The alternating-current component can’t get along in the coil because its frequency is so high that the coil impedes the motion of the electrons so much as practically to stop them. On the other hand these electrons can easily run into the waiting-room offered by the condenser and then run out again as soon as it is time.
When the current in the plate circuit is large all the electrons which aren’t needed for the steady stream through the telephone receiver run into one plate of the condenser. Of course, at that same instant an equal number leave the other plate and start off toward the B-battery and the filament. An instant later, when the current in the plate circuit is small, electrons start to come out of the 141plate and to join the stream through the receiver so that this stream is kept steady.
This steady stream of electrons, which is passing through the receiver winding, is larger than it would be if there was no incoming radio signal. The result is a stronger pull on the diaphragm of the receiver. The moment the signal starts this diaphragm is pulled over toward the magnet and it stays pulled over as long as the signal lasts. When the signal ceases it flies back. We would hear then a click when the signal started and another when it stopped.
If we wanted to distinguish dots from dashes this wouldn’t be at all satisfactory. So in the next letter I’ll show you what sort of changes we can make in the apparatus. To understand what effect these changes will have you need, however, to understand pretty well most of this letter.
142LETTER 15
RADIO-TELEPHONY
Dear Lad:
Before we start on the important subject matter of this letter let us make a short review of the preceding two letters.