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 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_. [Illustration: Fig 28] One thing this current-measuring instrument tells us is the direction of the electron stream through itself. It shows that the momentary stream of electrons goes through the coil from _d_ to _c_, that is in the opposite direction to the stream in the part _ab_. Now prepare to do a little close thinking. Read over carefully all I have told you about this experiment. You see that the moment the battery starts a stream of electrons from _a_ towards _b_, something causes a momentary, that is a temporary, movement of electrons from _d_ to _c_. We say that starting a stream of electrons from _a_ to _b_ sets up or "induces" a stream of electrons from _d_ to _c_.