The Why of Wireless

Radio Record, Volume I, Issue 10, 23 September 1927, Unnumbered Page

The Why of Wireless

Interesting Series Setting Out Scientific Facts Simply

(By

Electron

PARTY LI. It has been pointed out that the quantity ‘of electricity stored in a condenser depends on the pressur: between the plates. It also obviously depends on the area of the plates in close proximity fo one another since the crowding or condensing effect is due extirely to this proximity. We can vary the capacity, therefore, by moving the plates’ towards or away from each other. The action of a concenser is very similar to a spring. If a pull is ap--plied to a spring it wiil stretch, and the amount of stretch (within limits, of course, just like the space between the plates of a condenser), is proportional to the force. The interesting thing about a spring is that in every way irs effect is quite opposite to that of a heavy truck. When & pressure is applied the spring moves instantly-in fact, it mast be stretched before any resistance is feli-and then Slows down, whereas that of the weight is to gradually increase in speed. Also, if an oscillating force is applied the spring will stretcl and coimpress with anca

INDUCTANCE AND CAPACITY COMBINED. If then we fasten a truck to a spring we should expect some interesting results, and if we do we will not be disappointed, because the two effects are apposite and will neutralise each other. Ti you can, perform the following experiment, as it will give an excellent illustration of what takes place in every receiving set. Take a fairly flexible spring or piece _ of elastic rubber band, and fasten to it a weight which will cause it to stretch perceptibly. Hold the spring by its upper end and move it up and down slowly. Nothing much will hap-pen-the movement of the weight will be the same as that of your hand. Inctease the frequency gradually and eventually you will find that the movement of the weight is many times greater than that of your hand. At this frequency the effect of the spring is completely neutralised by that of the weight, and the only resistance to be overcome is that of friction. If friction could be climinated entirely the movement theoretically would be infinite.

This is the state of affairs which exists in motor-cars at what is called the "critical speed,’’ when the speed of the éngitie is such that the "springiness" ef the frame and its weight together have a natural frequency equal to this engine speed. At this point any out of balance of the engine will cause a pronounced vibration. The same thing occurs in swing bridges, and frequentty a broken vase in a room will "respond" to the vibrations set up by a patticutar note on the piano. Coming back to the experiment. If the frequency is now still further increased it wili be found that the movement of the weight is greatly decreased. This is because the effect of the weight at the higher speeds predomimates over that of the spring, and acts like a ‘"‘choke."

GET THE RIGHT FREQUENCY. We see, therefore, that there is one frequency at which the arrangement pill vibrate with ease, or, in other words, to which it will "respond." To all other frequencies it will respond feebly if at all, We say that it is *ttuned" to this frequency, and we can alter this frequency by altering either the weight or the stiffness of the spring If the weight is increased the natural or fundamental frequency is fowered, that is, the arrangement will respond to frequencies below that of the original. If the spring is made stiffpr (smaller) the natural frequency is taised. fn the same way, therefore, if we in-. grease the size of the coil or increase the capacity of a wireless set, the qvave-length of the station to which our pet will respond must increase, whereeas if we put in a smaller coil or defrease the capacity we will be able to ‘rear shorter waves. | Im the mechanical model had there been friction in the spring there would have been movement at all frequencies, and also, at the natural frequency, owjug to the fact that the spring is high-

ly stretclied and then compressed the effect of friction is to decrease the extent of swing or ‘"‘amplitude." So aiso in the wireless receiver. If there 1s resistance anywhere of appreciable aniount the effect will be to make the tuning ‘‘flat’’ so that as we turn the condenser dials the signals will become gradually louder instead of suddenly, as should be the case in a sharply-tuned set. Krom this it will be seen that ‘‘flat tuning’ is a fault of the receiving set and not of the transmitter. A poor quality condenser is a frequent soutce ot flat tuning. THE TRANSMITTER, It has been stated that all wireless sets consist of a coil aud a condenser. Some readers will wonder at this, as it is possible to have a crystal set with no variable condenser, ‘This is so, but then in this case we have to consider the whole set and not merely the ‘‘box of tricks.’ If this is done, it will be realised that the aerial wire and the ‘earth’? can and do iorm a condenser, and this type of condenser and coil is used on practically all sets. ‘The transmitting station has also got au aerial like this, and very powerful currents produced in a way which need not concern us at present are ‘induced’ into the coil in the way described in a previous article. ‘These powerful oscillating currents rush up into the aerial and down agai just as they do in all receiving sets, and, as has been mentioned, these currents have a strong magnetic effect like all electric currents. This magnetism rising and falling rapidly round about the aerial causes waves to be produced which radiate in all directions Now it must be understood that the "waves" we speak about in wireless are magnetic waves. ‘hey themselves are not electrical because no electricity can ~

pass througn oraimary air, which is a perlect moulrator. Lis will perhaps vacar up a aimculty wach malty peopac experience, ihey say, "Ow 1s It Lilac tule WITCLESS Call BEL LUTOURL Le 1UsulaoN ON My aeria: wire s’’ Lhey ao nor ocem to thunk that it the ‘wireless’ cau get throuyn several myles ot air it Will uot stop at a coduny of varlisn one-thousanuch of av ich tick, THE RECEIVER. These magnetuc waves travel through Space at a speed of 1db,UUU miles per secoud, and siuice thle dislauce between two successive .waves 1S, in the case o1 4YA, 440 metres (a metre being about dv inches) it can be calculated tuat the trequency, that 1s the number ot waves passig any pot per second, is approximately 70,000. These magnetic waves when they strike the metal of the aerial produce a current im it im exactly the same way aj any other kind of magnetism wojgd iiduce a current in a coil through which it passes. These currents flow up and down the aerial in exactly the same Way as the original currents flowed in the transmitting station’s aerial, If the receiving aerial is tuned to the fre- quency of the transmitter the currents will flow up and down with little hindrance. Even when the receiving station is close to the transmitter the amount of power received is almost inconceivably small, and we must take care that none of it escapes due to faulty insulators or a liigh resistance in the earth connection. But when we are situated several hundred miles from the transmitter it is impossible to hear distinctly, iéat all, so that some method of magnifying or amplifying the received signal becomes necessary. At present, however, we will assume that the current in the aerial is -- —

big enough to operate the telephones directly. MODULATION. There is one main difference between (a) the ordinary domestic telephone, and (b) the wireless telephone, and that is (a) when no speech is being transmitted there ts a steady though small current flowing through the microphone and receiver, and when speech is being transmitted this current is increased or decreased according to the air waves hitting the miscrophone diaphragm. (b) In the wireless transmitter when no speech is being broadcast there is this high-frequency current always flowing in the aerial, and when speech is being transmitted this current is increased or decreased, or as the technical expert says it is ‘‘modulated." This. modulation is a difficult thing to explain to the lay reader, but perhaps an analogy may inake the matter clearer. A visit to the sea shore will enable anyone to see that more than one kind of wave may be om the surface of the sea at one time. We may have, for. example, a heavy swell on the surface of which there may be other waves of smaller wave-length, and it will be noticed that both kinds can exist separately without one interfering with the other. If you could imagine that the little waves ate "carried" on the back of the big wave, as it were, then you will understand what is meant by the carrier wave, that is, the wave whose -function is to provide the means for the audio or audible wave to travel. Another analogy more near the truth may be demonstrated quite easily. Lay a piece of rope ot heavy cord about 15 feet long on the ground in a straight line, take liold of one end in one hand and jerk the hand up and then down quickly. A wave will be seen to run --

along the rope. Try the same experiment, but moving the hand slowly. A wave will not be propagated. lf we now wish to send a slow rise and fall along the rope, the only way we can do it is to emit a series of quick waves gradually increasing then decreasing in "amplitude," the short, quick waves are the carrier waves and the long slow rise and fall which is sent or carried by them is the audio