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 course, very weak and hence a detector tube cannot be used to generate oscillations that are useful for other purposes than heterodyne receptors and measurements.

There is a vacuum tube amplifier [Footnote: This is the _radiation_ UV-201, made by the Radio Corporation of America, Woolworth Bldg., New York City.] made that will stand a plate potential of 100 volts, and this can be used as a generator of oscillations by energizing it with a 110 volt direct current from your lighting service. Or in a pinch you can use five standard B batteries to develop the plate voltage, but these will soon run down. But whatever you do, never use a current from a lighting circuit on a tube of any kind that has a rated plate potential of less than 100 volts.

The Apparatus You Need.--For this experimental continuous wave telegraph transmitter get the following pieces of apparatus: (1) one _single coil tuner with three clips_; (2) one _.002 mfd. fixed condenser_; (3) three _.001 mfd. condensers_; (4) one _adjustable grid leak_; (5) one _hot-wire ammeter_; (6) one _buzzer_; (7) one _dry cell_; (8) one _telegraph key_; (9) one _100 volt plate vacuum tube amplifier_; (10) one _6 volt storage battery_; (11) one _rheostat_; (12) one _oscillation choke coil_; (13) one _panel cut-out_ with a _single-throw, double-pole switch_, and a pair of _fuse sockets_ on it.

The Tuning Coil.--You can either make this tuning coil or buy one. To make it get two disks of wood 3/4-inch thick and 5 inches in diameter and four strips of hard wood, or better, hard rubber or composition strips, such as _bakelite_, 1/2-inch thick, 1 inch wide and 5-3/4 inches long, and screw them to the disks as shown at A in Fig. 75. Now wrap on this form about 25 turns of No. 8 or 10, Brown and Sharpe gauge, bare copper wire with a s.p.a.ce of 1/8-inch between each turn.

Get three of the smallest size terminal clips, see B, and clip them on to the different turns, when your tuning coil is ready for use. You can buy a coil of this kind for $4.00 or $5.00.

The Condensers.--For the aerial series condenser get one that has a capacitance of .002 mfd. and that will stand a potential of 3,000 volts. [Footnote: The U C-1014 _Faradon_ condenser made by the Radio Corporation of America will serve the purpose.] It is shown at C. The other three condensers, see D, are also of the fixed type and may have a capacitance of .001 mfd.; [Footnote: List No. 266; fixed receiving condenser, sold by the Manhattan Electrical Supply Co.] the blocking condenser should preferably have a capacitance of 1/2 a mfd. In these condensers the leaves of the sheet metal are embedded in composition.

The aerial condenser will cost you $2.00 and the others 75 cents each.

[Ill.u.s.tration: (A) Fig. 75.--Apparatus for Experimental C. W.

Telegraph Transmitter.]

[Ill.u.s.tration: Fig. 75.--Apparatus for Experimental C. W. Telegraph Transmitter.]

The Aerial Ammeter.--This instrument is also called a _hot-wire_ ammeter because the oscillating currents flowing through a piece of wire heat it according to their current strength and as the wire contracts and expands it moves a needle over a scale. The ammeter is connected in the aerial wire system, either in the aerial side or the ground side--the latter place is usually the most convenient. When you tune the transmitter so that the ammeter shows the largest amount of current surging in the aerial wire system you can consider that the oscillation circuits are in tune. A hot-wire ammeter reading to 2.5 amperes will serve your needs, it costs $6.00 and is shown at E in Fig. 75.

[Ill.u.s.tration: United States Naval High Power Station, Arlington Va.

General view of Power Room. At the left can be seen the Control Switchboards, and overhead, the great 30 K.W. Arc Transmitter with Accessories.]

The Buzzer and Dry Cell.--While a heterodyne, or beat, receptor can receive continuous wave telegraph signals an ordinary crystal or vacuum tube detector receiving set cannot receive them unless they are broken up into trains either at the sending station or at the receiving station, and it is considered the better practice to do this at the former rather than at the latter station. For this small transmitter you can use an ordinary buzzer as shown at F. A dry cell or two must be used to energize the buzzer. You can get one for about 75 cents.

The Telegraph Key.--Any kind of a telegraph key will serve to break up the trains of sustained oscillations into dots and dashes. The key shown at G is mounted on a composition base and is the cheapest key made, costing $1.50.

The Vacuum Tube Oscillator.--As explained before you can use any amplifying tube that is made for a plate potential of 100 volts. The current required for heating the filament is about 1 ampere at 6 volts. A porcelain socket should be used for this tube as it is the best insulating material for the purpose. An amplifier tube of this type is shown at H and costs $6.50.

The Storage Battery.--A storage battery is used to heat the filament of the tube, just as it is with a detector tube, and it can be of any make or capacity as long as it will develop 6 volts. The cheapest 6 volt storage battery on the market has a 20 to 40 ampere-hour capacity and sells for $13.00.

The Battery Rheostat.--As with the receptors a rheostat is needed to regulate the current that heats the filament. A rheostat of this kind is shown at I and is listed at $1.25.

The Oscillation Choke Coil.--This coil is connected in between the oscillation circuits and the source of current which feeds the oscillator tube to keep the oscillations set up by the latter from surging back into the service wires where they would break down the insulation. You can make an oscillation choke coil by winding say 100 turns of No. 28 Brown and Sharpe gauge double cotton covered magnet wire on a cardboard cylinder 2 inches in diameter and 2-1/2 inches long.

Transmitter Connectors.--For connecting up the different pieces of apparatus of the transmitter it is a good scheme to use _copper braid_; this is made of braided copper wire in three sizes and sells for 7,15 and 20 cents a foot respectively. A piece of it is pictured at J.

The Panel Cut-Out.--This is used to connect the cord of the 110-volt lamp socket with the transmitter. It consists of a pair of _plug cutouts and a single-throw, double-pole_ switch mounted on a porcelain base as shown at K. In some localities it is necessary to place these in an iron box to conform to the requirements of the fire underwriters.

Connecting Up the Transmitting Apparatus.--The way the various pieces of apparatus are connected together is shown in the wiring diagram.

Fig. 76. Begin by connecting one post of the ammeter with the wire that leads to the aerial and the other post of it to one end of the tuning coil; connect clip _1_ to one terminal of the .002 mfd. 3,000 volt aerial condenser and the other post of this with the ground.

[Ill.u.s.tration: Fig. 76--Experimental C.W. Telegraph Transmitter]

Now connect the end of the tuning coil that leads to the ammeter with one end of the .001 mfd. grid condenser and the other end of this with the grid of the vacuum tube. Connect the telegraph key, the buzzer and the dry cell in series and then shunt them around the grid condenser.

Next connect the plate of the tube with one end of the .001 mfd.

blocking condenser and the other end of this with the clip _2_ on the tuning coil.

Connect one end of the filament with the + or positive electrode of the storage battery, the - or negative electrode of this with one post of the rheostat and the other post of the latter with the other end of the filament; then connect clip _3_ with the + or positive side of the storage battery. This done connect one end of the choke coil to the conductor that leads to the plate and connect the other end of the choke coil to one of the taps of the switch on the panel cut-out.

Connect the + or positive electrode of the storage battery to the other switch tap and between the switch and the choke coil connect the protective condenser across the 110 volt feed wires. Finally connect the lamp cord from the socket to the plug fuse taps when your experimental continuous wave telegraph transmitter is ready to use.

A 100 Mile C. W. Telegraph Transmitter.--Here is a continuous wave telegraph transmitter that will cover distances up to 100 miles that you can rely on. It is built on exactly the same lines as the experimental transmitter just described, but instead of using a 100 volt plate amplifier as a makeshift generator of oscillations it employs a vacuum tube made especially for setting up oscillations and instead of having a low plate voltage it is energized with 350 volts.

The Apparatus You Need.--For this transmitter you require: (1) one _oscillation transformer_; (2) one _hot-wire ammeter_; (3) one _aerial series condenser_; (4) one _grid leak resistance_; (5) one _chopper_; (6) one _key circuit choke coil_; (7) one _5 watt vacuum tube oscillator_; (8) one _6 volt storage battery_; (9) one _battery rheostat_; (10) one _battery voltmeter_; (11) one _blocking condenser_; (12) one _power circuit choke coil_, and (13) one _motor-generator_.

The Oscillation Transformer.--The tuning coil, or _oscillation transformer_ as this one is called, is a conductively coupled tuner--that is, the primary and secondary coils form one continuous coil instead of two separate coils. This tuner is made up of 25 turns of thin copper strip, 3/8 inch wide and with its edges rounded, and this is secured to a wood base as shown at A in Fig. 77. It is fitted with one fixed tap and three clips to each of which a length of copper braid is attached. It has a diameter of 6-1/4 inches, a height of 7-7/8 inches and a length of 9-3/8 inches, and it costs $11.00.

[Ill.u.s.tration: Fig. 77.--Apparatus of 100 Mile C. W. Telegraph Transmitter.]

The Aerial Condenser.--This condenser is made up of three fixed condensers of different capacitances, namely .0003, .0004 and .0005 mfd., and these are made to stand a potential of 7500 volts. The condenser is therefore adjustable and, as you will see from the picture B, it has one terminal wire at one end and three terminal wires at the other end so that one, two or three condensers can be used in series with the aerial. A condenser of this kind costs $5.40.

The Aerial Ammeter.--This is the same kind of a hot-wire ammeter already described in connection with the experimental set, but it reads to 5 amperes.

The Grid and Blocking Condensers.--Each of these is a fixed condenser of .002 mfd. capacitance and is rated to stand 3,000 volts. It is made like the aerial condenser but has only two terminals. It costs $2.00.

The Key Circuit Apparatus.--This consists of: (1) the _grid leak_; (2) the _chopper_; (3) the _choke coil_, and (4) the _key_. The grid leak is connected in the lead from the grid to the aerial to keep the voltage on the grid at the right potential. It has a resistance of 5000 ohms with a mid-tap at 2500 ohms as shown at C. It costs $2.00.

The chopper is simply a rotary interrupter driven by a small motor. It comprises a wheel of insulating material in which 30 or more metal segments are set in an insulating disk as shown at D. A metal contact called a brush is fixed on either side of the wheel. It costs about $7.00 and the motor to drive it is extra. The choke coil is wound up of about 250 turns of No. 30 Brown and Sharpe gauge cotton covered magnet wire on a spool which has a diameter of 2 inches and a length of 3-1/4 inches.

The 5 Watt Oscillator Vacuum Tube.--This tube is made like the amplifier tube described for use with the preceding experimental transmitter, but it is larger, has a more perfect vacuum, and will stand a plate potential of 350 volts while the plate current is .045 ampere. The filament takes a current of a little more than 2 amperes at 7.5 volts. A standard 4-tap base is used with it. The tube costs $8.00 and the porcelain base is $1.00 extra. It is shown at E.

The Storage Battery and Rheostat.--This must be a 5-cell battery so that it will develop 10 volts. A storage battery of any capacity can be used but the lowest priced one costs about $22.00. The rheostat for regulating the battery current is the same as that used in the preceding experimental transmitter.

The Filament Voltmeter.--To get the best results it is necessary that the voltage of the current which heats the filament be kept at the same value all of the time. For this transmitter a direct current voltmeter reading from 0 to 15 volts is used. It is shown at F and costs $7.50. The Oscillation Choke Coil.--This is made exactly like the one described in connection with the experimental transmitter.

The Motor-Generator Set.--Where you have only a 110 or a 220 volt direct current available as a source of power you need a _motor-generator_ to change it to 350 volts, and this is an expensive piece of apparatus. It consists of a single armature core with a motor winding and a generator winding on it and each of these has its own commutator. Where the low voltage current flows into one of the windings it drives its as a motor and this in turn generates the higher voltage current in the other winding. Get a 100 watt 350 volt motor-generator; it is shown at F and costs about $75.00.

The Panel Cut-Out.--This switch and fuse block is the same as that used in the experimental set.

The Protective Condenser.--This is a fixed condenser having a capacitance of 1 mfd. and will stand 750 volts. It costs $2.00.

Connecting Up the Transmitting Apparatus.--From all that has gone before you have seen that each piece of apparatus is fitted with terminal, wires, taps or binding posts. To connect up the parts of this transmitter it is only necessary to make the connections as shown in the wiring diagram Fig. 78.

[Ill.u.s.tration: Fig. 78.--5 to 50 Watt C. W. Telegraph Transmitter.

(With Single Oscillation Tube.)]

A 200 Mile C. W. Telegraph Transmitter.--To make a continuous wave telegraph transmitter that will cover distances up to 200 miles all you have to do is to use two 5 watt vacuum tubes in _parallel_, all of the rest of the apparatus being exactly the same. Connecting the oscillator tubes up in parallel means that the two filaments are connected across the leads of the storage battery, the two grids on the same lead that goes to the aerial and the two plates on the same lead that goes to the positive pole of the generator. Where two or more oscillator tubes are used only one storage battery is needed, but each filament must have its own rheostat. The wiring diagram Fig. 79 shows how the two tubes are connected up in parallel.

[Ill.u.s.tration: Fig. 79.--200 Mile C.W. Telegraph Transmitter (With Two Tubes in Parallel.)]