And now we can see how to make an audion produce an alternating current or as we sometimes say "make an audion oscillator." We shall set up an audion with its A-battery as in Fig. 34. Between the grid and the filament we put a coil and a condenser. Notice that they are in parallel, as we say. In the plate-filament circuit we connect the B-battery and a switch, _S_, and another coil. This coil in the plate circuit of the audion we place close to the other coil so that the two coils are just like the coils _ab_ and _cd_ of which I have been telling you. The moment any current flows in coil _ab_ there will be a current flow in the coil _cd_. (An induced electron stream.) Of course, as long as the switch in the B-battery is open no current can flow. The moment the switch _S_ is closed the B-battery makes the plate positive with respect to the filament and there is a sudden surge of electrons round the plate circuit and through the coil from _a_ to _b_. You know what that does to the coil _cd_. It induces an electron stream from _d_ towards _c_. Where do these electrons come from? Why, from the grid and the plate 1 of the condenser. Where do they go? Most of them go to the waiting-room offered by plate 2 of the condenser and some, of course, to the filament. What is the result? The grid becomes positive and the filament negative. [Illustration: Fig 35] This is the crucial moment in our study. Can you tell me what is going to happen to the stream of electrons in the plate circuit? Remember that just at the instant when we closed the switch the grid was neither positive nor negative. We were at the point of zero volts on the audion characteristic of Fig. 35. When we close the switch the current in the plate circuit starts to jump from zero mil-amperes to the number of mil-amperes which represents the point where Zero Volt St. crosses Audion Characteristic. But this jump in plate current makes the grid positive as we have just seen. So the grid will help the plate call electrons and that will make the current in the plate circuit still larger, that is, result in a larger stream of electrons from _a_ to _b_. This increase in current will be matched by an increased effect in the coil _cd_, for you remember how you and "Brownie" behaved. And that will pull more electrons away from plate 1 of the condenser and send them to the waiting-room of 2. All this makes the grid more positive and so makes it call all the more effectively to help the plate move electrons. [Illustration: Pl. V.--Variometer (top) and Variable Condenser (bottom) of the General Radio Company. Voltmeter and Ammeter of the Weston Instrument Company.]