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. [Illustration: Fig 47a] 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. [Illustration: Fig 47b] 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 can push and pull, that is on its e. m. f., and upon how much capacity the condenser has. [Illustration: Fig 47c] 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. [Illustration: Fig 47d] 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 that they go bumping around without getting any nearer plate 2. Of course, they spend all their energy this way, getting every one all stirred up and heating the wire.