Part 19 (2/2)

The high voltages which are used in the plate circuits of these high-powered audions are obtained froenerators instead of batteries

You remember from Letter 20 that an e net are suddenly changed in their positions, one being turned with reference to the other A generator is aan ecoils and by strong electroine, by a water wheel, or by an electric ive steady streams of electrons, that is for d-c currents, or to act as alternators

[Illustration: Fig 124]

Suppose we have, as shown in Fig 124, a d-c generator supplying current to a vacuuenerator passes through an iron-cored choke coil, ure

Between this coil and the plate circuit we connect across the line a telephone transmitter To make a system which ork efficiently we shall have to suppose that this transh resistance, say about the same as the internal resistance, _R_{p}_, of the tube and also that it can carry as large a current

Of the current which cooes to the tube and the rest to the transmitter If the resistance of the transmitter is increased it can't take as much current The coil, _L_{a}_, however, because of its inductance, tends to keep the sah itself For just an instant then the current in _L_{a}_ keeps steady even though the transmitter doesn't take its share The result istube On the other hand if the transmitter takes more current, because its resistance is decreased, the choke coil, _L_{a}_, will momentarily tend to keep the current steady so that what the trans tube

That's one way of looking at what happens We knoever, froet an increase in the a tube we e to its plate circuit That is what really happens when the transmitter increases in resistance and so doesn't take its full share of the current The reason is this: When the transmitter resistance is increased the current in the transh the current in _L_{a}_ is going to decrease That's the way it looks to the electrons; and you knohat electrons do in an inductive circuit when they think they shall have to stop They induce each other to keep on for a moment For a moment they act just as if there was so We say, therefore, that there is an extra e m f, and we call this an e m

f of self-induction All this time there has been active on the plate circuit of the tube the e enerator To this there is added at the instant when the transmitter resistance increases, the e

m f of self-induction in the coil, _L_{a}_ and so the total e m f

applied to the tube is momentarily increased This increased e m f, of course, results in an increased a current which the oscillator is supplying to the trans antenna

When the transer current should flow through the choke coil, the electrons are asked to speed up in going through the coil At first they object and during that instant they express their objection by an e e For an instant, then, the voltage of the oscillating tube is lowered and its alternating-current output is s 125]

For the purpose of bringing about such threatened changes in current, and hence such e m f's of self-induction, the carbon transmitter is not suitable because it has too s ability The plate circuit of a vacuum tube will serve admirably You know fro the plate voltage we can, by applying a voltage to the grid, change the current through the plate circuit Now if it was a wire resistance hich ere dealing and we should be able to obtain a change in current without changing the voltage acting on this ould say that we had changed the resistance We can say, therefore, that the internal resistance of the plate circuit of a vacuu 125 I have substituted the plate circuit of an audion for the transed to vary its resistance by changing the potential of the grid This we do by irid the e

m f developed in the secondary of a transformer, to the primary of which is connected a battery and a carbon transh the primary varies in accordance with the sounds spoken into the transmitter And for all the reasons which we have just finished studying there are si tube in the transure you will notice a small air-core coil, _L_{R}_, between the oscillator and the modulator tube This coil has a se impedance to radio-frequency currents The result is, it does not let the alternating currents of the oscillating tube flow into the modulator These currents are confined to their own circuit, where they are useful in establishi+ng similar currents in the antenna On the other hand, the coil _L_{R}_ doesn't seriously impede low-frequency currents and therefore it does not prevent variations in the current which are at audio-frequency It does not interfere with the changes in current which accompany the variations in the resistance of the plate circuit of the modulator

That is, it has too little impedance to act like _L_{a}_ and so it permits the modulator to vary the output of the oscillator

[Illustration: Fig 126]

The oscillating circuit of Fig 125 includes part of the antenna It differs also frorid and plate circuits are coupled I'll explain by Fig 126

The trans set which I have just described involves many of the principles of the most modern sets If you understand its operation you can probably reason out for yourself any of the other sets of which you will hear from time to time

LETTER 23

AMPLIFICATION AT INTERMEDIATE FREQUENCIES

DEAR SON:

In theI have already covered all the important principles There is one more system, however, which you will need to know This is spoken of either as the ”super-heterodyne” or as the ”intermediate-frequency amplification” method of reception