If you look again at Fig. 28 you will see what I mean. Suppose the switch is closed and a steady stream of electrons is flowing through the coil from a to b. There will be no current in the other part of the coil. Now open the switch. There will be a motion of the needle of the current-measuring instrument, showing a momentary current. The direction of this motion, however, shows that the momentary stream of electrons goes through the coil from c to d.
Do you see what this means? The moment the battery is disconnected there is nothing driving the electrons in the part ab and they slow down. Immediately, and just for an instant, a stream of electrons starts off in the part cd in the same direction as if the battery was driving them along.
86Now look again at Fig. 29. If the battery is suddenly disconnected there is a momentary rush of electrons in the same direction as the battery was driving them. Just as the self-inductance of a coil opposes the starting of a stream of electrons, so it opposes the stopping of a stream which is already going.
So far we haven’t said much about making an audion produce alternating e. m. f.’s and thus making it useful for radio-telephony. Before radio was possible all these things that I have just told you, and some more too, had to be known. It took hundreds of good scientists years of patient study and experiment to find out those ideas about electricity which have made possible radio-telephony.
Two of these ideas are absolutely necessary for the student of radio-communication. First: A condenser is a gap in a circuit where there are waiting-rooms for the electrons. Second: Electrons form habits. It’s hard to get them going through a coil of wire, harder than through a straight wire, but after they are going they don’t like to stop. They like it much less if they are going through a coil instead of a straight wire.
In my next letter I’ll tell you what happens when we have a coil and a condenser together in a circuit.
The “henry” has nothing to do with a well-known automobile. It was named after Joseph Henry, a professor years ago at Princeton University.