What happens then is something like this: Electrons are freed at the filament; the plate and the grid both call them and they start off in a rush. Some of them are stopped by the wires of the grid but most of them go on by to the plate. The grid is mostly open space, you know, and the electrons move as fast as lightning. They are going too fast in the general direction of the grid to stop and look for its few and small wires. When the grid is positive the grid helps the plate to call electrons away from the filament. Making the grid positive, therefore, increases the stream of electrons _between filament and plate_; that is, increases the current in the plate circuit. We could get the same effect so far as concerns an increased plate current by using more batteries in series in the plate circuit so as to pull harder. But the grid is so close to the filament that a single battery cell in the grid circuit can call electrons so strongly that it would take several extra battery cells in the plate circuit to produce the same effect. [Illustration: Fig 9] If we reverse the grid battery, as in Fig. 9, so as to make the grid negative, then, instead of attracting electrons the grid repels them. Nowhere near as many electrons will stream across to the plate when the grid says, "No, go back." The grid is in a strategic position and what it says has a great effect. When there is no battery connected to the grid it has no possibility of influencing the electron stream which the plate is attracting to itself. We say, then, that the grid is uncharged or is at "zero potential," meaning that it is zero or nothing in possibility. But when the grid is charged, no matter how little, it makes a change in the plate current. When the grid says "Come on," even though very softly, it has as much effect on the electrons as if the plate shouted at them, and a lot of extra electrons rush for the plate. But when the grid whispers "Go back," many electrons which would otherwise have gone streaking off to the plate crowd back toward the filament. That's how the audion works. There is an electron stream and a wonderfully sensitive way of controlling the stream. LETTER 7 HOW TO MEASURE AN ELECTRON STREAM (This letter may be omitted on the first reading.) DEAR YOUTH: If we are to talk about the audion and how its grid controls the current in the plate circuit we must know something of how to measure currents. An electric current is a stream of electrons. We measure it by finding the rate at which electrons are traveling along through the circuit. What do we mean by the word "rate?" You know what it means when a speedometer says twenty miles an hour. If the car should keep going just as it was doing at the instant you looked at the speedometer it would go twenty miles in the next hour. Its rate is twenty miles an hour even though it runs into a smash the next minute and never goes anywhere again except to the junk heap.