The Why of Wireless

Radio Record, Volume I, Issue 11, 30 September 1927, Unnumbered Page

The Why of Wireless

Interesting Series Setting Out Scientific Facts Simolv

(By

Electron

in this series of articles "Electron," who is a Bachelor of Science, is explaining in simple language the fundamental principles of wireless. This is the fourth of what is proving an interesting series, which has yet to be concluded.

f . Ralena and the corborundum Galena Part IV. THE DETECTOR. We have now reached the stage of the medulated or compound wave flowing up and down the receiving set. ‘The question now is-where will we place the ‘phones? If we place them in the aerial or anywhere in the main wire, no signals will be heard. Why? Because’ of two reasons. (1) The telephones have hundreds of turns of wire round the magnets and if that-is placed. in the aerial the result will be immediately to alter the "tuning" of the set. (2) It has been pointed out before that the telephones, consisting as they do of many turns of wire, will form a "choke" coil and will absolutely prevent the carrier wave from flowing. We might get over the first objection by placing the ’plones as shown it figure la, but the second objection is a much greater one. ‘This was the trouble which caused all*the delay to early wireless investigators. Many differetit methods were tried, but the most successful by-far was the crystal. When connected as in sketch 1b, several crystals when properly connected have the peculiar property of allowing electricity to pass fairly easily in otie direction, but almost totally preventing it from flowing in the reverse direction. the Jerystal acts therefore like a nonretutn valve and the result is to reconvert the high frequency modulated current into a low frequency audio current.similar to what flowed in the thicrophone in the studio. This is shown in figure III. The currents are in the first place little independent ‘pulses as shown by the areas shaded vertically in figure TI. These pulses are smoothed ont into the curve under the slant shading of the same figure lst ‘in the same wav as the explosions in the cylinders of a motor-car are Smonthed ont into a steady force. This 13 the reason for the so called telePhong condenser shown in the ficure and ‘frequently recommended in various circuits. . However, it is rarely necesSarv.as the flexible leads to the ’phones Or londspeaker form a condenser themBelvés. , The reason for the crystal behaving mt this way is not clearly known, sever. al reasons. havine been put forward. Several crystals and compositions have been used, but at the present time only wo forms have anv large rvogue-the seld under various trade names and 3s a ‘compound of fead and sulphur, "Being of a slate prev colour. The usual -4nethod of mounting is to fasten the '. ervstal in a‘metal cup with screws, ‘ clams, or by means of a metal of low melting point. The other contact is v means of a wire of verv small diameter. In fact, the smaller this contact is the better will the crystal function. Carhorundum or silicon carbide is a manufactured article consisting of carbon and silicon. Its main advantage is that the pressure of the contacts can ‘be auite heavv-several pounds, in fact -so that there is no daneer of the ‘rontacts slipping. This gives us the permanent detector, that is, a detector whose adinstment is made once for all. In the galena crystal the fine delicate contact for the "cat-whisker" has a tendency to slip, and as some points are more sensitive than others ‘this leads to a somewhat tedious tint for sensitive spots. The main disadyantage of the carborundum detector is that -for maximuni sensitiveness a conStatit electrical pressure from a local batterv has to he supplied, and this reduces the simplicity of the set. THE EDISON EFFECT. When Edison was earrvinge ont his ‘experiments with electric lamps he introduced into one of the lamns a little sheet of ‘metal connected with the outSide of the globe, as in figure 4. He found that on connecting this plate to one end of the plowing filament with a wire that a small electric current flowed from the. filament to the wire connected to the plate, but not it the reverse direction. Now. this ig remarkable, because the snace round the lamp is practically a nerfect vacuum, so that it would seem that it fs impossible for electricity to flow | throuch a perfect insulator, ‘

Kidison, however, saw uo immediate use for the discovery, and after merely noting it, proceeded with the perfect-_ ing of the lamp for lighting purposes. | THE FLEMING YVALYE, | Several years later Professor Fleming, | of Iingland, took up the study of this. "Tidison effect,’ and after going into the matter thoroughly came to the conelusion that the reason for this peculiar current was that the glowing filament is so intensely hot that the atoms composing it ate in a state of great agitation something like a pot of boiling water. ‘The result of this intense atomic agitation is that many electrons are shot off into space and many o them finally arrive on the cool metal plate. We have, therefore, a stream nf electrons shot off from the hot filament and arriving on the plate. This, as has been’ explained before, constitutes an electric current and in the conventional way we say the current flows from the plate to the filament. Tt will be remembered that this happening was hinted at in the first of this series. If it is grasped that the elec-

tron flow from the filament to the plate and the conventional direction of the current is in the opposite direction, then the main difficulty has‘ been surmounted, Note that this effect takes place only when the filament is lit. It is the heat which causes the electron flow. Also the effect can only be noticed if the plate and filament are in an almost perfect vacuum, , Fleming, being occupied with wireless, saw at once the immense importance of this new principle. Owing to the stream of electrons passing in one direction only it is clear that if an oscillating electrical pressure is applied to the plate and filament the apparatus. will act as a non return valve just like the crystal, Fig. 5. | The "valve," as it is called, has sev~ eral great advantages over the crystal. (1) There are no sensitive or dull spots, ‘80 that ‘no peatching with ,the catwhisker is necessary. (2) As long as

. the valve is alight we know that the set is ready for reception. (8) The cutrent from the aerial is supplemented by the electron flow from the filament, so that the valve has a magnifying or amplifying effect as well as its legitiimate one of rectifying. OTHER USES OF FLEMING VALVE. The Fleming valve just described was soon displaced by an improvement, but before we ‘go on to discuss this it will be advisable to study other applications of the pure Fleming valve. Its main use is to change alternating or oscillating current into steady or direct current and therefore it has come into prominence as a simple and cheap way of convetting the alternating current of the domestic supply into direct current for charging accumulators. ‘(hese rectifiers go under various trade names, such as ""Tungar," "Rectigon," etc.; but fundamentally they are simple Fleming valves constructed to deliver a fairly heavy current at a low voltage This type of valve is also used in transinitting stations to produce the high direct pressure necessary to oper-

ate the transmitting valves. THE DE FOREST VALVE OR TRIODE. The Vleming valve is sometimes called the two electrode valve because tliere are two separate constituents-the filament and the plate. It was found by de Forest, of America, that the usefulness of the apparatus could be very much increased by adding a third electrode midway between these two. This third electrode is made of wire gauze or a spiral of fine wire, as is shown in figure 6, and from its appearance is called the grid. Tigure 6a Shows the working parts of a small valve with the front half of the plate removed. The plate is always made to surround the filament entirely in order to collect as many of the electrons as possible. Figure 6b shows how the valve is drawn in diagrams for the sake of cleatness, but, of course, this would be very wasteful because of the |

loss of electrons downwards. The efliciency of this type of valve is increased greatly by use of a second battery connected between the plate and the filament as shown in figure 7. This battery is connected with its positive terminal connected to the plate, and this will bé™seen to draw all the electrons towards it. The pressure of this battery may vary from 20 to 100 volts, depending on the valve and the use to which it is put. We have then a stream of electrons flowing steadily from filament to plate, and, of conrse, tliese electrons have to pass through the spaces in the grid. Normally the grid is neutral, that is, it takes no active part in the flow of the electrons, but if the grid is made negative the effect will be to repel the electrons, which will, therefore be unable to pass through to the plate. The three electrode valve therefore is more than a non-return valve. It is rather somewhat like a tap which can turn on or off at will a strong flow of water. . Notice (hat the aerial current does not pass through the ’phones any

( longer-it is merely used to operate the grid, and it should be further noted that the grid is "pressure operated," that is, it does not require current to operate it, but merely a small increase or decrease of electrical pressure. — In one type of valve if the pressure across the plate and filament is 100 volts, it is found that’ a pressure of 10 volts between the grid and filament will be sufficient to .totally stop the plate current, so that a pressure of 10 volts on the grid is equal to 100 of the plate. : We have therefore a means of stepping up the voltage'10 times. _ If now the grid is made positive with regard to the filament, the grid will behave like a smali plate, and will attract still more electrons from thie -filament. The disadvantage af. this state of affairs is that when the grid becomes positive some electrons will flow on to it and will remain there, Note that, when an electron alights on. the grid it will not be shot off towards the |

plate unless the grid is white hot as well as the filament. As this is never the case, it follows that when electrons’ are attracted to the positive grid they will remain there, keeping the grid positive and preventing it from varying in pressure as it should. When the grid becomes positive, therefore the: Signals become distorted, and the mu-~ sical quality is much poorer. Care should always be taken to keep the prid always negative with regard.. to the filament, and this is usually done by meaus of a third battery placed as in figure 8. Tor cottvenience it conversation, the batteries are given distinguishing letters. The battery used for lighting the filament is usually of. a four or six volts, and is called the. "A" battery. ‘The second, for supplying the pressure to the plate of 20 to 100° volts, is called the "B" battery, and the third, for supplying the negative . bias to the grid, is called the "Cc" battery. (‘lo be continued.) °