons to-wards the grid and the plate. (There areseveral types of valve, but this is one of thepopular amateur types.)

Now, electricity can only flow as betweenpositive and negative poles. That is to say,that positive and negative mutually attract ;but positive repels positive, and negativerepels negative.

If we connect one end of a wire to the plateterminal of a valve and the other to one ofthe filament terminals with a suitable batteryinterposed, and connect the filament terminalsto another battery so that the filament glows,a rush of negative electrons takes place fromthe filament towards the grid and the plate,the stream of electrons constituting a path orconductor, and allowing the current to flow inthe plate circuit through the valve. And if wethen interpose a source of alternating currentin the grid circuit, owing to the electrons


projected from the filament being "negative,"they are repelled by the negative impulses ofthe alternating current, which only allow im-pulses to pass in one direction, and in thisway rectify the current.

But the valve can also be made to act asa magnifier or amplifier, and to impart thevariations of the feeble oncoming aerial cur-rent to a stronger current in the plate circuit.

If we connect one end of a wire to thenegative terminal of a third battery and theother to the grid terminal, and the positivebattery terminal to the filament terminal byanother wire, we shall have a third circuitin which a negative charge flows into thegrid, and if we can vary this negative chargewe may increase it to such an extent thatit will repel the stream of electrons fromthe filament and prevent them from pas-sing the grid and reaching the plate. Byincreasing the grid charge sufficiently itwill overcome the stream of electronsentirely. On the other hand, if we connectthe positive terminal of the battery tothe grid, the positive charge will attractand assist the flow of electrons. By asuitable arrangement of circuits a strongcurrent in the plate circuit can be madeto oscillate in sympathy with the aerialcurrents. This more powerful current vary-ing just as the aerial current varies, it followsthat all the interruptions in the aerial current

WIRELESS REALLY EXPLAINED 77(which interruptions correspond to the origi-nal sound waves) are impressed upon thecurrent in the plate circuit, and are ultimatelyreproduced in the telephone as air vibrationsor sound waves again. This magnification ofthe feeble aerial currents is called " ampli-fication."

That is the simplest application of thevalve as a detector, rectifier, and magnifieror amplifier of the aerial currents, but thereare other applications involving too manydiagrams and too much explanation forus to deal with here. Suffice it, therefore,to mention that the valve may be usedsolely as a rectifier or as an amplifier, oras both, and that many valves can be usedin a receiving set, each one amplifying stillfurther the current magnified and passedon to it by the preceding valve, until thefeeble aerial current has been magnifiedhundreds or thousands of times. Again, bya certain arrangement of circuits, valves canbe made to remagnify what they havealready amplified. In some cases, a crystalis used as the rectifier and the valve as anampli tier.

Unfortunately, valve receivers entail theuse of an accumulator for supplying currentfor the filament, and batteries (which maycomprise a large number of small dry -cells)for supplying current to the plate circuit.This means a great deal more expense andtrouble than is involved in crystal sets, but


the results are of course far more satisfactory.For instance, using only an indoor or frameaerial, an amateur valve set, comprisingseveral valves in series, will enable signals tobe received in England from America.

CHAPTER XWIRELESS WAVES AND TELEPHONY

IN Chapter IV. it was explained how Hertzfirst discovered that electro-magnetic wavescould be generated and detected, and how,many years later, Marconi invented themeans of applying this great discovery towireless telegraphy.

Curious as it may at first appear, wirelesstelegraphy-the transmission of signals-was a fairly easy matter even over longdistances compared with telephony-thetransmission of speech or music.

In Chapter III. it was explained how awritten message is translated into the Morsecode by perforating a paper tape with certaincombinations of holes, and that by passingthe tape through an instrument called atransmitter these holes cause interruptions inan electric current. Thus every inter-ruption in the current causes correspondinginterruptions in the wireless waves generatedat the aerial, and at the receiving end thecurrent generated by the waves in the aerialis impressed with the same series of inter-ruptions which can readily be reconvertedinto the Morse code and so into a writtenmessage again.

But when we come to transmitting speech,79


the Morse code is of no use to us. Wecannot say that a dot or dash shall representa particular sound. Again, there is aninfinite variety of sounds.

Instead, therefore, of punching a papertape as in the case of telegraphy, in wirelesstelephony-the transmission of sound-ameans had to be found of impressing uponthe ether waves vibrations corresponding tothe sound waves.

To do this necessitates a continuous andunbroken train of ether waves which canbe modulated by means of sound waves, sothat the slightest modulation or merging ofthe sound waves produces a correspondingeffect upon the train of ether waves. Thesecontinuous ether waves are called " carrierwaves."

We will first follow out the process ofordinary telephony from end to end.

A telephone transmitter comprises a littlecase in which there is loosely packed a smallquantity of carbon granules, and throughthese granules an electric current from abattery flows. At one end the carbongranules bear against an extremely thinmetallic plate or diaphragm fitted in themouthpiece of the telephone. By liftingthe receiver off the hook, we close an electriccircuit and a current begins to flow steadilythrough the granules. But the moment webegin to speak, the sound waves of our voiceimpinging upon the diaphragm (which is


exceedingly delicate and sensitive) cause itto vibrate and to press against the granulesbehind it with a force that varies exactlyaccording to the modulations of the soundwaves of our voice. Now, the flow of thecurrent of electricity varies according tohow tightly the granules of carbon arepacked; consequently the varying pressureof the diaphragm upon the granules causesthe flow of the current to vary accordingly.

The electric current passes along land wiresto a receiver, which the person we are talkingto holds to his ear. In this receiver thereis a little electro-magnet energized by thecurrent, and close to it a diaphragm whichis attracted by the magnet. As the strengthof the current varies, the impact alsovaries in strength, with the consequencethat the diaphragm moves to and fro withvarying force. With every such movementthe diaphragm hits the air and reproducesexactly the original sound waves of ourvoice.

In the case of wireless telephony, we beginand end in the same way. But instead ofthe mouth -piece and ear -piece being con-nected by wire, the electric current, uponwhich the modulations of our voice have beenimpressed, is led to a transmitting valve,which causes it to impart its modulations toanother current, thousands of times stronger,which speeds into the aerial. This currentsets UP an unbroken train of ether (carrier)

6


waves, but every modulation in it causes acorresponding variation in the regularity ofthe ether waves. The current produced bythese waves in an aerial also bears the samemodulations, and eventually the currentflowing through the 'phones of our wirelessreceiver reproduces the original sound wavesagain.

The principle on which music is trans-mitted is just the same.

A loud speaker is merely a means of magni-fying and distributing the sound waves atthe receiving end, and is not an essentialfeature of wireless, although a good loudspeaker may be a useful one, in that it enablesany number of persons in a room to hear,instead of only one or two who have to donhead 'phones in order to listen -in.

!

CHAPTER XIGENERAL HINTS AND NOTES

SEE that your aerial is insulated properly,and does not come into contact with anythingelse; also that all joints and wire connectionsare properly made.

Treat your set as you would treat a boxof eggs-it is the safest plan. Rememberthat valves are expensive things to replace.Dropping or knocking a variable condensermay alter the spacing of the plates and renderit practically useless.

Never use accumulators to the end of thecharge. Keep them regularly charged; evenif you do not use them for a fortmght, givethem a small charge now and then. Accu-mulators last years if they are taken care of,but deteriorate rapidly if allowed to rundown. Keep the terminals always clean andthe screws hard up against the ends of thewires.

When you have fixed up your set and haverecovered from the first shock of delight,study the subject of wireless more deeply,carry out experiments for yourself, alwaysaiming at improvements, learn all you canabout wireless developments, and what othersare doing; then perhaps some day you may

83


make a wonderful discovery, for wireless isstill in its cradle.

Do not handle a crystal unless absolutelynecessary, and never do so with greasy hands.Grease and dirt greatly impair its sensitive-ness.

If you are not getting satisfactory resultswhen listening -in with a crystal set, re -adjustthe wire or metal point against the crystal.Most crystals are more sensitive in some spotsthan in others, and you will need to find oneof these sensitive spots to get the best results.Occasionally it is well to dip a clean stiffbrush into alcohol and scrub the crystalgently all over.

Remember that the current coming fromthe aerial is exceedingly feeble. Althoughit oscillates at a speed of, perhaps, amillion times a second, it may possess only amillionth of the strength of an electric torchbattery. Consequently you cannot affordthe least chance of leakage. That is whyinsulators are employed for the aerial, andinsulated wire is used indoors. Use largecopper wire for " earthing," and make surethat all contacts are good, and that the wireis well secured to " earth."

If intermittent, sizzling noises occur, theyare probably due to atmospherics (i.e.,electrical disturbances in the air), and youcannot do anything but possess your soulin patience until they stop. But if they arepersistent and continuous, they are probably


due to something else. It may be due toa loose connection somewhere, or even toan accumulator that has been overcharged,and has not quite stopped gassing.

It is important for the telephone wires tobe attached, so that the current flows in theright direction, otherwise the telephonereceivers will be demagnetized and ruinedin time.

When tuning -in, continue the adjustmentuntil you hear a clear steady note. If youpossess a valve set, and are receiving a con-cert quite well, but to the accompaniment ofhissing sounds, it probably means that yourset is oscillating-i.e., sending out wavesfrom your aerial to the annoyance of otherlisteners. Correct this until you get thesteady note, remembering that you have noright to interfere with the enjoyment ofother people.

Do not expect too much of a crystal set.If you are within ten to twenty miles of asufficiently powerful broadcasting station,you may derive great enjoyment from it,but if you are more ambitious you will needa valve set.

If you invest in a valve set, buy one onthe unit system, so that you can add an extraamplifying unit at any time in case you wantto increase the range or to tune -in to lesspowerful stations.

Never use a gas pipe as an " earth "-thereis risk of fire or explosion in so doing.


There are many other precautions anddon'ts that might be added, most of whichyou will learn by experience.

There is still a good deal of mystery aboutaerials, and we have yet a great deal to learnabout them. Although we know that cer-tain results can be obtained with certaintypes of aerial, experimenters are continuallydiscovering new arrangements, which givesatisfactory results, and-especially if youpossess a two or three valve receiving set-you will find a great deal of fascination inexperimenting with indoor aerials.

You cannot do better than begin by con-structing the crystal set described in ChapterVIII. It is the type which is most readilyunderstood, and when you understand it,you will easily be able to construct otherinductance coils for yourself, to cover otherranges of wave -length. Later on you willstudy the mysteries of vario-couplers, and sogradually work up to a higher class instru-ment. Freak sets, although for the mostpart not very useful, offer an interestingfield for experiment. Freak sets have beenmade smaller than matchboxes.

If at any time you contemplate purchasinga ready-made set, do not believe everythingmakers tell you. Remember that, like otherthings, wireless receiving sets are made tosell, and although there are leading firmswhose reputation depends upon the qualityof what they sell, there is always plenty of


cheap and out-of-date stock on the market-especially of foreign manufacture. Yourbest course is to ask the advice of a friendwho has had experience, as to what to buy,where to go, what to pay, and what you mayexpect in the way of reception.

The principal morning newspapers publishdaily the broadcasting programmes and thewave -lengths of the various stations, so thatyou have merely to tune -in to the rightwave -length at the right time, in order toenjoy the concert, lecture, or children'sbedtime stories.

CHAPTER XIITHE MARVELS OF WIRELESS

THE real wonders of wireless lie, not in themere fact that we can listen -in to musicand speech at a distance without the use ofwires, as in other aspects of this branch ofscience. For there is really nothing to thephysicist more wonderful in wireless tele-graphy and telephony than in telegraphy andtelephony carried on by wire. Nor is itparticularly wonderful to construct freaksets in rings and matchboxes.Let us consider, then, a few of the realwonders of wireless.Into an overhead wire an electric currentspeeds with the velocity of light-i86,000miles a second. To and fro the current

surges, changing its direction hundreds ofthousands of times a second, and with everyalternation sets up an invisible electro-magnetic wave which sets off and in one -sixteenth of a second has struck every aerialin the world, setting up a ripple of electricityin each one tuned to receive it.

In a room in London a woman stands,singing a song. Every note is impressed as88


a modulation in that vast torrent of waves,and in about one five -hundredth of a secondit is heard by thousands of listeners all overEngland. Following one upon another athundreds of thousands to the second, eachwave expands like a vast unseen globe,striking an electrically charged, invisiblescreen thirty or forty miles overhead, whichsends it hurtling downward again and again,until at last it dies out in the ocean ofether.

It is comforting to reflect that wireless isnot merely an amusement for the rich. Itis an amazing reflection that for two or threepounds a wireless set may be constructed orpurchased and that broadcast concerts maybe enjoyed in hundreds of thousands ofhumble homes. And it is an almost ludicrousfact that essential parts of receiving appara-tus are exposed for sale in the street oncostermongers' barrows !

Yet what a terrible waste of energy isthere !-more than enough to convey mes-sages to every spot on earth.

Yes, and that loss of energy. brings us tothe next wonder-directional wireless.

Long before wireless telegraphy and tele-phony were invented it was known thatelectro-magnetic waves could be reflectedand focussed like those of light. And somen set themselves to generate wirelesswaves and direct them in beams, instead ofallowing them to expand in all directions.


And now they carry on telegraphy betweenLondon and the Continent with wirelessbeams, missing out all other stations withwhich they have no wish to communicate.They have even made a " wireless light-house " that revolves and sends out beamsof wireless waves at regular intervals, thatcan be picked up by ships at sea, thoughthey be fogbound and out of sight ofland.

During the war, they found out how tolocate transmitting stations-known as" direction finding," so that guns and aero-planes could be turned against them, and itoften happened that transmitting stationshad to be abandoned or moved to otherpositions in consequence.

The pilot of an aeroplane, lost in darknessor in a dense fog, speaks into his wirelesstelephone. He doesn't waste words, but justenquires where he is-apparently a curiousthing to ask-and listens. In a matter ofseconds the reply conies-more clearly thanif the speaker were seated beside him. Itis the voice of a man in the office of adistant aerodrome, far below in the fog ordarkness, whose businecs it is to listen -infor lost airmen and direct them. In a fewwords he tells the pilot just where he waswhen he spoke.

Wireless control of airships and of vesselsat sea is a fait accompli, and before long itmay be no uncommon thing to see a crewless

WIRELESS REALLY EXPLAINED 91aeroplane or airship rise from the groundand, responding in every movement to thetouch of switches in a control room on theground, set off and deliver mails in Paris orwherever else desired.

The same may be done with ships-asteamer might easily leave an English portwithout a soul on board and cross to France,guided in the same manner.

In the next Great War such applica-tions of wireless will inevitably be utilizedfor purposes of dealing death and destruc-tion.

Wireless is already in one sense a trans-mission of power without wires, but thereare those who believe that it will ultimatelybe employed to transmit sufficient power tooperate motors directly. Here the small-engined glider offers opportunity. Notmerely should we then see aeroplanes inflight controlled from the round, but aero-planes without fuel deriving their drivingpower by wireless from transmitting stationsbelow.

If you tune -in to Eiffel Tower wave -lengthat the right time morning or evening, at10.45 you will receive the time signal. Nottoo interesting perhaps ? Or possibly youthink that signal is sent out for you to setyour watch by ? Well, there are moreimportant watches than yours and mine thatwill be set by it. Far out at sea, fog -boundor storm -tossed and at the mercy of the


waves, a ship is in distress. She has lost herbearings and cannot wireless her where-abouts to would-be rescuers. Suddenly herwireless operator picks up the time signal,and in a few seconds the navigating officerhas found his longitude and knows exactlyhis position east or west.

In the same way the longitude of any placein the world can be found with the aid oftime signals: a receiving operator or anotherman with him could place his hand on theground and say, " Through this spot meridianso-and-so passes," or " This spot is 170degrees 45 minutes 3o seconds east ofGreenwich," and from that the exact dis-tance would be known. Before the adventof wireless that could only be done by layinga telegraph wire to every separate spot to belocated.

And now, what of the future ? Who cantell what wonders there are ahead ?

Wireless is still in its infancy. We livein an age of waves and vibrations-life itselfmay be but a spell of vibrations that riseto a maximum amplitude and then dieaway.

The field for research is a big one and fullof mystery and promise.

The day may come when telepathy-thought transference-will be practised asreadily as wireless telephony; when television-the sight of distant objects beyond the

WIRELESS REALLY EXPLAINED 93range of our eyes now-will enable us to seepeople in far-off countries and to witnessevents there as they occur; and when weshall discover that greatest secret of all-thesecret of life.

1

t

94 WIRELESS REALLY EXPLAINED WIRELESS REALLY EXPLAINED 9J̀

MORSE CODEALPHABET.

FIGURES.I ..... 6 - . .2 . . Elm. MIN 7 ... .3 . ....... 8 ......... . .4 . . .., 9....

5 0. - ...

PUNCTUATION AND OTHER SIGNS.Full Stop (.) . .....Note of Interrogation, or request} . .. .for repetition (?)Note of Exclamation (I) ....... . . - -Hyphen or dash (-) - . . . ...Bar indicating fraction (/') ,. . .Call (Preliminary) ... ..Double dash (separating preamble

from address, address from text, ... . . ....and text from signature)

ErrorEnd of transmission ... . .Invitation to transmit ..Wait . . .Received SignalEnd of Work . .. ..All Stations . . - - ." TR " (prefix for preliminary corre-1

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