The Microphone Transmitter.--The best kind of a microphone to use with this and other telephone transmitting sets is a _Western Electric No.

284-W_. [Footnote: Made by the Western Electric Company, Chicago, Ill.] This is known as a solid back transmitter and is the standard commercial type used on all long distance Bell telephone lines. It articulates sharply and distinctly and there are no current variations to distort the wave form of the voice and it will not buzz or sizzle.

It is shown in Fig. 84 and costs $2.00. Any other good microphone transmitter can be used if desired.

[Ill.u.s.tration: Fig. 84.--Standard Microphone Transmitter.]

Connecting Up the Apparatus.--Begin by connecting the leading-in wire with one of the terminals of the microphone transmitter, as shown in the wiring diagram Fig. 85, and the other terminal of this to one end of the tuning coil. Now connect _clip 1_ of the tuning coil to one of the posts of the hot-wire ammeter, the other post of this to one end of aerial condenser and, finally, the other end of the latter with the water pipe or other ground. The microphone can be connected in the ground wire and the ammeter in the aerial wire and the results will be practically the same.

[Ill.u.s.tration: Fig. 85.--Wiring Diagram of Short Distance Wireless Telephone Set. (Microphone in Aerial Wire.)]

Next connect one end of the grid condenser to the post of the tuning coil that makes connection with the microphone and the other end to the grid of the tube, and then shunt the grid leak around the condenser. Connect the + or _positive_ electrode of the storage battery with one terminal of the filament of the vacuum tube, the other terminal of the filament with one post of the rheostat and the other post of this with the - or _negative_ electrode of the battery.

This done, connect _clip 2_ of the tuning coil to the + or _positive_ electrode of the battery and bring a lead from it to one of the switch taps of the panel cut-out.

Now connect _clip 3_ of the tuning coil with one end of the blocking condenser, the other end of this with one terminal of the choke coil and the other terminal of the latter with the other switch tap of the cut-out. Connect the protective condenser across the direct current feed wires between the panel cut-out and the choke coil. Finally connect the ends of a lamp cord to the fuse socket taps of the cut-out, and connect the other ends to a lamp plug and screw it into the lamp socket of the feed wires. Screw in a pair of 5 ampere _fuse plugs_, close the switch and you are ready to tune the transmitter and talk to your friends.

A 25 to 50 Mile Wireless Telephone Transmitter--With Direct Current Motor Generator.--Where you have to start with 110 or 220 volt direct current and you want to transmit to a distance of 25 miles or more you will have to install a _motor-generator_. To make this transmitter you will need exactly the same apparatus as that described and pictured for the _100 Mile C. W. Telegraph Transmitting Set_ in Chapter XVI, except that you must subst.i.tute a _microphone transmitter_ and a _telephone induction coil_, or a _microphone transformer_, or still better, a _magnetic modulator_, for the telegraph key and chopper.

The Apparatus You Need.--To reiterate; the pieces of apparatus you need are: (1) one _aerial ammeter_ as shown at E in Fig. 75; (2) one _tuning coil_ as shown at A in Fig. 77; (3) one _aerial condenser_ as shown at B in Fig. 77; (4) one _grid leak_ as shown at C in Fig. 77; (5) one _grid, blocking_ and _protective condenser_; (6) one _5 watt oscillator tube_ as shown at E in Fig. 77; (7) one _rheostat_ as shown at I in Fig. 75; (8) one _10 volt (5 cell) storage battery_; (9) one _choke coil_; (10) one _panel cut-out_ as shown at K in Fig. 75, and (11) a _motor-generator_ having an input of 110 or 220 volts and an output of 350 volts.

In addition to the above apparatus you will need: (12) a _microphone transmitter_ as shown in Fig. 84; (13) a battery of four dry cells or a 6 volt storage battery, and either (14) a _telephone induction coil_ as shown in Fig. 86; (15) a _microphone transformer_ as shown in Fig.

87; or a _magnetic modulator_ as shown in Fig. 88. All of these parts have been described, as said above, in Chapter XVI, except the microphone modulators.

[Ill.u.s.tration: Fig. 86.--Telephone Induction Coil. (Used with Microphone Transmitter.)]

[Ill.u.s.tration: Fig. 87.--Microphone Transformer. (Used with Microphone Transmitter.)]

[Ill.u.s.tration: Fig. 88.--Magnetic Modulator. (Used with Microphone Transmitter.)]

The Telephone Induction Coil.--This is a little induction coil that transforms the 6-volt battery current after it has flowed through and been modulated by the microphone transmitter into alternating currents that have a potential of 1,000 volts of more. It consists of a primary coil of _No. 20 B. and S._ gauge cotton covered magnet wire wound on a core of soft iron wires while around the primary coil is wound a secondary coil of _No. 30_ magnet wire. Get a _standard telephone induction coil_ that has a resistance of 500 or 750 ohms and this will cost you a couple of dollars.

The Microphone Transformer.--This device is built on exactly the same principle as the telephone induction coil just described but it is more effective because it is designed especially for modulating the oscillations set up by vacuum tube transmitters. As with the telephone induction coil, the microphone transmitter is connected in series with the primary coil and a 6 volt dry or storage battery.

In the better makes of microphone transformer, there is a third winding, called a _side tone_ coil, to which a headphone can be connected so that the operator who is speaking into the microphone can listen-in and so learn if his transmitter is working up to standard.

The Magnetic Modulator.--This is a small closed iron core transformer of peculiar design and having a primary and a secondary coil wound on it. This device is used to control the variations of the oscillating currents that are set up by the oscillator tube. It is made in three sizes and for the transmitter here described you want the smallest size, which has an output of 1/2 to 1-1/2 amperes. It costs about $10.00.

How the Apparatus Is Connected Up.--The different pieces of apparatus are connected together in exactly the same way as the _100 Mile C. W.

Telegraph Set_ in Chapter XVI except that the microphone transmitter and microphone modulator (whichever kind you use) is subst.i.tuted for the telegraph key and chopper.

Now there are three different ways that the microphone and its modulator can be connected in circuit. Two of the best ways are shown at A and B in Fig. 89. In the first way the secondary terminals of the modulator are shunted around the grid leak in the grid circuit as at A, and in the second the secondary terminals are connected in the aerial as at B. Where an induction coil or a microphone transformer is used they are shunted around a condenser, but this is not necessary with the magnetic modulator. Where a second tube is used as in Fig. 90 then the microphone and its modulator are connected with the grid circuit and _clip 3_ of the tuning coil.

[Ill.u.s.tration: Fig. 89.--Wiring Diagram of 25 to 50 Mile Wireless Telephone. (Microphone Modulator Shunted Around Grid-Leak Condenser.)]

[Ill.u.s.tration: (B) Fig. 89.--Microphone Modulator Connected in Aerial Wire.]

[Ill.u.s.tration: Fig. 90.--Wiring Diagram of 50 to 100 Mile Wireless Telephone Transmitting Set.]

A 50 to 100 Mile Wireless Telephone Transmitter--With Direct Current Motor Generator.--As the initial source of current available is taken to be a 110 or 220 volt direct current a motor-generator having an output of 350 volts must be used as before. The only difference between this transmitter and the preceding one is that: (1) two 5 watt tubes are used, the first serving as an _oscillator_ and the second as a _modulator_; (2) an _oscillation choke coil_ is used in the plate circuit; (3) a _reactance coil_ or _reactor_, is used in the plate circuit; and (4) a _reactor_ is used in the grid circuit.

The Oscillation Choke Coil.--You can make this choke coil by winding about 275 turns of _No. 28 B. and S. gauge_ cotton covered magnet wire on a spool 2 inches in diameter and 4 inches long. Give it a good coat of sh.e.l.lac varnish and let it dry thoroughly.

The Plate and Grid Circuit Reactance Coils.--Where a single tube is used as an oscillator and a second tube is employed as a modulator, a _reactor_, which is a coil of wire wound on an iron core, is used in the plate circuit to keep the high voltage direct current of the motor-generator the same at all times. Likewise the grid circuit reactor is used to keep the voltage of the grid at a constant value.

These reactors are made alike and a picture of one of them is shown in Fig. 91 and each one will cost you $5.75.

[Ill.u.s.tration: Fig. 91.--Plate and Grid Circuit Reactor.]

Connecting up the Apparatus.--All of the different pieces of apparatus are connected up as shown in Fig. 89. One of the ends of the secondary of the induction coil, or the microphone transformer, or the magnetic modulator is connected to the grid circuit and the other end to _clip 3_ of the tuning coil.

A 100 to 200 Mile Wireless Telephone Transmitter--With Direct Current Motor Generator.--By using the same connections shown in the wiring diagrams in Fig. 89 and a single 50 watt oscillator tube your transmitter will then have a range of 100 miles or so, while if you connect up the apparatus as shown in Fig. 90 and use two 50 watt tubes you can work up to 200 miles. Much of the apparatus for a 50 watt oscillator set where either one or two tubes are used is of the same size and design as that just described for the 5 watt oscillator sets, but, as in the C. W. telegraph sets, some of the parts must be proportionately larger. The required parts are (1) the _50 watt tube_; (2) the _grid leak resistance_; (3) the _filament rheostat_; (4) the _filament storage battery_; and (5) the _magnetic modulator_. All of these parts, except the latter, are described in detail under the heading of a _500 Mile C. W. Telegraph Transmitting Set_ in Chapter XVI, and are also pictured in that chapter.

It is not advisable to use an induction coil for the modulator for this set, but use, instead, either a telephone transformer, or better, a magnetic modulator of the second size which has an output of from 1-1/2 to 3-1/2 amperes. The magnetic modulator is described and pictured in this chapter.

A 50 to 100 Mile Wireless Telephone Transmitting Set--With 110 Volt Alternating Current.--If you have a 110 volt [Footnote: Alternating current for lighting purposes ranges from 102.5 volts to 115 volts, so we take the median and call it 110 volts.] alternating current available you can use it for the initial source of energy for your wireless telephone transmitter. The chief difference between a wireless telephone transmitting set that uses an alternating current and one that uses a direct current is that: (1) a _power transformer_ is used for stepping up the voltage instead of a motor-generator, and (2) a _vacuum tube rectifier_ must be used to convert the alternating current into direct current.

The Apparatus You Need.--For this telephone transmitting set you need: (1) one _aerial ammeter_; (2) one _tuning coil_; (3) one _telephone modulator_; (4) one _aerial series condenser_; (5) one _4 cell dry battery_ or a 6 volt storage battery; (6) one _microphone transmitter_; (7) one _battery switch_; (8) one _grid condenser_; (9) one _grid leak_; (10) two _5 watt oscillator tubes with sockets_; (11) one _blocking condenser_; (12) one _oscillation choke coil_; (13) two _filter condensers_; (14) one _filter reactance coil_; (15) an _alternating current power transformer_, and (16) two _20 watt rectifier vacuum tubes_.

All of the above pieces of apparatus are the same as those described for the _100 Mile C. W. Telegraph Transmitter_ in Chapter XVII, except: (a) the _microphone modulator_; (b) the _microphone transmitter_ and (c) the _dry_ or _storage battery_, all of which are described in this chapter; and the new parts which are: (d) the _rectifier vacuum tubes_; (e) the _filter condensers_; and (f) the _filter reactance coil_; further and finally, the power transformer has a _third_ secondary coil on it and it is this that feeds the alternating current to the rectifier tubes, which in turn converts it into a pulsating direct current.

The Vacuum Tube Rectifier.--This rectifier has two electrodes, that is, it has a filament and a plate like the original vacuum tube detector, The smallest size rectifier tube requires a plate potential of 550 volts which is developed by one of the secondary coils of the power transformer. The filament terminal takes a current of 7.5 volts and this is supplied by another secondary coil of the transformer.

This rectifier tube delivers a direct current of 20 watts at 350 volts. It looks exactly like the 5 watt oscillator tube which is pictured at E in Fig. 77. The price is $7.50.

The Filter Condensers.--These condensers are used in connection with the reactance coil to smooth out the pulsating direct current after it has pa.s.sed through the rectifier tube. They have a capacitance of 1 mfd. and will stand 750 volts. These condensers cost about $2.00 each.

The Filter Reactance Coil.--This reactor which is shown in Fig. 92, has about the same appearance as the power transformer but it is somewhat smaller. It consists of a coil of wire wound on a soft iron core and has a large inductance, hence the capacitance of the filter condensers are proportionately smaller than where a small inductance is used which has been the general practice. The size you require for this set has an output of 160 milliamperes and it will supply current for one to four 5 watt oscillator tubes. This size of reactor costs $11.50.

[Ill.u.s.tration: Fig. 92.--Filter Reactor for Smoothing out Rectified Currents.]

Connecting Up the Apparatus.--The wiring diagram in Fig. 93 shows how the various pieces of apparatus for this telephone transmitter are connected up. You will observe: (1) that the terminals of the power transformer secondary coil which develops 10 volts are connected to the filaments of the oscillator tubes; (2) that the terminals of the other secondary coil which develops 10 volts are connected with the filaments of the rectifier tubes; (3) that the terminals of the third secondary coil which develops 550 volts are connected with the plates of the rectifier tubes; (4) that the pair of filter condensers are connected in parallel and these are connected to the mid-taps of the two filament secondary coils; (5) that the reactance coil and the third filter condenser are connected together in series and these are shunted across the filter condensers, which are in parallel; and, finally, (6) a lead connects the mid-tap of the 550-volt secondary coil of the power transformer with the connection between the reactor and the third filter condenser.

[Ill.u.s.tration: Fig 93.--100 to 200 Mile Wireless Telephone Transmitter.]

A 100 to 200 Mile Wireless Telephone Transmitting Set--With 110 Volt Alternating Current.--This telephone transmitter is built up of exactly the same pieces of apparatus and connected up in precisely the same way as the one just described and shown in Fig. 93.

Apparatus Required.--The only differences between this and the preceding transmitter are: (1) the _magnetic modulator_, if you use one, should have an output of 3-1/2 to 5 amperes; (2) you will need two _50 watt oscillator tubes with sockets_; (3) two _150 watt rectifier tubes with sockets_; (4) an _aerial ammeter_ that reads to _5 amperes_; (5) three _1 mfd. filter condensers_ in parallel; (6) _two filter condensers of 1 mfd. capacitance_ that will stand _1750 volts_; and (6) a _300 milliampere filter reactor_.

The apparatus is wired up as shown in Fig. 93.

CHAPTER XIX

THE OPERATION OF VACUUM TUBE TRANSMITTERS

The three foregoing chapters explained in detail the design and construction of (1) two kinds of C. W. telegraph transmitters, and (2) two kinds of wireless telephone transmitters, the difference between them being whether they used (A) a direct current, or (B) an alternating current as the initial source of energy. Of course there are other differences between those of like types as, for instance, the apparatus and connections used (_a_) in the key circuits, and (_b_) in the microphone circuits. But in all of the transmitters described of whatever type or kind the same fundamental device is used for setting up sustained oscillations and this is the _vacuum tube_.

The Operation of the Vacuum Tube Oscillator.--The operation of the vacuum tube in producing sustained oscillations depends on (1) the action of the tube as a valve in setting up the oscillations in the first place and (2) the action of the grid in amplifying the oscillations thus set up, both of which we explained in Chapter XIV.

In that chapter it was also pointed out that a very small change in the grid potential causes a corresponding and larger change in the amount of current flowing from the plate to the filament; and that if a vacuum tube is used for the production of oscillations the initial source of current must have a high voltage, in fact the higher the plate voltage the more powerful will be the oscillations.