Letters of a Radio-Engineer to His Son

by John Mills

You know that this action of the battery will go on until there are so many electrons in the negative plate of the condenser that they prevent the battery from adding any more electrons to that plate. The same thing happens at the other condenser plate. The positive terminal of the battery calls electrons away from the condenser plate which it is making positive until so many electrons have left that the protons in the atoms of the plate are calling for electrons to stay home just as loudly and effectively as the positive battery-terminal is calling them away.

When both these conditions are reached–and they are both reached at the same time–then the battery has to stop driving electrons around the circuit. The battery has not enough e. m. f. to drive any more electrons. Why? Because the condenser has now just enough e. m. f. with which to oppose the battery.

It would be well to learn at once the right words 80to use in describing this action. We say that the battery sends a “charging current” around its circuit and “charges the condenser” until it has the same e. m. f. When the battery is first connected to the condenser there is lots of space in the waiting-rooms so there is a great rush or surge of electrons into one plate and away from the other. Just at this first instant the charging current, therefore, is large but it decreases rapidly, for the moment electrons start to pile up on one plate of the condenser and to leave the other, an e. m. f. builds up on the condenser. This e. m. f., of course, opposes that of the battery so that the net e. m. f. acting to move electrons round the circuit is no longer that of the battery, but is the difference between the e. m. f. of the battery and that of the condenser. And so, with each added electron, the e. m. f. of the condenser increases until finally it is just equal to that of the battery and there is no net e. m. f. to act.

What would happen if we should then disconnect the battery? The condenser would be left with its extra electrons in the negative plate and with its positive plate lacking the same number of electrons. That is, the condenser would be left charged and its e. m. f. would be of the same number of volts as the battery.

Now suppose we connect a short wire between the plates of the condenser as in Fig. 26. The electrons rush home from the negative plate to the positive plate. As fast as electrons get home 81the e. m. f. decreases. When they are all back the e. m. f. has been reduced to zero. Sometimes we say that “the condenser discharges.” The “discharge current” starts with a rush the moment the conducting path is offered between the two plates. The e. m. f. of the condenser falls, the discharge current grows smaller, and in a very short time the condenser is completely discharged.

That’s what happens when there is a short conducting path for the discharge current. If that were all that could happen I doubt if there would be any radio communication to-day. But if we connect a coil of wire between two plates of a charged condenser, as in Fig. 27, then something of great interest happens. To understand you must know something more about electron streams.

Suppose we should wind a few turns of wire on a cylindrical core, say on a stiff cardboard tube. We shall use insulated wire. Now start from one end of the coil, say a, and follow along the coiled wire for a few turns and then scratch off the insulation and solder onto the coil two wires, b, and c, as shown in Fig. 28. The further end of the coil we shall call d. Now let’s arrange a battery and switch so that we can send a current through the part of the coil between a and b. Arrange also a current-measuring instrument so as to show if any current is flowing in the part of the coil between c and d. For this purpose we shall use a kind of current-measuring 82 instrument which I have not yet explained. It is different from the hot-wire type described in Letter 7 for it will show in which direction electrons are streaming through it.

The diagram of Fig. 28 indicates the apparatus of our experiment. When we close the switch, S, the battery starts a stream of electrons from a towards b. Just at that instant the needle, or pointer, of the current instrument moves. The needle moves, and thus shows a current in the coil cd; but it comes right back again, showing that the current is only momentary. Let’s say this again in different words. The battery keeps steadily forcing electrons through the circuit ab but the instrument in the circuit cd shows no current in that circuit except just at the instant when current starts to flow in the neighboring circuit ab.