Part 15 (1/2)

In adjusting the coupling of the two coils of Fig 92 we stopped short of allowing the tube circuit to oscillate and to generate a high frequency If we had gone on increasing the coupling we should have reached a position where steady oscillations would begin Usually this is marked by a little click in the receiver The reason is that when the tube oscillates the average current in the plate circuit is not the same as the steady current which ordinarily flows between filae, therefore, in the average current in the plate circuit when the tube starts to oscillate You ree current in the plate circuit So the receiver diaphrages position as the tube starts to oscillate and a listener hears a little click

The frequency of the alternating current which the tube produces depends upon the tuned circuit formed by _L_ and _C_ Suppose that this frequency is not the sa antenna is tuned What will happen?

There will be i e m

f's, one due to the tube's own oscillations and the other inco station The two e m f 's are both active at once so that at each instant the e rid is really the sum of these two e m f's Suppose at sorid positive A little later one of theative while the other is still trying to make it positive And later still when the first e rid positive the second will be trying to ether but with different lengths of step Even if they start together with their left feet they are soon so coht foot while the other is putting down his left A little later, but just for an instant, they are in step again And so it goes They are in step for a moment and then completely out of step Suppose one of them makes ten steps in the time that the other makes nine In that time they will be once in step and once completely out of step If one ht this will happen twice

The sa happens in the audion detector circuit when two e htly in frequency are sirid If one e h ten coht cycles, then during that time they will twice be exactly in step, that is, ”in phase” as we say Twice in that time they will be exactly out of step, that is, exactly ”opposite in phase” Twice in that time the two e rid and twice they will exactly oppose Unless they are equal in amplitude there will still be a net e m f even when they are exactly opposed The result of all this is that the average current in the plate circuit of the detector will alternately increase and decrease twice during this time

The listener will then hear a note of a frequency equal to the difference between the frequencies of the two e rid of the detector Suppose the inconal has a frequency of 100,000 cycles a second but that the detector tube is oscillating in its own circuit at the rate of 99,000 cycles per second, then the listener will hear a note of 1000 cycles per second One thousand times each second the two e m f's will be exactly in phase and one thousand times each second they will be exactly opposite in phase The voltage applied to the grid will be a maximum one thousand times a second and alternately a minimum We can think of it, then, as if there were inal which varied in intensity one thousand ti variation in the current through the telephone receiver and thus give rise to a musical note of about two octaves above92 will let us detect signals which are not varying in intensity And consequently this is the nals which are sent out by such a ”continuous wave transmitter” as I showed you at the end of Letter 13

When the key of a C-W trans-antenna an alternating current This current doesn't vary in strength It is there as long as the sender has his key down

Because, however, of the effect which I described above there will be an audible note fro at a frequency within two or three thousand cycles of that of the trans continuous wave signals is called the ”heterodyne””force” and the other parttwo different electron--station and the other produced locally at the receiving station

Neither by itself will produce any sound, except a click when it starts

Both together produce a musical sound in the telephone receiver; and the frequency of that note is the difference of the two frequencies

There are a number of words used to describe this circuit with some of which you should be familiar It is sometimes called a ”feed-back”

circuit because part of the output of the audion is fed back into its input side More generally it is known as the ”regenerative circuit”

because the tube keeps on generating an alternating current The little coil which is used to feed back into the grid circuit some of the effects from the plate circuit is sometimes called a ”tickler” coil

It is not necessary to use a grid condenser in a feed-back circuit but it is perhaps the usual enerative circuit is used The whole value of the regenerative circuit so far as receiving is concerned is in the high efficiency which it peret just as loud signals by using another tube instead of enerative circuit the tube is perfor but it is also aer than it is without changing the shape of its picture, that is without changing its ”wave for we ain at the audion and see how it acts You know that a change in the grid potential e an audion in a circuit which will tell us a little93 shows the circuit

This circuit is the same as we used to find the audion characteristic except that there is a clip for varying the number of batteries in the plate circuit and a voltrid at zero potential and the usual number of batteries in the plate circuit The voltmeter tells us the e m f We read the ammeter in the plate circuit and note what that current is Then we shi+ft the slider in the grid circuit so as to give the grid a ses We can nowthe current in this circuit back to its original value Of course, if we rid positive we move the clip so as to use fewer cells of the B-battery On the other hand if we ative we shall need more e m f in the plate circuit In either case we shall find that we need to e of the plate circuit than we have rid circuit

[Illustration: Fig 93]

Usually we perforet more accurate results We read the voltmeter in the plate circuit and the ae the nu in the plate circuit That changes the plate current The next step is to shi+ft the slider in the grid circuit until we have again the original value of current in the plate circuit Suppose that the tube is ordinarily run with a plate voltage of 40 volts and we start with that e m f on the plate Suppose thatmake it 50 volts and then vary the position of the slider in the grid circuit until the ae irid circuit Suppose it reads 2 volts What does that94]

It rid circuit have the same effect on the plate current as ten volts in the plate circuit If we apply a volt to the grid circuit we get five tie an effect in the plate circuit as ould if the volt were applied there We get a greater effect, the effect of rid

We say that the tube acts as an ”aer effect than the number of volts which we apply would ordinarily entitle us to

Now let's take a simple case of the use of an audion as an a circuit hich we find that the signals are not easily understood because they are too weak Let this be the receiving circuit of Fig 88 which I a

94

We have replaced the telephone receiver by a ”transforether An alternating current in one will give rise to an alternating current in the other