87LETTER 11
A “C-W” TRANSMITTER
Dear Son:
Let’s look again at the coils of Fig. 28 which we studied in the last letter. I have reproduced them here so you won’t have to turn back. When electrons start from a towards b there is a momentary stream of electrons from d towards c. If the electron stream through ab were started in the opposite direction, that is from b to a the induced stream in the coil cd would be from c towards d.
It all reminds me of two boys with a hedge or fence between them as in Fig. 30. One boy is after the other. Suppose you were being chased; you know what you’d do. If your pursuer started off 88with a rush towards one end of the hedge you’d “beat it” towards the other. But if he started slowly and cautiously you would start slowly too. You always go in the opposite direction, dodging back and forth along the paths which you are wearing in the grass on opposite sides of the hedge. If he starts to the right and then slows up and starts back, you will start to your right, slow up, and start back. Suppose he starts at the center of the hedge. First he dodges to the right, and then back through the center as far to the left, then back again and so on. You follow his every change.
I am going to make a picture of what you two do. Let’s start with the other fellow. He dodges or alternates back and forth. Some persons would say he “oscillates” back and forth in the same path. As 89he does so he induces you to move. I am on your side of the hedge with a moving-picture camera. My camera catches both of you. Fig. 31 shows the way the film would look if it caught only your heads. The white circle represents the tow-head on my side of the hedge and the black circle, young Brown who lives next door. Of course, the camera only catches you each time the shutter opens but it is easy to draw a complete picture of what takes place as time goes on. See Fig. 32.
