41An ordinary electric light bulb, however, is not an audion although it is like one in the emission of electrons from its filament. That reminds me that last night as I was waiting for a train I picked up one of the Radio Supplements which so many newspapers are now running. There was a column of enquiries. One letter told how its writer had tried to use an ordinary electric light bulb to receive radio signals.
He had plenty of electrons in it but no way to control them and make their motions useful. In an audion besides the filament there are two other things. One is a little sheet or plate of metal with a connecting wire leading out through the glass walls and the other is a little wire screen shaped like a gridiron and so called a “grid.” It also has a connecting wire leading through the glass. Fig. 4 shows an audion. It will be most convenient, however, to represent an audion as in Fig. 5. There you see the filament, F, with its two terminals brought out from the tube, the plate, P, and between these the grid, G.
These three parts of the tube are sometimes called “elements.” Usually, however, they are called “electrodes” and that is why the audion is spoken of as the “three-electrode vacuum tube.” An electrode is what we call any piece of metal or wire which is so placed as to let us get at electrons (or 42ions) to control their motions. Let us see how it does so.
To start with, we shall forget the grid and think of a tube with only a filament and a plate in it–a two-electrode tube. We shall represent it as in Fig. 6 and show the battery which heats the filament by some lines as at A. In this way of representing a battery each cell is represented by a short heavy line and a longer lighter line. The heavy line stands for the negative plate and the longer line for the positive plate. We shall call the battery which heats the filament the “filament battery” or sometimes the “A-battery.” As you see, it is formed by several battery cells connected in series.
Sometime later I may tell you how to connect battery cells together and why. For the present all you need to remember is that two batteries are in series if the positive plate of one is connected to the negative plate of the other. If the batteries are alike they will pull an electron just twice as hard as either could alone.
43To heat the filament of an audion, such as you will probably use in your set, will require three storage-battery cells, like the one I described in my fourth letter, all connected in series. We generally use storage batteries of about the same size as those in the automobile. If you will look at the automobile battery you will see that it is made of three cells connected in series. That battery would do very well for the filament circuit.
Pl. IV.–Radiotron (Courtesy of Radio Corporation of America).