Part 8 (1/2)

Its discharge ative until it is zero volts and there we are--back practically where we started The plate current is increasing and the grid is getting positive, and we're off on another ”cycle” as we say During a cycle the plate current increases to a ain to its initial value

[Illustration: Fig 36]

This letter has a longer continuous train of thought than I usually ask you to follow But before I stop I want to give you soood this is in radio

What about the current which flows in coil _cd_? It's an alternating current, isn't it? First the electrons streaain from _c_ towards _d_

Suppose we set up another coil like _CD_ in Fig 36 It would have an alternating current induced in it If this coil was connected to an antenna there would be radio waves sent out The switch _S_ could be used for a key and kept closed longer or shorter intervals depending upon whether dashes or dots were being set I'll tell you s of a ”C-W Transmitter,”

that is a ”continuous wave transraphy

It would be worth while to go over this letter again using a pencil and tracing in the various circuits the electron streams which I have described

LETTER 12

INDUCTANCE AND CAPACITY

DEAR SIR:

In the last letter I didn't stop to draw you a picture of the action of the audion oscillator which I described I a two pencils and drawing simultaneously two curves One curve shohat happens to the current in the plate circuit The other sho the voltage of the grid changes Both curves start froether show just what happens in the tube fro 37 shows the two curves You will notice how I have drawn thee and the plate current are related, as I have told you, and the audion characteristic is just a convenient way of showing the relationshi+p If we know the current in the plate circuit we can find the voltage of the grid and vice versa

As tirows to itsup and down between these two extre alternately, having its maximum positive value when the plate current is a ative value when the plate current is zero Look at the two curves and see this for yourself

[Illustration: Fig 37]

Noant to tell you so about how fast these oscillations occur We start by learning tords One is ”cycle” hich you are already partly familiar and the other is ”frequency” Take cycle first

Starting from zero the current increases to a ain for the sah ”a cycle of values” It doesn't make any difference where we start froh all its different values until we are back at the same value as we started with and ready to start all over, then we have followed through a cycle of values

Once you get the idea of a cycle, and the

31 will help you to understand, then the other idea is easy By ”frequency” we mean the number of cycles each second The electric current which we use in lighting our house goes through sixty cycles a second That means the current reverses its direction 120 ti currents which have very high frequencies

In shi+p sets the frequency is either 500,000 or 1,000,000 cycles per second A sets usually have oscillators which run at well over a e stations use lower frequencies

You'll find, however, that the newspaper announcements of the various broadcast stations do not tell the frequency but instead tell the ”wave length” I aoing to give you a sith”

and you'll have the frequency For exaths of 300three hundred ives one million and that is one of the frequencies which I told you were used by shi+p sets Dividing by six hundred gives 500,000 or just half the frequency You can re waves have low frequencies, but sets with short waves have high frequencies The frequency and the wave length don't change in the sae in opposite ways or inversely, as we say The higher the frequency the shorter the wave length

I'll tell you about wave lengths later First let's see how to control the frequency of an audion oscillator like that of Fig 38

[Illustration: Fig 38]