118But suppose that the two circuits do not have the same natural frequencies, that is the condenser and inductance in one circuit are so large that it just naturally takes more time for an oscillation in that circuit than in the other. It is like learning to dance. You know about how well you and your partner would get along if you had one frequency of oscillation and she had another. That’s what happens in a case like this.
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, 119except as you drag her around the floor. You are trying to follow another tune. As a couple you have a difficult time going anywhere under these conditions. But it would be all right if you both had the same tune.
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 120 plates. If we do, we find one position of these plates, that is one setting, corresponding to one value of capacity in the condenser, where the current in the circuit is a maximum. This is the setting of the condenser for which the circuit has the same tune or natural frequency as the circuit cd. Sometimes we say that the circuits are now in resonance. We also refer to the curve of values of current and condenser positions as a “tuning curve.” Such a curve is shown in Fig. 51.
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 121want to tune for the long wave-length stations, that is, those with a low frequency.