How the Vacuum Tube Acts as an Amplifier.--If you connect up the filament and the plate of a three electrode tube with the batteries and do not connect in the grid, you will find that the electrons which are thrown off by the filament will not get farther than the grid regardless of how high the voltage is that you apply to the plate. This is due to the fact that a large number of electrons which are thrown off by the filament strike the grid and give it a negative charge, and consequently, they cannot get any farther. Since the electrons do not reach the plate the current from the B battery cannot flow between it and the filament.

Now with a properly designed amplifier tube a very small negative voltage on the grid will keep a very large positive voltage on the plate from sending a current through the tube, and oppositely, a very small positive voltage on the grid will let a very large plate current flow through the tube; this being true it follows that any small variation of the voltage from positive to negative on the grid and the other way about will vary a large current flowing from the plate to the filament.

In the Morse telegraph the relay permits the small current that is received from the distant sending station to energize a pair of magnets, and these draw an armature toward them and close a second circuit when a large current from a local battery is available for working the sounder. The amplifier tube is a variable relay in that the feeble currents set up by the incoming waves constantly and proportionately vary a large current that flows through the headphones. This then is the principle on which the amplifying tube works.

The Operation of a Simple Vacuum Tube Receiving Set.--The way a simple vacuum tube detector receiving set works is like this: when the filament is heated to brilliancy it gives off electrons as previously described. Now when the electric waves impinge on the aerial wire they set up oscillations in it and these surge through the primary coil of the loose coupled tuning coil, a diagram of which is shown at B in Fig. 41.

The energy of these oscillations sets up oscillations of the same frequency in the secondary coil and these high frequency currents whose voltage is first positive and then negative, surge in the closed circuit which includes the secondary coil and the variable condenser. At the same time the alternating positive and negative voltage of the oscillating currents is impressed on the grid; at each change from + to - and back again it allows the electrons to strike the plate and then shuts them off; as the electrons form the conducting path between the filament and the plate the larger direct current from the B battery is permitted to flow through the detector tube and the headphones.

Operation of a Regenerative Vacuum Tube Receiving Set.--By feeding back the pulsating direct current from the B battery through the tickler coil it sets up other and stronger oscillations in the secondary of the tuning coil when these act on the detector tube and increase its sensitiveness to a remarkable extent. The regenerative, or _feed back_, action of the receiving circuits used will be easily understood by referring back to B in Fig. 47.

When the waves set up oscillations in the primary of the tuning coil the energy of them produces like oscillations in the closed circuit which includes the secondary coil and the condenser; the alternating positive and negative voltages of these are impressed on the grid and these, as we have seen before, cause similar variations of the direct current from the B battery which acts on the plate and which flows between the latter and the filament.

This varying direct current, however, is made to flow back through the third, or tickler coil of the tuning coil and sets up in the secondary coil and circuits other and larger oscillating currents and these augment the action of the oscillations produced by the incoming waves. These extra and larger currents which are the result of the feedback then act on the grid and cause still larger variations of the current in the plate voltage and hence of the current of the B battery that flows through the detector and the headphones. At the same time the tube keeps on responding to the feeble electric oscillations set up in the circuits by the incoming waves. This regenerative action of the battery current augments the original oscillations many times and hence produce sounds in the headphones that are many times greater than where the vacuum tube detector alone is used.

Operation of Autodyne and Heterodyne Receiving Sets.--On page 109 [Chapter VII] we discussed and at A in Fig. 36 is shown a picture of two tuning forks mounted on sounding boxes to illustrate the principle of electrical tuning. When a pair of these forks are made to vibrate exactly the same number of times per second there will be a condensation of the air between them and the sound waves that are sent out will be augmented. But if you adjust one of the forks so that it will vibrate 256 times a second and the other fork so that it will vibrate 260 times a second then there will be a phase difference between the two sets of waves and the latter will augment each other 4 times every second and you will hear these rising and falling sounds as _beats_.

Now electric oscillations set up in two circuits that are coupled together act in exactly the same way as sound waves produced by two tuning forks that are close to each other. Since this is true if you tune one of the closed circuits so that the oscillations in it will have a frequency of a 1,000,000 and tune the other circuit so that the oscillations in it have a frequency of 1,001,000 a second then the oscillations will augment each other 1,000 times every second.

As these rising and falling currents act on the pulsating currents from the B battery which flow through the detector tube and the headphones you will hear them as beats. A graphic representation of the oscillating currents set up by the incoming waves, those produced by the heterodyne oscillator and the beats they form is shown in Fig. 73. To produce these beats a receptor can use: (1) a single vacuum tube for setting up oscillations of both frequencies when it is called an _autodyne_, or _self-heterodyne_ receptor, or (2) a separate vacuum tube for setting up the oscillations for the second circuit when it is called a _heterodyne_ receptor.

[Illustration: Fig. 73.--How the Heterodyne Receptor Works.]

The Autodyne, or Self-Heterodyne Receiving Set.--Where only one vacuum tube is used for producing both frequencies you need only a regenerative, or feed-back receptor; then you can tune the aerial wire system to the incoming waves and tune the closed circuit of the secondary coil so that it will be out of step with the former by 1,000 oscillations per second, more or less, the exact number does not matter in the least. From this you will see that any regenerative set can be used for autodyne, or self-heterodyne, reception.

The Separate Heterodyne Receiving Set.--The better way, however, is to use a separate vacuum tube for setting up the heterodyne oscillations. The latter then act on the oscillations that are produced by the incoming waves and which energize the grid of the detector tube. Note that the vacuum tube used for producing the heterodyne oscillations is a _generator_ of electric oscillations; the latter are impressed on the detector circuits through the variable coupling, the secondary of which is in series with the aerial wire as shown in Fig. 74. The way in which the tube acts as a generator of oscillations will be told in Chapter XVIII.

[Illustration: Fig. 74.--Separate Heterodyne Oscillator.]

CHAPTER XVI

CONTINUOUS WAVE TELEGRAPH TRANSMITTING SETS WITH DIRECT CURRENT

In the first part of this book we learned about spark-gap telegraph sets and how the oscillations they set up are _damped_ and the waves they send out are _periodic_. In this and the next chapter we shall find out how vacuum tube telegraph transmitters are made and how they set up oscillations that are _sustained_ and radiate waves that are _continuous_.

Sending wireless telegraph messages by continuous waves has many features to recommend it as against sending them by periodic waves and among the most important of these are that the transmitter can be: (1) more sharply tuned, (2) it will send signals farther with the same amount of power, and (3) it is noiseless in operation. The disadvantageous features are that: (1) a battery current is not satisfactory, (2) its circuits are somewhat more complicated, and (3) the oscillator tubes burn out occasionally. There is, however, a growing tendency among amateurs to use continuous wave transmitters and they are certainly more up-to-date and interesting than spark gap sets.

Now there are two practical ways by which continuous waves can be set up for sending either telegraphic signals or telephonic speech and music and these are with: (a) an _oscillation arc lamp_, and (b) a _vacuum tube oscillator_. The oscillation arc was the earliest known way of setting up sustained oscillations, and it is now largely used for commercial high power, long distance work. But since the vacuum tube has been developed to a high degree of efficiency and is the scheme that is now in vogue for amateur stations we shall confine our efforts here to explaining the apparatus necessary and how to wire the various parts together to produce several sizes of vacuum tube telegraph transmitters.

Sources of Current for Telegraph Transmitting Sets.--Differing from a spark-gap transmitter you cannot get any appreciable results with a low voltage battery current to start with. For a purely experimental vacuum tube telegraph transmitter you can use enough B batteries to operate it but the current strength of these drops so fact when they are in use, that they are not at all satisfactory for the work.

You can, however, use 110 volt direct current from a lighting circuit as your initial source of power to energize the plate of the vacuum tube oscillator of your experimental transmitter. Where you have a 110 volt _direct current_ lighting service in your home and you want a higher voltage for your plate, you will then have to use a motor-generator set and this costs money. If you have 110 volt _alternating current_ lighting service at hand your troubles are over so far as cost is concerned for you can step it up to any voltage you want with a power transformer. In this chapter will be shown how to use a direct current for your source of initial power and in the next chapter how to use an alternating current for the initial power.

An Experimental Continuous Wave Telegraph Transmitter.--You will remember that in Chapter XV we learned how the heterodyne receiver works and that in the separate heterodyne receiving set the second vacuum tube is used solely to set up oscillations. Now while this extra tube is used as a generator of oscillations these are, of