Part 14 (2/2)

It is sufficient to say that when the key is depressed the set sends out radio signals at the rate usually of 1000 signals a second Every ti station the current in the telephone receiver is sudden reduced; and in the tis the tube back to a condition where it can receive the next signal While the sending key is depressed the current in the receiver is decreasing and increasing once for every signal which is being transm of the telephone receiver makes one vibration

What the listener then hears is ato that number of vibrations a second, that is, a note with a frequency of one thousand cycles per second He hears a note of frequency about that of two octaves above middle _C_ on the piano

There are usually other notes present at the same time and the sound is not like that of any91]

If the key is held down a long ti time If it is depressed only about a third of that time so as to send a dot, the listener hears the note for a shorter ti 91 I have drawn a sketch to show the e rid of a detector and to sho the plate current varies if there is a condenser and leak in the grid circuit I have only shown three signals in succession If the operator sends at the rate of about twenty words a nals in succession

The frequency of the alternations in one of the little signals will depend upon the wave length which the sending operator is using If he uses the wave length of 600 meters, as shi+p stations do, he will send with a radio frequency of 500,000 cycles a second Since the signals are at the rate of a thousand a second each one is made up of 500 complete cycles of the current in the antenna It would be impracticable therefore to show you a conal froure quite corid-condenser and its leak are so chosen as to ell for signals from a 500-cycle spark set they will also ell for the notes in human speech which are about 1000 cycles a second in frequency

The detecting circuit will not, however, work so well for the other notes which are in the human voice and are necessary to speech For example, if notes of about 2000 cycles a second are involved in the speech which is being transh On the other hand, for the very lowest notes in the voice the leak ork too fast and such variations in the signal current will not be detected as efficiently as are those of 1000 cycles a second

You can see that there is always a little favoritisrid-condenser detector It doesn't exactly reproduce the variations in intensity of the radio signal which werestation It distorts a little As aive it that distortion because it is so efficient It h the telephone when it receives a signal that we can use it to receive nals fro stations, than we can receive with the arrangement described in Letter 14

LETTER 18

AMPLIFIERS AND THE REGENERATIVE CIRCUIT

MY DEAR RECEIVER:

There is one way ofan audion even more efficient as a detector than the method described in the last letter And that is tocircuit as in Fig 92 It is exactly like that of Fig 90 of the previous letter except for the fact that the current in the plate circuit passes through a little coil, _L_{t}_, which is placed near the coil _L_ and so can induce in it an e m

f which will correspond in intensity and wave form to the current in the plate circuit

If we should take out the grid condenser and its leak this circuit would be like that of Fig 54 in Letter 13 which we used for a generator of high-frequency alternating currents You rerid circuit produces a large effect in the plate circuit Because the plate circuit is coupled to the grid circuit the grid is again affected and so there is a still larger effect in the plate circuit And so on, until the current in the plate circuit is swinging from zero to its maximum possible value

What happens depends upon how closely the coils _L_ and _L_{t}_ are coupled, that is, upon howcurrent in one can affect the other If they are turned at right angles to each other, so that there is no possible ”

Start with the coils at right angles to each other and turn _L_{t}_ so as to bring its windings more and more parallel to those of _L_

If ant _L_{t}_ to have a large effect on _L_ its windings should be parallel and also in the sa 54 of Letter 13 to which we just referred As we approach nearer to that position the current in _L_{t}_ induces more and more e m f

in coil _L_ For some position of the two coils, and the actual position depends on the tube we are using, there will be enough effect frorid circuit so that there will be continuous oscillations

[Illustration: Fig 92]

We want to stop just short of this position There will then be no continuous oscillations; but if any changes do take place in the plate current they will affect the grid And these changes in the grid voltage will result in still larger changes in the plate current

Now suppose that there is conal with, speech significance The current in the plate circuit varies accordingly So does the current in coil _L_{t}_ which is in the plate circuit But this current induces an e m f in coil _L_ and this adds to the e reater variation in the plate current This goes on as long as there is an incorid circuit the result is a larger e nal could set up all by itself

You see nohy I said the tube talked to itself It repeats to itself whatever it receives It has a greater strength of signal to detect than if it didn't repeat Of course, it detects also just as I told you in the preceding letter