Letters of a Radio-Engineer to His Son

by John Mills

There is one way of making an audion even more efficient as a detector than the method described in the last letter. And that is to make it talk to itself.

Suppose we arrange a receiving circuit as in Fig. 92. It is exactly like that of Fig. 90 of the previous letter except for the fact that the current in the plate circuit passes through a little coil, L_T, which is placed near the coil L and so can induce in it an e. m. f. which will correspond in intensity and wave form to the current in the plate circuit.

If we should take out the grid condenser and its leak this circuit would be like that of Fig. 54 in Letter 13 which we used for a generator of high-frequency alternating currents. You remember how that circuit operates. A small effect in the grid circuit produces a large effect in the plate circuit. Because the plate circuit is coupled to the grid circuit the grid is again affected and so there is a still larger effect in the plate circuit. And so on, until the current in the plate circuit is swinging from zero to its maximum possible value.

What happens depends upon how closely the coils L and L_T are coupled, that is, upon how much the 177 changing current in one can affect the other. If they are turned at right angles to each other, so that there is no possible mutual effect we say there is “zero coupling.”

Start with the coils at right angles to each other and turn L_T so as to bring its windings more and more parallel to those of L. If we want L_T to have a large effect on L its windings should be parallel and also in the same direction just as they were in Fig. 54 of Letter 13 to which we just referred. As we approach nearer to that position the current in L_T induces more and more e. m. f. in coil L. For some position of the two coils, and the actual position depends on the tube we are using, there will be enough effect from the plate circuit upon the grid circuit so that there will be continuous oscillations.

We want to stop just short of this position. There will then be no continuous oscillations; but if any changes do take place in the plate current they will affect the grid. And these changes in the grid voltage will result in still larger changes in the plate current.

Now suppose that there is coming into the detector circuit of Fig. 92 a radio signal with, speech significance. 178The current in the plate circuit varies accordingly. So does the current in coil L_T which is in the plate circuit. But this current induces an e. m. f. in coil L and this adds to the e. m. f. of the incoming signal so as to make a greater variation in the plate current. This goes on as long as there is an incoming signal. Because the plate circuit is coupled to the grid circuit the result is a larger e. m. f. in the grid circuit than the incoming signal could set up all by itself.

You see now why I said the tube talked to itself. It repeats to itself whatever it receives. It has a greater strength of signal to detect than if it didn’t repeat. Of course, it detects also just as I told you in the preceding letter.

In adjusting the coupling of the two coils of Fig. 92 we stopped short of allowing the tube circuit to oscillate and to generate a high frequency. If we had gone on increasing the coupling we should have reached a position where steady oscillations would begin. Usually this is marked by a little click in the receiver. The reason is that when the tube oscillates the average current in the plate circuit is not the same as the steady current which ordinarily flows between filament and plate. There is a sudden change, therefore, in the average current in the plate circuit when the tube starts to oscillate. You remember that what affects the receiver is the average current in the plate circuit. So the receiver diaphragm suddenly changes position as the tube starts to oscillate and a listener hears a little click.