  Dear Radio Enthusiast: I want to tell you about receiving sets and their tuning. In the last letter I told you what determines the frequency of oscillation of an audion oscillator. It was the condenser and inductance which you studied in connection with Fig. 36. That’s what determines the frequency and also what makes the oscillations. All the tube does is to keep them going. Let’s see why this is so. Start first, as in Fig. 47a, with a very simple circuit of a battery and a non-inductive resistance, that is, a wire wound like that of Fig. 40 in the previous letter, so that it has no inductance. The battery must do work forcing electrons through that wire. It has the ability, or the energy as we say. Now connect a condenser to the battery as in Fig. 47b. The connecting wires are very short; and so practically all the work which the battery does is in storing electrons in the negative plate of the condenser and robbing the positive plate. The battery displaces a certain number of electrons in the waiting-rooms of the condenser. How many, depends upon how hard it Remove the battery and connect the charged condenser to the resistance as in Fig. 47c. The electrons rush home. They bump and jostle their way along, heating the wire as they go. They have a certain amount of energy or ability to do work because they are away from home and they use it all up, bouncing along on their way. When once they are home they have used up all the surplus energy which the battery gave them. Try it again, but this time, as in Fig. 47d, connect the charged condenser to a coil which has inductance. The electrons don’t get started as fast because of the inductance. But they keep going because the electrons in the wire form the habit. The result is that about the time enough electrons have got into plate 2 (which was positive), to satisfy all its lonely protons, the electrons in the wire are streaming along at a great rate. A lot of them keep going until they land on this plate and so make it negative. That’s the same sort of thing that happens in the case of the inductance and condenser in the oscillating audion circuit except for one important fact. There is nothing to keep electrons going to the 2 plate except this habit. And there are plenty of stay-at-home electrons to stop them as they rush along. They bump and jostle, but some of them are stopped or else diverted so Some of the energy which the electrons had when they were on plate 1 is spent, therefore, and there aren’t as many electrons getting to plate 2. When they turn around and start back, as you know they do, the same thing happens. The result is that each successive surge of electrons is smaller than the preceding. Their energy is being wasted in heating the wire. The stream of electrons gets smaller and smaller, and the voltage of the condenser gets smaller and smaller, until by-and-by there isn’t any stream and the condenser is left uncharged. When that happens, we say the oscillations have “damped out.” That’s one way of starting oscillations which damp out–to start with a charged condenser and connect an inductance across it. There is another way which leads us to some important ideas. Look at Fig. 48. There is an inductance and a condenser. Near the coil is another coil which has a battery and a key in circuit with it. The coils are our old friends of Fig. 33 in Letter 10. Suppose we close the switch S. It starts a current through the coil ab which goes on steadily as soon as it really gets going. While it is starting, however, it induces an electron stream Now take away this coil ab with its battery and substitute the oscillator of Fig. 36. What’s going to happen? We have two circuits in which oscillations can occur. See Fig. 49. One circuit is associated with an audion and some batteries which keep supplying it with energy so that its oscillations are continuous. The other circuit is near enough to the first to be influenced by what happens in that circuit. We say it is “coupled” to it, because whatever happens in the first circuit induces an effect in the second circuit. Suppose first that in each circuit the inductance and capacity have such values as to produce oscillations of the same frequency. Then the moment we start the oscillator we have the same effect in both circuits. Let me draw the picture a little differently (Fig. 50) so that you can see this more easily. I have merely made the coil ab in two parts, one of which can affect cd in the oscillator and the other the coil L of the second circuit. If circuit L-C takes longer for each oscillation than does circuit ab its electron stream is always working at cross purposes with the electron stream in ab which is trying to lead it. Its electrons start off from one condenser plate to the other and before they have much more than got started the stream in ab tries to call them back to go in the other direction. It is practically impossible under these conditions to get a stream of any size going in circuit L-C. It is equally hard if L-C has smaller capacity and inductance than ab so that it naturally oscillates faster. I’ll tell you exactly what it is like. Suppose you and your partner are trying to dance without any piano or other source of music. She has one tune running through her head and she dances to that, If we want the electron stream in coil ab to have a large guiding effect on the stream in coil L-C we must see that both circuits have the same tune, that is the same natural frequency of oscillation. This can be shown very easily by a simple experiment. Suppose we set up our circuit L-C with an ammeter in it, so as to be able to tell how large an electron stream is oscillating in that circuit. Let us also make the condenser a variable one so that we can change the natural frequency or tune of the circuit. Now let’s see what happens to the current as we vary this condenser, changing the capacity and thus changing the tune of the circuit. If we use a variable plate condenser it will have a scale on top graduated in degrees and we can note the reading of the ammeter for each position of the movable That’s all there is to tuning–adjusting the capacity and inductance of a circuit until it has the same natural frequency as some other circuit with which we want it to work. We can either adjust the capacity as we just did, or we can adjust the inductance. In that case we use a variable inductance as in Fig. 52. If we want to be able to tune to any of a large range of frequencies we usually have to take out or put into the circuit a whole lot of mil-henries at a time. When we do we get these mil-henries of inductance from a coil which we call a “loading coil.” That’s why your friends add a loading coil when they When our circuit L-C of Fig. 49 is tuned to the frequency of the oscillator we get in it a maximum current. There is a maximum stream of electrons, and hence a maximum number of them crowded first into one and then into the other plate of the condenser. And so the condenser is charged to a maximum voltage, first in one direction and then in the other. Now connect the circuit L-C to the grid of an audion. If the circuit is tuned we’ll have the maximum possible voltage applied between grid and filament. In the plate circuit we’ll get an increase and then a decrease of current. You know that will happen for I prepared you for this moment by the last page of my ninth letter. I’ll tell you more about that current in the plate circuit in a later letter. I am connecting a telephone receiver in the plate circuit, and also a condenser, the latter for a reason to be explained later. The combination appears then as in Fig. 53. That figure shows a C-W transmitter and an audion detector. This is the sort of a detector Whenever we start the oscillating current in the transmitter we get an effect in the detector circuit, of which I’ll tell you more later. For the moment I am interested in showing you how the transmitter and the detector may be separated by miles and still there will be an effect in the detector circuit every time the key in the transmitter circuit is closed. This is how we do it. At the sending station, that is, wherever we locate the transmitter, we make a condenser using the earth, or ground, as one plate. We do the same thing at the receiving station where the detector circuit is located. To these condensers we connect inductances and these inductances we couple to our transmitter and receiver as shown in Fig. 54. The upper plate of the condenser in each Now we have our radio transmitting-station and a station for receiving its signals. You remember we can make dots and dashes by the key or switch in the oscillator circuit. When we depress the key we start the oscillator going. That sets up oscillations in the circuit with the inductance and the capacity formed by the antenna. If we want a real-sized stream of electrons up and down this antenna lead (the vertical wire), we must tune that circuit. That is why I have shown a variable inductance in the circuit of the transmitting antenna. What happens when these electrons surge back and forth between the horizontal wires and the ground, I don’t know. I do know, however, that if we tune the antenna circuit at the receiving station there will be a small stream of electrons surging back and forth in that circuit. Usually scientists explain what happens by saying that the transmitting station sends out waves in the ether and that these waves are received by the antenna system at the distant station. Wherever you put up a receiving station you will get the effect. It will be much smaller, however, the farther the two stations are apart. I am not going to tell you anything about wave motion in the ether because I don’t believe we know Somehow energy, the ability to do work, travels out from the sending antenna in all directions. Wherever you put up your receiving station you get more or less of this energy. Of course, energy is being sent out only while the key is depressed and the oscillator going. This energy travels just as fast as light, that is at the enormous speed of 186,000 miles a second. If you use meters instead of miles the speed is 300,000,000 meters a second. Now, how far will the energy which is sent out from the antenna travel during the time it takes for one oscillation of the current in the antenna? Suppose the current is oscillating one million times a second. Then it takes one-millionth of a second for one oscillation. In that time the energy will have traveled away from the antenna one-millionth part of the distance it will travel in a whole second. That is one-millionth of 300 million meters or 300 meters. The distance which energy will go in the time taken by one oscillation of the source of that energy is the wave length. In the case just given that distance is 300 meters. The wave length, then, of 300 meters corresponds to a frequency of one million. In fact if we divide 300 million meters by the frequency we get the wave length, and that’s the same rule as I gave you in the last letter. In further letters I’ll tell you how the audion works as a detector and how we connect a telephone |
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