199LETTER 20
TELEPHONE RECEIVERS AND OTHER ELECTROMAGNETIC DEVICES
Dear Son:
In an earlier letter when we first introduced a telephone receiver into a circuit I told you something of how it operates. I want now to tell why and also of some other important devices which operate for the same reason.
You remember that a stream of electrons which is starting or stopping can induce the electrons of a neighboring parallel circuit to start off in parallel paths. We do not know the explanation of this. Nor do we know the explanation of another fact which seems to be related to this fact of induction and is the basis for our explanations of magnetism.
If two parallel wires are carrying steady electron streams in the same general direction the wires attract each other. If the streams are oppositely directed the wires repel each other. Fig. 101 illustrates 200this fact. If the streams are not at all in the same direction, that is, if they are at right angles, they have no effect on each other.
These facts, of the attraction of electron streams which are in the same direction and repulsion of streams in opposite directions, are all that one need remember to figure out for himself what will happen under various conditions. For example, if two coils of wire are carrying currents what will happen is easily seen. Fig. 102 shows the two coils and a section through them.
Looking at this cross section we seem to have four wires, 1 and 2 of coil A and 3 and 4 of coil B. You see at once that if the coils are free to move they will move into the dotted positions shown in Fig 102, because wire 1 attracts wire 3 and repels wire 4, while wire 2 attracts wire 4 and repels wire 3. If necessary, and if they are free to move, the coils will turn completely around to get to this position. I have shown such a case in Fig. 103.