What's going to happen in the plate circuit? When there is no e. m. f. applied to the grid circuit, that is when the grid potential (possibilities) is zero, there is a definite current in the plate circuit. That current we can find from our characteristic of Fig. 23 for it is where the curve crosses Zero Volts. As the grid becomes positive the current rises above this value. When the grid is made negative the current falls below this value. The current, _I_{B}_, then is made alternately greater and less than the current when _E_{C}_ is zero. You might spend a little time thinking over this, seeing what happens when an alternating e. m. f. is applied to the grid of an audion, for that is going to be fundamental to our study of radio. [Footnote 3: A mil-ampere is a thousandth of an ampere just as a millimeter is a thousandth of a meter.] LETTER 10 CONDENSERS AND COILS DEAR SON: In the last letter we learned of an alternating e. m. f. The way of producing it, which I described, is very crude and I want to tell how to make the audion develop an alternating e. m. f. for itself. That is what the audion does in the transmitting set of a radio telephone. But an audion can't do it all alone. It must have associated with it some coils and a condenser. You know what I mean by coils but you have yet to learn about condensers. A condenser is merely a gap in an otherwise conducting circuit. It's a gap across which electrons cannot pass so that if there is an e. m. f. in the circuit, electrons will be very plentiful on one side of the gap and scarce on the other side. If there are to be many electrons waiting beside the gap there must be room for them. For that reason we usually provide waiting-rooms for the electrons on each side of the gap. Metal plates or sheets of tinfoil serve nicely for this purpose. Look at Fig. 25. You see a battery and a circuit which would be conducting except for the gap at _C_. On each side of the gap there is a sheet of metal. The metal sheets may be separated by air or mica or paraffined paper. The combination of gap, plates, and whatever is between, provided it is not conducting, is called a condenser. Let us see what happens when we connect a battery to a condenser as in the figure. The positive terminal of the battery calls electrons from one plate of the condenser while the negative battery-terminal drives electrons away from itself toward the other plate of the condenser. One plate of the condenser, therefore, becomes positive while the other plate becomes negative. [Illustration: Fig 25]