The "Why" Of Wireless

Radio Record, Volume I, Issue 9, 16 September 1927, Unnumbered Page

The "Why" Of Wireless

The Scientific Facts Set Out Simply

in this series of articles "Electron" is explaining in simple language the "why" of wireless, with the object of giving readers an understanding of its principles, so that they may intelligently get the best out of their sets.

HOW SOUND WAVES ACT, When a train is shunting several interesting things may be noticed. If the train is stationary and the engine is caused to move backwards suddenly the whole train will not move instantly. The first thing to happen is that the springs of the bufters between the engine and the first carriage will be compressed. This causes the first carriage to move backwards. ‘Then the springs between the first and second carriages become compressed, and the second carriage then moves, and so on. There is therefore a pulse of compression runs down the train at a speed depending on the stiffness of the springs and the weight of the individuat carriages If the engine had moved forwards instead of backwards a tension pulse would have run down the trai at the same speed, the springs being extended ,instead of being compressed lf ont the engine had moved backwards anid then for wards the result would be a compression wave running down the train closelv followed by a tension wave running down at the same speed Note that the two impulses do not cancel each other out. If the engine is now set in a state of vibration or oscillation the results would be a continual series of waves alternately compression and tension running down the train Tf the period of oscillation is great then we Sav that the freauency is small or low and there will be a large distance be tween the successive waves on the train We sav in this case that the ‘‘wavelenoth" is sreat Tf, on the other band the frequency is hioh, then the distance between waves or the wave-leneth is Bmiall In the case otf wireless waves the Bpced of the waves Is high, in the vicmuty of ldt,vuv miles per second, wuereas that olf sound is slow, being onty LLOU teet per secoud Electrical oscillation are produced in a special way to be described later, but sound waves are produced by merely vibrating some mechanical object such as a drum or stretched string quickly enough tor the air waves set up affecting our ears and enabling us to recognise "sound " Notes of high frequency are recognised as "high" | motes. such as the piccolo, flute, soprattu, etc., and low frequency as ‘ow’ notes, as the low notes of

an organ euphonium, bass voice, etc These notes are never "pure," that is, they never consist solely of a single wave of definite frequency, but are alwavs accompanied by other waves of frequencies, 3, 5, 7, or any odd number of times the original and fundamental frequency The difference between two similar notes played by the different instruments, such as violin and cornet, are simply in the number and proportion of. these ‘"‘overtones’" or ‘‘harmonics."" It is difficult to eet a mechanical object to move quicklv enough to respond to these exceedinelv ranid vibrations, with the result that in some Wireless receivers, and in manv gramophones, the "aualitv’? is mutilated, althouch the "notes" are renroduced nerfectly This is particularly the case with the letter "s.°% which consists of verv hich-frequency vibrations, and is bevond the canahilities of all but the verv hest gramenhones. The letter ‘s" is reproduced with ereat clearness hv most wireless sets, thns showine that it is suneriot to the maioritv of gramophones in reproducing power. HOW THE TRANSMITTER TRANSMITS.

. When any person sings, speaks, or plays an instrument, air waves of a special type are set up which travel through the air in approximately Straight lines. When these waves strike any object they make it oscillate at the Bame tate and in the same way as the original cause of their existence. In the telephone the transmitter or microhone consists of a thin metal or caron plate, behind which is a looselypacked quantity of carbon powder An electric battery is connected so as to pass a current through this powder fram the front plate to a block at the ‘back. Normally, this current is small, because the "resistance" of the powder is great, but when the plate is pressed itiwards the particles of the powder be-

img then compressed will make better. contact with ore another, and will allow a bigger current ot flow. ‘Lhe current dowing is theretore proportional to the moyenent of the microphone plate, AND THE RECEIVER RECEIVES When an electric current is flowing through a wire, and that wire is wound round a picce of iron or steel, as in a reel of cotton, the steel ‘‘core’"’ will become magnétised, while the current 1s tiowing, the Notth pole being at one end and the South pole at the other if the core is made of a piece of stecl which is already magnetised, then the current will have the ectfect of either strengthening or weakening the origina! magnet. This is important to know, because in valve sets there is always a fairly strong current flowing in the plate of the last valve, and passing through the phones or loudspeaker. If this current is passed through the speaker in the wrong direction it will have a tendency to demagnitise the magnet core and render the speaker useless. For this reason, therefore, most speaker cords have one of the conncections marked with a red thread, and this should always be connected to the plus side of the ‘B" battery. The telephone consists of a horse-shoe type of magnet, fitted with two little bobbins of wire as described. Over the poles of the magnet is placed a thin sheet of iron, which is always attracted to, but is never allowed to touch, the magnet. When a current passes through the coils the disc is attracted; and it will be seen that when the disc or diaphragm of the microphone moves inwards, that of the recciver moves

inwards also, if the current from the microphone 4s allowed to pass either directly or per the medium of the broadcasting station through the receiver. ‘The receiver will therefore produce sound waves exactly similar to those. which struck tle microphone, WHAT IS "FREQUENCY?" There are sevetal difficulties to the perfect reproduction of music, the most umportant of which is that all vibrating or oscillating bodies have a natura! frequency like that of a clock pendulum, a swing bridge or a string of a piano, These bodies will vibrate most easily when the object causing them to vibrate is itself vibrating at their natural frequency. In the case of a microphone or telephone, this would be disastrous since some notes would be exaggerated, and others would be suppressed. This fault is present in some poor loudspeakers, aud can always be easily recoguiscd by some notes, usually fairly low, always being predominant. It is got over in the transniitting microphone by stretching the diaphragm so tightly that its satural frequency is very ligh-almost at the upper limit of the waves which the human ear can recoguise, and as this is where most receivers are comparatively insensitive, the slight accentuating of these waves is an advantage. Loudspeakers get over the disadvantage by doing away with the magnets moving the disc di-rectly-they have the piece which its operated by them in the form of a very stiff flat spring, which has the same effect as the stretched diaphragm of the microphone, and the diaphragm is now

very thin and light often in the form of a cone of thin paper or other light materiat. There is uothing magical about the cone shape. It is merely used because it has more mechanical rigidity than a flat disc, and can therefore be made thinner and lighter, and can therefore respond to the higher frequencies more casily. The adjusting screw on most loudspeakers is to adjust the distance between the magnets and the operating disc or flat spring. The nearer the two are the stronger the magnetic pull, and consequently the londer the signal. If too near, however, the diaphragm will hit the magnets and cause distortion, and by means of the screw we can lave the two close together for weak signals to get maximem strength and yet separate them when getting loud music from tlic local station. induction, Tt has Leen pointed out that when an electric current flows round a coil of wire magnetised is produced. The converse is also true; that is, if the amoant of magnetism pzessing through a coil varies an electro-motive force will be set up in the coil, and a current will flow while the magnetism is changing. What is more, this E.M.F. is proportional both in magnitude and direction to the way in which the magnetism is varying. For example, when the magnet is inserted, the induced current will flow in a certain direction-when the magnet is withdrawn the current will reverse. A very common method of inducing currents is to pass an oscillating current round a coil, and the magnetism produced by this current is made

to pass through a neighbouring coil in which a similar current is thus induced, Chis is not a method of getting something for nothiag, because the current flowing in the second coil or secondary has itself a magnetic efiect which wiil react with the primary. Bven if we have a single coil through which an escillating current is flowing the maguctism produced will produce an oscillating K.M.F. in the coil itself, and this will always be found to oppose the current causing it, For instance, when the main current is rising the induced E.M.F. opposes this rise to a large extent. Note that it cannot altogether prevent the current flowing, bevause the induced E.M I. is due to the fact that the main current (and hence the magnetisin) is increasing. But it ean delay the rise considerably. When the main current is stopped the affect of the induced current is to oppose this stoppage, and it tries to keep the current flowing. This peculiar proverty is very similar to a heayy truck on wheels. When the truck is pushed it wiN not start instantly at full speed, but will accelerate slowly. When flowing at a steady specd it offers little resistance, but when it is required to stop, it will tend to keep on flowing. if an oscillating foree ot low frequency is applied to the truck, it will oscillate without much difhceulty, but if the frequency is raised the truck will be too heavy to respond, and will theretore reniain practically stationary, In the same way a coil of many turns will allow a low frequency current to How through it with little hindrance, but will effectually prevent high frequency currents from penetrating. When the putvose of the coil is to cause as large a delay as possible, and thus to "throttle"? down the current, it is called a "choke" coil. The interesting thing about a choke is that while acting like an insulator or son-conductor to high frequency curlents it will Iet "direct" currents flow through it with little hindrance. The uses of both choke coils and coils inducing an oscillating current into a neicibouring coil are numerous. in wireless, and will be explained in detaig er,

Capacity and Condensers. If two metal plates are placed close together separated by a thin sheet of paper soaked iu melted paraffin wax and connected to a battery, current will flow into the plates for an instant. It scems as if the electricity hoped that the paraffined paper would break down (as it sometimes does if the pressure becomes too great) and allows a continuous current to flow. The elections crowd themselves on the plate in the hope of getting across the gap just as a crowd of people will crowd on the side of a ferry steamer which 1s approaching a wharf The similarity goes further thau this, because if the distance between the two plates is decreased the electrons will pack themscives more and more closely. The ability of an atrangement like this to condense the electricity has led men to call it a ‘condenser,’ and, of course, condensers can be obtained in many different forms, The ‘‘capacity" of a condenser is the amount of electricity it can store when there is a pressure of one volt between the plates. If the quantity tiius stored is one conlomb we say the capacity is one "‘Farad.’’ This capacity is much too large for wircless purposes, and the unit chosen is one-mil-lionth of a Farad, called a ~"microFarad." ‘The usual variable condensers used ir. wireless are of the order of onethousaadth of a imnicro-farad, or .00L m.f, (To be continued.)