CHAPTER VIII

A SIMPLE VACUUM TUBE DETECTOR RECEIVING SET

While you can receive dots and dashes from spark wireless telegraph

stations and hear spoken words and music from wireless telephone

stations with a crystal detector receiving set such as described in

Chapter III, you can get stations that are much farther away and hear

them better with a _vacuum tube detector_ receiving set.

Though the vacuum tube detector requires two batteries to operate it

and the receiving circuits are somewhat more complicated than where a

crystal detector is used still the former does not have to be

constantly adjusted as does the latter and this is another very great

advantage. Taken all in all the vacuum tube detector is the most

sensitive and the most satisfactory of the detectors that are in use

at the present time.

Not only is the vacuum tube a detector of electric wave signals and

speech and music but it can also be used to _amplify_ them, that is,

to make them stronger and, hence, louder in the telephone receiver and

further its powers of amplification are so great that it will

reproduce them by means of a _loud speaker_, just as a horn amplifies

the sounds of a phonograph reproducer, until they can be heard by a

room or an auditorium full of people. There are two general types of

loud speakers, though both use the principle of the telephone

receiver. The construction of these loud speakers will be fully

described in a later chapter.

Assembled Vacuum Tube Receiving Sets.--You can buy a receiving set

with a vacuum tube detector from the very simplest type, which is

described in this chapter, to those that are provided with

_regenerative circuits_ and _amplifying_ tubes or both, which we shall

describe in later chapters, from dealers in electrical apparatus

generally. While one of these sets costs more than you can assemble a

set for yourself, still, especially in the beginning, it is a good

plan to buy an assembled one for it is fitted with a _panel_ on which

the adjusting knobs of the rheostat, tuning coil and condenser are

mounted and this makes it possible to operate it as soon as you get it

home and without the slightest trouble on your part.

You can, however, buy all the various parts separately and mount them

yourself. If you want the receptor simply for receiving then it is a

good scheme to have all of the parts mounted in a box or enclosed

case, but if you want it for experimental purposes then the parts

should be mounted on a base or a panel so that all of the connections

are in sight and accessible.

A Simple Vacuum Tube Receiving Set.--For this set you should use: (1)

a _loose coupled tuning coil,_ (2) a _variable condenser,_ (3) a

_vacuum tube detector,_ (4) an A or _storage battery_ giving 6 volts,

(5) a B or _dry cell battery_ giving 22-1/2 volts, (6) a _rheostat_

for varying the storage battery current, and (7) a pair of 2,000-ohm

_head telephone receivers_. The loose coupled tuning coil, the

variable condenser and the telephone receivers are the same as those

described in Chapter III.

The Vacuum Tube Detector. With Two Electrodes.--A vacuum tube in its

simplest form consists of a glass bulb like an incandescent lamp in

which a _wire filament_ and a _metal plate_ are sealed as shown in

Fig. 37, The air is then pumped out of the tube and a vacuum left or

after it is exhausted it is filled with nitrogen, which cannot burn.

[Illustration: Fig. 37.--Two Electrode Vacuum Tube Detectors.]

When the vacuum tube is used as a detector, the wire filament is

heated red-hot and the metal plate is charged with positive

electricity though it remains cold. The wire filament is formed into a

loop like that of an incandescent lamp and its outside ends are

connected with a 6-volt storage battery, which is called the A

battery; then the + or _positive_ terminal of a 22-1/2 volt dry cell

battery, called the B battery, is connected to the metal plate while

the - or _negative_ terminal of the battery is connected to one of the

terminals of the wire filament. The diagram, Fig. 37, simply shows how

the two electrode vacuum tube, the A or dry battery, and the B or

storage battery are connected up.

Three Electrode Vacuum Tube Detector.--The three electrode vacuum tube

detector shown at A in Fig. 38, is much more sensitive than the two

electrode tube and has, in consequence, all but supplanted it. In this

more recent type of vacuum tube the third electrode, or _grid_, as it

is called, is placed between the wire filament and the metal plate and

this allows the current to be increased or decreased at will to a very

considerable extent.

[Illustration: Fig. 38.--Three Electrode Vacuum Tube Detector and

Battery Connections.]

The way the three electrode vacuum tube detector is connected with the

batteries is shown at B. The plate, the A or dry cell battery and one

terminal of the filament are connected in _series_--that is, one after

the other, and the ends of the filament are connected to the B or

storage battery. In assembling a receiving set you must, of course,

have a socket for the vacuum tube. A vacuum tube detector costs from

$5.00 to $6.00.

The Dry Cell and Storage Batteries.--The reason that a storage battery

is used for heating the filament of the vacuum tube detector is

because the current delivered is constant, whereas when a dry cell

battery is used the current soon falls off and, hence, the heat of the

filament gradually grows less. The smallest A or 6 volt storage

battery on the market has a capacity of 20 to 40 ampere hours, weighs

13 pounds and costs about $10.00. It is shown at A in Fig. 39. The B

or dry cell battery for the vacuum tube plate circuit that gives

22-1/2 volts can be bought already assembled in sealed boxes. The

small size is fitted with a pair of terminals while the larger size is

provided with _taps_ so that the voltage required by the plate can be

adjusted as the proper operation of the tube requires careful

regulation of the plate voltage. A dry cell battery for a plate

circuit is shown at B.

[Illustration: Fig. 39.--A and B Batteries for Vacuum

Tube Detectors.]

The Filament Rheostat.--An adjustable resistance, called a _rheostat_,

must be used in the filament and storage battery circuit so that the

current flowing through the filament can be controlled to a nicety.

The rheostat consists of an insulating and a heat resisting form on

which is wound a number of turns of resistance wire. A movable contact

arm that slides over and presses on the turns of wire is fixed to the

knob on top of the rheostat. A rheostat that has a resistance of 6

ohms and a current carrying capacity of 1.5 amperes which can be

mounted on a panel board is the right kind to use. It is shown at A

and B in Fig. 40 and costs $1.25.

[Illustration: Fig. 40.--Rheostat for the A or Storage Battery

Current.]

Assembling the Parts.--Begin by placing all of the separate parts of

the receiving set on a board or a base of other material and set the

tuning coil on the left hand side with the adjustable switch end

toward the right hand side so that you can reach it easily. Then set

the variable condenser in front of it, set the vacuum tube detector at

the right hand end of the tuning coil and the rheostat in front of the

detector. Place the two sets of batteries back of the instruments and

screw a couple of binding posts _a_ and _b_ to the right hand lower

edge of the base for connecting in the head phones all of which is

shown at A in Fig. 41.

[Illustration: (A) Fig. 41.--Top View of Apparatus Layout for a Vacuum

Tube Detector Receiving Set.]

[Illustration: (B) Fig. 41.--Wiring Diagram of a Simple Vacuum Tube

Receiving Set.]

Connecting Up the Parts.--To wire up the different parts begin by

connecting the sliding contact of the primary coil of the loose

coupled tuning coil (this you will remember is the outside one that is

wound with fine wire) to the upper post of the lightning switch and

connect one terminal of this coil with the water pipe. Now connect the

free end of the secondary coil of the tuning coil (this is the inside

coil that is wound with heavy wire) to one of the binding posts of the

variable condenser and connect the movable contact arm of the

adjustable switch of the primary of the tuning coil with the other

post of the variable condenser.

Next connect the grid of the vacuum tube to one of the posts of the

condenser and then connect the plate of the tube to the _carbon

terminal_ of the B or dry cell battery which is the + or _positive

pole_ and connect the _zinc terminal_ of the - or _negative_ pole to

the binding post _a_, connect the post _b_ to the other side of the

variable condenser and then connect the terminals of the head phones

to the binding posts _a_ and _b_. Whatever you do be careful not to

get the plate connections of the battery reversed.

Now connect one of the posts of the rheostat to one terminal of the

filament and the other terminal of the filament to the - or _negative_

terminal of the A or storage battery and the + or _positive_ terminal

of the A or storage battery to the other post of the rheostat. Finally

connect the + or positive terminal of the A or storage battery with

the wire that runs from the head phones to the variable condenser, all

of which is shown in the wiring diagram at B in Fig. 41.

Adjusting the Vacuum Tube Detector Receiving Set.--A vacuum tube

detector is tuned exactly in the same way as the _Crystal Detector Set

No. 2_ described in Chapter III, in-so-far as the tuning coil and

variable condenser are concerned. The sensitivity of the vacuum tube

detector receiving set and, hence, the distance over which signals and

other sounds can be heard depends very largely on the sensitivity of

the vacuum tube itself and this in turn depends on: (1) the right

amount of heat developed by the filament, or _filament brilliancy_ as

it is called, (2) the right amount of voltage applied to the plate,

and (3) the extent to which the tube is exhausted where this kind of a

tube is used.

To vary the current flowing from the A or storage battery

through the filament so that it will be heated to the right degree you

adjust the rheostat while you are listening in to the signals or other

sounds. By carefully adjusting the rheostat you can easily find the

point at which it makes the tube the most sensitive. A rheostat is

also useful in that it keeps the filament from burning out when the

current from the battery first flows through it. You can very often

increase the sensitiveness of a vacuum tube after you have used it for

a while by recharging the A or storage battery.

The degree to which a vacuum tube has been exhausted has a very

pronounced effect on its sensitivity. The longer the tube is used the

lower its vacuum gets and generally the less sensitive it becomes.

When this takes place (and you can only guess at it) you can very

often make it more sensitive by warming it over the flame of a candle.

Vacuum tubes having a gas content (in which case they are, of course,

no longer vacuum tubes in the strict sense) make better detectors than

tubes from which the air has been exhausted and which are sealed off

in this evacuated condition because their sensitiveness is not

dependent on the degree of vacuum as in the latter tubes. Moreover, a

tube that is completely exhausted costs more than one that is filled

with gas.

CHAPTER IX

VACUUM TUBE AMPLIFIER RECEIVING SETS

The reason a vacuum tube detector is more sensitive than a crystal

detector is because while the latter merely _rectifies_ the

oscillating current that surges in the receiving circuits, the former

acts as an _amplifier_ at the same time. The vacuum tube can be used

as a separate amplifier in connection with either: (1) a _crystal

detector_ or (2) a _vacuum tube detector_, and (_a_) it will amplify

either the _radio frequency currents_, that is the high frequency

oscillating currents which are set up in the oscillation circuits or

(_b_) it will amplify the _audio frequency currents_, that is, the

_low frequency alternating_ currents that flow through the head phone

circuit.

To use the amplified radio frequency oscillating currents or amplified

audio frequency alternating currents that are set up by an amplifier

tube either a high resistance, called a _grid leak_, or an _amplifying

transformer_, with or without an iron core, must be connected with the

plate circuit of the first amplifier tube and the grid circuit of the

next amplifier tube or detector tube, or with the wire point of a

crystal detector. Where two or more amplifier tubes are coupled

together in this way the scheme is known as _cascade amplification._

Where either a _radio frequency transformer_, that is one without the

iron core, or an _audio frequency transformer_, that is one with the

iron core, is used to couple the amplifier tube circuits together

better results are obtained than where a high resistance grid leak is

used, but the amplifying tubes have to be more carefully shielded from

each other or they will react and set up a _howling_ noise in the head

phones. On the other hand grid leaks cost less but they are more

troublesome to use as you have to find out for yourself the exact

resistance value they must have and this you can do only by testing

them out.

A Grid Leak Amplifier Receiving Set. With Crystal Detector.--The

apparatus you need for this set includes: (1) a _loose coupled tuning

coil_, (2) a _variable condenser_, (3) _two fixed condensers_, (4) a

_crystal detector_, or better a _vacuum tube detector_, (5) an A or _6

volt storage battery_, (6) a _rheostat_, (7) a B or 22-1/2 _volt dry

cell battery_, (8) a fixed resistance unit, or _leak grid_ as it is

called, and (9) a pair of _head-phones_. The tuning coil, variable

condenser, fixed condensers, crystal detectors and head-phones are

exactly the same as those described in _Set No. 2_ in Chapter III.

The A and B batteries are exactly the same as those described in

Chapter VIII. The _vacuum tube amplifier_ and the _grid leak_ are the

only new pieces of apparatus you need and not described before.

The Vacuum Tube Amplifier.--This consists of a three electrode vacuum

tube exactly like the vacuum tube detector described in Chapter VIII

and pictured in Fig. 38, except that instead of being filled with a

non-combustible gas it is evacuated, that is, the air has been

completely pumped out of it. The gas filled tube, however, can be used

as an amplifier and either kind of tube can be used for either radio

frequency or audio frequency amplification, though with the exhausted

tube it is easier to obtain the right plate and filament voltages for

good working.

The Fixed Resistance Unit, or Grid Leak.--Grid leaks are made in

different ways but all of them have an enormously high resistance.

One way of making them consists of depositing a thin film of gold on a

sheet of mica and placing another sheet of mica on top to protect it

the whole being enclosed in a glass tube as shown at A in Fig. 42.

These grid leaks are made in units of from 50,000 ohms (.05 megohm) to

5,000,000 ohms (5 megohms) and cost from $1 to $2.

[Illustration: Fig. 42.--Grid Leaks and How to Connect Them up.]

As the _value_ of the grid leak you will need depends very largely

upon the construction of the different parts of your receiving set and

on the kind of aerial wire system you use with it you will have to try

out various resistances until you hit the right one. The resistance

that will give the best results, however, lies somewhere between

500,000 ohms (1/2 a megohm) and 3,000,000 ohms (3 megohms) and the

only way for you to find this out is to buy 1/2, 1 and 2 megohm grid

leak resistances and connect them up in different ways, as shown at B,

until you find the right value.

Assembling the Parts for a Crystal Detector Set.--Begin by laying the

various parts out on a base or a panel with the loose coupled tuning

coil on the left hand side, but with the adjustable switch of the

secondary coil on the right hand end or in front according to the way

it is made. Then place the variable condenser, the rheostat, the

crystal detector and the binding posts for the head phones in front of

and in a line with each other. Set the vacuum tube amplifier back of

the rheostat and the A and B batteries back of the parts or in any

other place that may be convenient. The fixed condensers and the grid

leak can be placed anywhere so that it will be easy to connect them in

and you are ready to wire up the set.

Connecting Up the Parts for a Crystal Detector.--First connect the

sliding contact of the primary of the tuning coil to the leading-in

wire and one of the end wires of the primary to the water pipe, as

shown in Fig. 43. Now connect the adjustable arm that makes contact

with one end of the secondary of the tuning coil to one of the posts

of the variable condenser; then connect the other post of the latter

with a post of the fixed condenser and the other post of this with the

grid of the amplifying tube.

[Illustration: Fig. 43.--Crystal Detector Receiving Set with Vacuum

Tube Amplifier (Resistance Coupled).]

Connect the first post of the variable condenser to the + or _positive

electrode_ of the A battery and its - or _negative electrode_ with the

rotating contact arm of the rheostat. Next connect one end of the

resistance coil of the rheostat to one of the posts of the amplifier

tube that leads to the filament and the other filament post to the +

or _positive electrode_ of the A battery. This done connect the

_negative_, that is, the _zinc pole_ of the B battery to the positive

electrode of the A battery and connect the _positive_, or _carbon

pole_ of the former with one end of the grid leak and connect the

other end of this to the plate of the amplifier tube.

To the end of the grid leak connected with the plate of the amplifier

tube connect the metal point of your crystal detector, the crystal of

the latter with one post of the head phones and the other post of them

with the other end of the grid leak and, finally, connect a fixed

condenser in _parallel_ with--that is across the ends of the grid

leak, all of which is shown in the wiring diagram in Fig. 43.

A Grid Leak Amplifying Receiving Set With Vacuum Tube Detector.--A

better amplifying receiving set can be made than the one just

described by using a vacuum tube detector instead of the crystal

detector. This set is built up exactly like the crystal detector

described above and shown in Fig. 43 up to and including the grid leak

resistance, but shunted across the latter is a vacuum tube detector,

which is made and wired up precisely like the one shown at A in Fig.

41 in the chapter ahead of this one. The way a grid leak and vacuum

tube detector with a one-step amplifier are connected up is shown at A

in Fig. 44. Where you have a vacuum tube detector and one or more

amplifying tubes connected up, or in _cascade_ as it is called, you

can use an A, or storage battery of 6 volts for all of them as shown

at B in Fig. 44, but for every vacuum tube you use you must have a B

or 22-1/2 volt dry battery to charge the plate with.

[Illustration: (A) Fig. 44--Vacuum Tube Detector Set with One Step

Amplifier (Resistance Coupled).]

[Illustration: (B) Fig. 44.--Wiring Diagram for Using One A or Storage

Battery with an Amplifier and a Detector Tube.]

A Radio Frequency Transformer Amplifying Receiving Set.--Instead of

using a grid leak resistance to couple up the amplifier and detector

tube circuits you can use a _radio frequency transformer_, that is, a

transformer made like a loose coupled tuning coil, and without an iron

core, as shown in the wiring diagram at A in Fig. 45. In this set,

which gives better results than where a grid leak is used, the

amplifier tube is placed in the first oscillation circuit and the

detector tube in the second circuit.

[Illustration: (A) Fig. 45.--Wiring Diagram for a Radio Frequency

Transformer Amplifying Receiving Set.]

[Illustration: (B) Fig. 45.--Radio Frequency Transformer.]

Since the radio frequency transformer has no iron core the high

frequency, or _radio frequency_ oscillating currents, as they are

called, surge through it and are not changed into low frequency, or

_audio frequency_ pulsating currents, until they flow through the

detector. Since the diagram shows only one amplifier and one radio

frequency transformer, it is consequently a _one step amplifier_;

however, two, three or more, amplifying tubes can be connected up by

means of an equal number of radio frequency transformers when you will

get wonderful results. Where a six step amplifier, that is, where six

amplifying tubes are connected together, or in _cascade_, the first

three are usually coupled up with radio frequency transformers and the

last three with audio frequency transformers. A radio frequency

transformer is shown at B and costs $6 to $7.

An Audio Frequency Transformer Amplifying Receiving Set.--Where audio

frequency transformers are used for stepping up the voltage of the

current of the detector and amplifier tubes, the radio frequency

current does not get into the plate circuit of the detector at all for

the reason that the iron core of the transformer chokes them off,

hence, the succeeding amplifiers operate at audio frequencies. An

audio frequency transformer is shown at A in Fig. 46 and a wiring

diagram showing how the tubes are connected in _cascade_ with the

transformers is shown at B; it is therefore a two-step audio frequency

receiving set.

[Illustration: (A) Fig. 46.--Audio Frequency Transformer.]

[Illustration: (B) Fig. 46--Wiring Diagram for an Audio Frequency

Transformer Amplifying Receiving Set. (With Vacuum Tube Detector and

Two Step Amplifier Tubes.)]

A Six Step Amplifier Receiving Set With a Loop Aerial.--By using a

receiving set having a three step radio frequency and a three step

audio frequency, that is, a set in which there are coupled three

amplifying tubes with radio frequency transformers and three

amplifying tubes with audio frequency transformers as described under

the caption _A Radio Frequency Transformer Receiving Set_, you can use

a _loop aerial_ in your room thus getting around the difficulties--if

such there be--in erecting an out-door aerial. You can easily make a

loop aerial by winding 10 turns of _No. 14_ or _16_ copper wire about

1/16 inch apart on a wooden frame two feet on the side as shown in

Fig. 47. With this six step amplifier set and loop aerial you can

receive wave lengths of 150 to 600 meters from various high power

stations which are at considerable distances away.

[Illustration: (A) Fig. 47.--Six Step Amplifier with Loop Aerial.]

[Illustration: (B) Fig. 47.--Efficient Regenerative Receiving Set.

(With Three Coil Loose Coupler Tuner.)]

How to Prevent Howling.--Where radio frequency or audio frequency

amplifiers are used to couple your amplifier tubes in cascade you must

take particular pains to shield them from one another in order to

prevent the _feed back_ of the currents through them, which makes the

head phones or loud speaker _howl_. To shield them from each other the

tubes should be enclosed in metal boxes and placed at least 6 inches

apart while the transformers should be set so that their cores are at

right angles to each other and these also should be not less than six

inches apart.

CHAPTER X

REGENERATIVE AMPLIFICATION RECEIVING SETS

While a vacuum tube detector has an amplifying action of its own, and

this accounts for its great sensitiveness, its amplifying action can

be further increased to an enormous extent by making the radio

frequency currents that are set up in the oscillation circuits react

on the detector.

Such currents are called _feed-back_ or _regenerative_ currents and

when circuits are so arranged as to cause the currents to flow back

through the detector tube the amplification keeps on increasing until

the capacity of the tube itself is reached. It is like using steam

over and over again in a steam turbine until there is no more energy

left in it. A system of circuits which will cause this regenerative

action to take place is known as the _Armstrong circuits_ and is so

called after the young man who discovered it.

Since the regenerative action of the radio frequency currents is

produced by the detector tube itself and which sets up an amplifying

effect without the addition of an amplifying tube, this type of

receiving set has found great favor with amateurs, while in

combination with amplifying tubes it multiplies their power

proportionately and it is in consequence used in one form or another

in all the better sets.

There are many different kinds of circuits which can be used to

produce the regenerative amplification effect while the various kinds

of tuning coils will serve for coupling them; for instance a two or

three slide single tuning coil will answer the purpose but as it does

not give good results it is not advisable to spend either time or

money on it. A better scheme is to use a loose coupler formed of two

or three honeycomb or other compact coils, while a _variocoupler_ or a

_variometer_ or two will produce the maximum regenerative action.

The Simplest Type of Regenerative Receiving Set. With Loose Coupled

Tuning Coil.--While this regenerative set is the simplest that will

give anything like fair results it is here described not on account of

its desirability, but because it will serve to give you the

fundamental idea of how the _feed-back_ circuit is formed.

For this set you need: (1) a _loose-coupled tuning coil_ such as

described in Chapter III, (2) a _variable condenser_ of _.001 mfd._

(microfarad) capacitance; (3) one _fixed condenser_ of _.001 mfd._;

(4) one _fixed condenser_ for the grid leak circuit of _.00025 mfd._;

(5) a _grid leak_ of 1/2 to 2 megohms resistance; (6) a _vacuum tube

detector_; (7) an _A 6 volt battery_; (8) a _rheostat_; (9) a _B 22

1/2 volt battery_; and (10) a pair of _2000 ohm head phones_.

Connecting Up the Parts.--Begin by connecting the leading-in wire of

the aerial with the binding post end of the primary coil of the loose

coupler as shown in the wiring diagram Fig. 48 and then connect the

sliding contact with the water pipe or other ground. Connect the

binding post end of the primary coil with one post of the variable

condenser, connect the other post of this with one of the posts of the

_.00025 mfd._ condenser and the other end of this with the grid of the

detector tube; then around this condenser shunt the grid leak

resistance.

[Illustration: Fig. 48.--Simple Regenerative Receiving Set. (With

Loose Coupler Tuner.)]

Next connect the sliding contact of the primary coil with the other

post of the variable condenser and from this lead a wire on over to

one of the terminals of the filament of the vacuum tube; to the other

terminal of the filament connect one of the posts of the rheostat and

connect the other post to the - or negative electrode of the A

battery and then connect the + or positive electrode of it to the

other terminal of the filament.

Connect the + or positive electrode of the A battery with one post of

the .001 mfd. fixed condenser and connect the other post of this to

one of the ends of the secondary coil of the tuning coil and which is

now known as the _tickler coil_; then connect the other end of the

secondary, or tickler coil to the plate of the vacuum tube. In the

wiring diagram the secondary, or tickler coil is shown above and in a

line with the primary coil but this is only for the sake of making the

connections clear; in reality the secondary, or tickler coil slides to

and fro in the primary coil as shown and described in Chapter III.

Finally connect the _negative_, or zinc pole of the _B battery_ to one

side of the fixed condenser, the _positive_, or carbon, pole to one of

the terminals of the head phones and the other terminal of this to the

other post of the fixed condenser when your regenerative set is

complete.

An Efficient Regenerative Receiving Set. With Three Coil Loose

Coupler.--To construct a really good regenerative set you must use a

loose coupled tuner that has three coils, namely a _primary_, a

_secondary_ and a _tickler coil_. A tuner of this kind is made like an

ordinary loose coupled tuning coil but it has a _third_ coil as shown

at A and B in Fig. 49. The middle coil, which is the _secondary_, is

fixed to the base, and the large outside coil, which is the _primary_,

is movable, that is it slides to and fro over the middle coil, while

the small inside coil, which is the _tickler_, is also movable and can

slide in or out of the middle _coil_. None of these coils is variable;

all are wound to receive waves up to 360 meters in length when used

with a variable condenser of _.001 mfd_. capacitance. In other words

you slide the coils in and out to get the right amount of coupling and

you tune by adjusting the variable condenser to get the exact wave

length you want.

[Illustration: (A) Fig. 49.--Diagram of a Three Coil Coupler.]

[Illustration: (B) Fig. 49.--Three Coil Loose Coupler Tuner.]

With Compact Coils.--Compact coil tuners are formed of three fixed

inductances wound in flat coils, and these are pivoted in a mounting

so that the distance between them and, therefore, the coupling, can be

varied, as shown at A in Fig. 50. These coils are wound up by the

makers for various wave lengths ranging from a small one that will

receive waves of any length up to 360 meters to a large one that has a

maximum of 24,000 meters. For an amateur set get three of the smallest

coils when you can not only hear amateur stations that send on a 200

meter wave but broadcasting stations that send on a 360 meter wave.

[Illustration: Fig. 50.--Honeycomb Inductance Coil.]

These three coils are mounted with panel plugs which latter fit into a

stand, or mounting, so that the middle coil is fixed, that is,

stationary, while the two outside coils can be swung to and fro like a

door; this scheme permits small variations of coupling to be had

between the coils and this can be done either by handles or by means

of knobs on a panel board. While I have suggested the use of the

smallest size coils, you can get and use those wound for any wave

length you want to receive and when those are connected with

variometers and variable condensers, and with a proper aerial, you

will have a highly efficient receptor that will work over all ranges

of wave lengths. The smallest size coils cost about $1.50 apiece and

the mounting costs about $6 or $7 each.

The A Battery Potentiometer.--This device is simply a resistance like

the rheostat described in connection with the preceding vacuum tube

receiving sets but it is wound to 200 or 300 ohms resistance as

against 1-1/2 to 6 ohms of the rheostat. It is, however, used as well

as the rheostat. With a vacuum tube detector, and especially with one

having a gas-content, a potentiometer is very necessary as it is only

by means of it that the potential of the plate of the detector can be

accurately regulated. The result of proper regulation is that when the

critical potential value is reached there is a marked increase in the

loudness of the sounds that are emitted by the head phones.

As you will see from A in Fig. 51 it has three taps. The two taps

which are connected with the ends of the resistance coil are shunted

around the A battery and the third tap, which is attached to the

movable contact arm, is connected with the B battery tap, see B, at

which this battery gives 18 volts. Since the A battery gives 6 volts

you can vary the potential of the plate from 18 to 24 volts. The

potentiometer must never be shunted around the B battery or the latter

will soon run down. A potentiometer costs a couple of dollars.

[Illustration: (A) Fig. 51.--The Use of the Potentiometer.]

The Parts and How to Connect Them Up.--For this regenerative set you

will need: (1) a _honeycomb_ or other compact _three-coil tuner_, (2)

two _variable_ (_.001_ and _.0005 mfd_.) _condensers_; (3) a _.00025

mfd. fixed condenser_; (4) a _1/2 to 2 megohm grid leak_; (5) a _tube

detector_; (6) a _6 volt A battery_; (7) _a rheostat_; (8) a

_potentiometer_; (9) an _18_ or _20 volt B battery_; (10) a _fixed

condenser_ of _.001 mfd. fixed condenser_; and (11) a _pair of 2000

ohm head phones_.

To wire up the parts connect the leading-in wire of the aerial with

the primary coil, which is the middle one of the tuner, and connect

the other terminal with the ground. Connect the ends of the secondary

coil, which is the middle one, with the posts of the variable

condenser and connect one of the posts of the latter with one post of

the fixed .00025 mfd. condenser and the other post of this with the

grid; then shunt the grid leak around it. Next connect the other post

of the variable condenser to the - or _negative_ electrode of the _A

battery_; the + or _positive_ electrode of this to one terminal of the

detector filament and the other end of the latter to the electrode of

the A battery.

Now connect one end of the tickler coil with the detector plate and

the other post to the fixed .001 mfd. condenser, then the other end of

this to the positive or carbon pole of the B battery.

This done shunt the potentiometer around the A battery and run a wire

from the movable contact of it (the potentiometer) over to the 18 volt

tap, (see B, Fig. 51), of the B battery.

Finally, shunt the head phones and the .001 mfd. fixed condenser and

you are ready to try out conclusions.

A Regenerative Audio Frequency Amplifier Receiving Set.--The use of

amateur regenerative cascade audio frequency receiving sets is getting

to be quite common. To get the greatest amplification possible with

amplifying tubes you have to keep a negative potential on the grids.

You can, however, get very good results without any special charging

arrangement by simply connecting one post of the rheostat with the

negative terminal of the filament and connecting the _low potential_

end of the secondary of the tuning coil with the - or negative

electrode of the A battery. This scheme will give the grids a negative

bias of about 1 volt. You do not need to bother about these added

factors that make for high efficiency until after you have got your

receiving set in working order and understand all about it.

The Parts and How to Connect Them Up.--Exactly the same parts are

needed for this set as the one described above, but in addition you

will want: (1) two more _rheostats_; (2) _two_ more sets of B 22-1/2

_volt batteries_; (3) _two amplifier tubes_, and (4) _two audio

frequency transformers_ as described in Chapter IX and pictured at A

in Fig. 46.

To wire up the parts begin by connecting the leading-in wire to one

end of the primary of the tuning coil and then connect the other end

of the coil with the ground. A variable condenser of .001 mfd.

capacitance can be connected in the ground wire, as shown in Fig. 52,

to good advantage although it is not absolutely needed. Now connect

one end of the secondary coil to one post of a _.001 mfd._ variable

condenser and the other end of the secondary to the other post of the

condenser.

[Illustration: Fig. 52.--Regenerative Audio Frequency Amplifier

Receiving Set.]

Next bring a lead (wire) from the first post of the variable condenser

over to the post of the first fixed condenser and connect the other

post of the latter with the grid of the detector tube. Shunt 1/2 to 2

megohm grid leak resistance around the fixed condenser and then

connect the second post of the variable condenser to one terminal of

the detector tube filament. Run this wire on over and connect it with

the first post of the second rheostat, the second post of which is

connected with one terminal of the filament of the first amplifying

tube; then connect the first post of the rheostat with one end of the

secondary coil of the first audio frequency transformer, and the other

end of this coil with the grid of the first amplifier tube.

Connect the lead that runs from the second post of variable condenser

to the first post of the third rheostat, the second post of which is

connected with one terminal of the second amplifying tube; then

connect the first post of the rheostat with one end of the secondary

coil of the second audio frequency transformer and the other end of

this coil with the grid of the second amplifier tube.

This done connect the - or negative electrode of the A battery

with the second post of the variable condenser and connect the + or

positive electrode with the free post of the first rheostat, the other

post of which connects with the free terminal of the filament of the

detector. From this lead tap off a wire and connect it to the free

terminal of the filament of the first amplifier tube, and finally

connect the end of the lead with the free terminal of the filament of

the second amplifier tube.

Next shunt a potentiometer around the A battery and connect the

third post, which connects with the sliding contact, to the negative

or zinc pole of a B battery, then connect the positive or

carbon pole of it to the negative or zinc pole of a second B

battery and the positive or carbon pole of the latter with one end of

the primary coil of the second audio frequency transformer and the

other end of it to the plate of the first amplifying tube. Run the

lead on over and connect it to one of the terminals of the second

fixed condenser and the other terminal of this with the plate of the

second amplifying tube. Then shunt the headphones around the

condenser.

Finally connect one end of the tickler coil of the tuner with the

plate of the detector tube and connect the other end of the tickler to

one end of the primary coil of the first audio frequency transformer

and the other end of it to the wire that connects the two B

batteries together.

CHAPTER XI

SHORT WAVE REGENERATIVE RECEIVING SETS

A _short wave receiving set_ is one that will receive a range of

wave lengths of from 150 to 600 meters while the distance over which

the waves can be received as well as the intensity of the sounds

reproduced by the headphones depends on: (1) whether it is a

regenerative set and (2) whether it is provided with amplifying tubes.

High-grade regenerative sets designed especially for receiving amateur

sending stations that must use a short wave length are built on the

regenerative principle just like those described in the last chapter

and further amplification can be had by the use of amplifier tubes as

explained in Chapter IX, but the new feature of these sets is the use

of the _variocoupler_ and one or more _variometers_. These tuning

devices can be connected up in different ways and are very popular

with amateurs at the present time.

Differing from the ordinary loose coupler the variometer has no

movable contacts while the variometer is provided with taps so that

you can connect it up for the wave length you want to receive. All you

have to do is to tune the oscillation circuits to each other is to

turn the _rotor_, which is the secondary coil, around in the _stator_,

as the primary coil is called in order to get a very fine variation of

the wave length. It is this construction that makes _sharp tuning_

with these sets possible, by which is meant that all wave lengths are

tuned out except the one which the receiving set is tuned for.

A Short Wave Regenerative Receiver--With One Variometer and Three

Variable Condensers.--This set also includes a variocoupler and a

_grid coil_. The way that the parts are connected together makes it a

simple and at the same time a very efficient regenerative receiver for

short waves. While this set can be used without shielding the parts

from each other the best results are had when shields are used.

The parts you need for this set include: (1) one _variocoupler_; (2)

one _.001 microfarad variable condenser_; (3) one _.0005 microfarad

variable condenser_; (4) one _.0007 microfarad variable condenser_;

(5) _one 2 megohm grid leak_; (6) one _vacuum tube detector_; (7) one

_6 volt A battery_; (8) one _6 ohm_, 1-1/2 _ampere rheostat_; (9) one

_200 ohm potentiometer_; (10) one 22-1/2 _volt B battery_; (11) one

_.001 microfarad fixed condenser_, (12) one pair of _2,000 ohm

headphones_, and (13) a _variometer_.

The Variocoupler.--A variocoupler consists of a primary coil wound on

the outside of a tube of insulating material and to certain turns of

this taps are connected so that you can fix the wave length which your

aerial system is to receive from the shortest wave; i.e., 150 meters

on up by steps to the longest wave, i.e., 600 meters, which is the

range of most amateur variocouplers that are sold in the open market.

This is the part of the variocoupler that is called the _stator_.

The secondary coil is wound on the section of a ball mounted on a

shaft and this is swung in bearings on the stator so that it can turn

in it. This part of the variocoupler is called the _rotor_ and is

arranged so that it can be mounted on a panel and adjusted by means of

a knob or a dial. A diagram of a variocoupler is shown at A in Fig.

53, and the coupler itself at B. There are various makes and

modifications of variocouplers on the market but all of them are about

the same price which is $6.00 or $8.00.

[Illustration: Fig. 53.--How the Variocoupler is Made and Works.]

The Variometer.--This device is quite like the variocoupler, but with

these differences: (1) the rotor turns in the stator, which is also

the section of a ball, and (2) one end of the primary is connected

with one end of the secondary coil. To be really efficient a

variometer must have a small resistance and a large inductance as well

as a small dielectric loss. To secure the first two of these factors

the wire should be formed of a number of fine, pure copper wires each

of which is insulated and the whole strand then covered with silk.

This kind of wire is the best that has yet been devised for the

purpose and is sold under the trade name of _litzendraht_.

A new type of variometer has what is known as a _basket weave_, or

_wavy wound_ stator and rotor. There is no wood, insulating compound

or other dielectric materials in large enough quantities to absorb the

weak currents that flow between them, hence weaker sounds can be heard

when this kind of a variometer is used. With it you can tune sharply

to waves under 200 meters in length and up to and including wave

lengths of 360 meters. When amateur stations of small power are

sending on these short waves this style of variometer keeps the

electric oscillations at their greatest strength and, hence, the

reproduced sounds will be of maximum intensity. A wiring diagram of a

variometer is shown at A in Fig. 54 and a _basketball_ variometer is

shown complete at B.

[Illustration: Fig. 54.--How the Variometer is Made and Works.]

Connecting Up the Parts.--To hook-up the set connect the leading-in

wire to one end of the primary coil, or stator, of the variocoupler

and solder a wire to one of the taps that gives the longest wave

length you want to receive. Connect the other end of this wire with

one post of a .001 microfarad variable condenser and connect the other

post with the ground as shown in Fig. 55. Now connect one end of the

secondary coil, or rotor, to one post of a .0007 mfd. variable

condenser, the other post of this to one end of the grid coil and the

other end of this with the remaining end of the rotor of the

variocoupler.

[Illustration: Fig. 55.--Short Wave Regenerative Receiving Set (one

Variometer and three Variable Condensers.)]

Next connect one post of the .0007 mfd. condenser with one of the

terminals of the detector filament; then connect the other post of

this condenser with one post of the .0005 mfd. variable condenser and

the other post of this with the grid of the detector, then shunt the

megohm grid leak around the latter condenser. This done connect the

other terminal of the filament to one post of the rheostat, the other

post of this to the - or negative electrode of the 6 volt A

battery and the + or positive electrode of the latter to the other

terminal of the filament.

Shunt the potentiometer around the A battery and connect the sliding

contact with the - or zinc pole of the B battery and the + or carbon

pole with one terminal of the headphone; connect the other terminal to

one of the posts of the variometer and the other post of the

variometer to the plate of the detector. Finally shunt a .001 mfd.

fixed condenser around the headphones. If you want to amplify the

current with a vacuum tube amplifier connect in the terminals of the

amplifier circuit shown at A in Figs. 44 or 45 at the point where

they are connected with the secondary coil of the loose coupled tuning

coil, in those diagrams with the binding posts of Fig. 55 where the

phones are usually connected in.

Short Wave Regenerative Receiver. With Two Variometers and Two

Variable Condensers.--This type of regenerative receptor is very

popular with amateurs who are using high-grade short-wave sets. When

you connect up this receptor you must keep the various parts well

separated. Screw the variocoupler to the middle of the base board or

panel, and secure the variometers on either side of it so that the

distance between them will be 9 or 10 inches. By so placing them the

coupling will be the same on both sides and besides you can shield

them from each other easier.

For the shield use a sheet of copper on the back of the panel and

place a sheet of copper between the parts, or better, enclose the

variometers and detector and amplifying tubes if you use the latter in

sheet copper boxes. When you set up the variometers place them so that

their stators are at right angles to each other for otherwise the

magnetic lines of force set up by the coils of each one will be

mutually inductive and this will make the headphones or loud speaker

_howl_. Whatever tendency the receptor has to howl with this

arrangement can be overcome by putting in a grid leak of the right

resistance and adjusting the condenser.

The Parts and How to Connect Them Up.--For this set you require: (1)

one _variocoupler_; (2) two _variometers_; (3) one _.001 microfarad

variable condenser_; (4) one _.0005 microfarad variable condenser_;

(5) one _2 megohm grid leak resistance_; (6) one _vacuum tube

detector_; (7) one _6 volt A battery_; (8) one _200 ohm

potentiometer_; (9) one _22-1/2 volt B battery_; (10) one _.001

microfarad fixed condenser_, and (11) one pair of _2,000 ohm

headphones_.

To wire up the set begin by connecting the leading-in wire to the

fixed end of the primary coil, or _stator_, of the variocoupler, as

shown in Fig. 56, and connect one post of the .001 mfd. variable

condenser to the stator by soldering a short length of wire to the tap

of the latter that gives the longest wave you want to receive. Now

connect one end of the secondary coil, or _rotor_, of the variocoupler

with one post of the .0005 mfd. variable condenser and the other part

to the grid of the detector tube. Connect the other end of the rotor

of the variocoupler to one of the posts of the first variometer and

the other post of this to one of the terminals of the detector

filament.

[Illustration: Fig. 56.--Short Wave Regenerative Receiving Set (two

Variometers and two Variable Condensers.)]

Connect this filament terminal with the - or negative electrode of the

A battery and the + or positive electrode of this with one post

of the rheostat and lead a wire from the other post to the free

terminal of the filament. This done shunt the potential around the

A battery and connect the sliding contact to the - or zinc pole

of the B battery and the + or carbon pole of this to one

terminal of the headphones, while the other terminal of this leads to

one of the posts of the second variometer, the other post of which is

connected to the plate of the detector tube. If you want to add an

amplifier tube then connect it to the posts instead of the headphones

as described in the foregoing set.

CHAPTER XII

INTERMEDIATE AND LONG WAVE REGENERATIVE RECEIVING SETS

All receiving sets that receive over a range of wave lengths of from

150 meters to 3,000 meters are called _intermediate wave sets_ and all

sets that receive wave lengths over a range of anything more than

3,000 meters are called _long wave sets_. The range of intermediate

wave receptors is such that they will receive amateur, broadcasting,

ship and shore Navy, commercial, Arlington's time and all other

stations using _spark telegraph damped waves_ or _arc_ or _vacuum tube

telephone continuous waves_ but not _continuous wave telegraph

signals_, unless these have been broken up into groups at the

transmitting station. To receive continuous wave telegraph signals

requires receiving sets of special kind and these will be described in

the next chapter.

Intermediate Wave Receiving Sets.--There are two chief schemes

employed to increase the range of wave lengths that a set can receive

and these are by using: (1) _loading coils_ and _shunt condensers_,

and (2) _bank-wound coils_ and _variable condensers_. If you have a

short-wave set and plan to receive intermediate waves with it then

loading coils and fixed condensers shunted around them affords you the

way to do it, but if you prefer to buy a new receptor then the better

way is to get one with bank-wound coils and variable condensers; this

latter way preserves the electrical balance of the oscillation

circuits better, the electrical losses are less and the tuning easier

and sharper.

Intermediate Wave Set With Loading Coils.--For this intermediate wave

set you can use either of the short-wave sets described in the

foregoing chapter. For the loading coils use _honeycomb coils_, or

other good compact inductance coils, as shown in Chapter X and having

a range of whatever wave length you wish to receive. The following

table shows the range of wave length of the various sized coils when

used with a variable condenser having a .001 microfarad _capacitance_,

the approximate _inductance_ of each coil in _millihenries_ and prices

at the present writing:

TABLE OF CHARACTERISTICS OF HONEYCOMB COILS

Approximate Wave

Length in Meters in

Millihenries

Inductance .001 mfd. Variable Mounted

Appx. Air Condenser. on Plug

.040 130-- 375 $1.40

.075 180-- 515 1.40

.15 240-- 730 1.50

.3 330-- 1030 1.50

.6 450-- 1460 1.55

1.3 660-- 2200 1.60

2.3 930-- 2850 1.65

4.5 1300-- 4000 1.70

6.5 1550-- 4800 1.75

11. 2050-- 6300 1.80

20. 3000-- 8500 2.00

40. 4000--12000 2.15

65. 5000--15000 2.35

100. 6200--19000 2.60

125. 7000--21000 3.00

175. 8200--24000 3.50

These and other kinds of compact coils can be bought at electrical

supply houses that sell wireless goods. If your aerial is not very

high or long you can use loading coils, but to get anything like

efficient results with them you must have an aerial of large

capacitance and the only way to get this is to put up a high and long

one with two or more parallel wires spaced a goodly distance apart.

The Parts and How to Connect Them Up.--Get (1) _two honeycomb or other

coils_ of the greatest wave length you want to receive, for in order

to properly balance the aerial, or primary oscillation circuit, and

the closed, or secondary oscillation circuit, you have to tune them to

the same wave length; (2) two _.001 mfd. variable condensers_, though

fixed condensers will do, and (3) two small _single-throw double-pole

knife switches_ mounted on porcelain bases.

To use the loading coils all you have to do is to connect one of them

in the aerial above the primary coil of the loose coupler, or

variocoupler as shown in the wiring diagram in Fig. 57, then shunt one

of the condensers around it and connect one of the switches around

this; this switch enables you to cut in or out the loading coil at

will. Likewise connect the other loading coil in one side of the

closed, or secondary circuit between the variable .0007 mfd. condenser

and the secondary coil of the loose coupler or variocoupler as shown

in Fig. 53. The other connections are exactly the same as shown in

Figs. 44 and 45.

[Illustration: Fig. 57.--Wiring Diagram Showing Fixed Loading Coils

for Intermediate Wave Set.]

An Intermediate Wave Set With Variocoupler Inductance Coils.--By using

the coil wound on the rotor of the variocoupler as the tickler the

coupling between the detector tube circuits and the aerial wire system

increases as the set is tuned for greater wave lengths. This scheme

makes the control of the regenerative circuit far more stable than it

is where an ordinary loose coupled tuning coil is used.

When the variocoupler is adjusted for receiving very long waves the

rotor sets at right angles to the stator and, since when it is in this

position there is no mutual induction between them, the tickler coil

serves as a loading coil for the detector plate oscillation circuit.

Inductance coils for short wave lengths are usually wound in single

layers but _bank-wound coils_, as they are called are necessary to get

compactness where long wave lengths are to be received. By winding

inductance coils with two or more layers the highest inductance values

can be obtained with the least resistance. A wiring diagram of a

multipoint inductance coil is shown in Fig. 58. You can buy this

intermediate wave set assembled and ready to use or get the parts and

connect them up yourself.

[Illustration: Fig. 58.--Wiring Diagram for Intermediate Wave Receptor

with one Variocoupler and 12 section Bank-wound Inductance Coil.]

The Parts and How to Connect Them Up.--For this regenerative

intermediate wave set get: (1) one _12 section triple bank-wound

inductance coil_, (2) one _variometer_, and (3) all the other parts

shown in the diagram Fig. 58 except the variocoupler. First connect

the free end of the condenser in the aerial to one of the terminals of

the stator of the variocoupler; then connect the other terminal of the

stator with one of the ends of the bank-wound inductance coil and

connect the movable contact of this with the ground.

Next connect a wire to the aerial between the variable condenser and

the stator and connect this to one post of a .0005 microfarad fixed

condenser, then connect the other post of this with the grid of the

detector and shunt a 2 megohm grid leak around it. Connect a wire to

the ground wire between the bank-wound inductance coil and the ground

proper, i.e., the radiator or water pipe, connect the other end of

this to the + electrode of the A battery and connect this end also to

one of the terminals of the filament. This done connect the other

terminal of the filament to one post of the rheostat and the other

post of this to the - or negative side of the A battery.

To the + electrode of the A battery connect the - or zinc pole of the

B battery and connect the + or carbon pole of the latter with one post

of the fixed .001 microfarad condenser. This done connect one terminal

of the tickler coil which is on the rotor of the variometer to the

plate of the detector and the other terminal of the tickler to the

other post of the .001 condenser and around this shunt your

headphones. Or if you want to use one or more amplifying tubes connect

the circuit of the first one, see Fig. 45, to the posts on either side

of the fixed condenser instead of the headphones.

A Long Wave Receiving Set.--The vivid imagination of Jules Verne never

conceived anything so fascinating as the reception of messages without

wires sent out by stations half way round the world; and in these days

of high power cableless stations on the five continents you can

listen-in to the messages and hear what is being sent out by the

Lyons, Paris and other French stations, by Great Britain, Italy,

Germany and even far off Russia and Japan.

A long wave set for receiving these stations must be able to tune to

wave lengths up to 20,000 meters. Differing from the way in which the

regenerative action of the short wave sets described in the preceding

chapter is secured and which depends on a tickler coil and the

coupling action of the detector in this long wave set, [Footnote: All

of the short wave and intermediate wave receivers described, are

connected up according to the wiring diagram used by the A. H. Grebe

Company, Richmond Hill, Long Island, N. Y.] this action is obtained by

the use of a tickler coil in the plate circuit which is inductively

coupled to the grid circuit and this feeds back the necessary amount

of current. This is a very good way to connect up the circuits for the

reason that: (1) the wiring is simplified, and (2) it gives a single

variable adjustment for the entire range of wave lengths the receptor

is intended to cover.

The Parts and How to Connect Them Up.--The two chief features as far

as the parts are concerned of this long wave length receiving set are

(1) the _variable condensers_, and (2) the _tuning inductance coils_.

The variable condenser used in series with the aerial wire system has

26 plates and is equal to a capacitance of _.0008 mfd._ which is the

normal aerial capacitance. The condenser used in the secondary coil

circuit has 14 plates and this is equal to a capacitance of _.0004

mfd_.

There are a number of inductance coils and these are arranged so that

they can be connected in or cut out and combinations are thus formed

which give a high efficiency and yet allow them to be compactly

mounted. The inductance coils of the aerial wire system and those of

the secondary coil circuit are practically alike. For wave lengths up

to 2,200 meters _bank litz-wound coils_ are used and these are

wound up in 2, 4 and 6 banks in order to give the proper degree of

coupling and inductance values.

Where wave lengths of more than 2,200 meters are to be received

_coto-coils_ are used as these are the "last word" in inductance coil

design, and are especially adapted for medium as well as long wave

lengths. [Footnote: Can be had of the Coto Coil Co., Providence, R. I.]

These various coils are cut in and out by means of two five-point

switches which are provided with auxiliary levers and contactors for

_dead-ending_ the right amount of the coils. In cutting in coils for

increased wave lengths, that is from 10,000 to 20,000 meters, all of

the coils of the aerial are connected in series as well as all of the

coils of the secondary circuit. The connections for a long wave

receptor are shown in the wiring diagram in Fig. 59.

[Illustration: Fig. 59.--Wiring Diagram Showing Long Wave Receptor

with Variocouplers and Bank-wound Inductance Coils]

CHAPTER XIII

HETERODYNE OR BEAT LONG WAVE TELEGRAPH RECEIVING SET

Any of the receiving sets described in the foregoing chapters will

respond to either: (1) a wireless telegraph transmitter that uses a

spark gap and which sends out periodic electric waves, or to (2) a

wireless telephone transmitter that uses an arc or a vacuum tube

oscillator and which sends out continuous electric waves. To receive

wireless _telegraph_ signals, however, from a transmitter that uses an

arc or a vacuum tube oscillator and which sends out continuous waves,

either the transmitter or the receptor must be so constructed that the

continuous waves will be broken up into groups of audio frequency and

this is done in several different ways.

There are four different ways employed at the present time to break up

the continuous waves of a wireless telegraph transmitter into groups

and these are: (_a_) the _heterodyne_, or _beat_, method, in which

waves of different lengths are impressed on the received waves and so

produces beats; (_b_) the _tikker_, or _chopper_ method, in which the

high frequency currents are rapidly broken up; (_c_) the variable

condenser method, in which the movable plates are made to rapidly

rotate; (_d_) the _tone wheel_, or _frequency transformer_, as it is

often called, and which is really a modified form of and an

improvement on the tikker. The heterodyne method will be described in

this chapter.

What the Heterodyne or Beat Method Is.--The word _heterodyne_ was

coined from the Greek words _heteros_ which means _other_, or

_different_, and _dyne_ which means _power_; in other words it means

when used in connection with a wireless receptor that another and

different high frequency current is used besides the one that is

received from the sending station. In music a _beat_ means a regularly

recurrent swelling caused by the reinforcement of a sound and this is

set up by the interference of sound waves which have slightly

different periods of vibration as, for instance, when two tones take

place that are not quite in tune with each other. This, then, is the

principle of the heterodyne, or beat, receptor.

In the heterodyne, or beat method, separate sustained oscillations,

that are just about as strong as those of the incoming waves, are set

up in the receiving circuits and their frequency is just a little

higher or a little lower than those that are set up by the waves

received from the distant transmitter. The result is that these

oscillations of different frequencies interfere and reinforce each

other when _beats_ are produced, the period of which is slow enough to

be heard in the headphones, hence the incoming signals can be heard

only when waves from the sending station are being received. A fuller

explanation of how this is done will be found in Chapter XV.

The Autodyne or Self-Heterodyne Long-Wave Receiving Set.--This is the

simplest type of heterodyne receptor and it will receive periodic

waves from spark telegraph transmitters or continuous waves from an

arc or vacuum tube telegraph transmitter. In this type of receptor the

detector tube itself is made to set up the _heterodyne oscillations_

which interfere with those that are produced by the incoming waves

that are a little out of tune with it.

With a long wave _autodyne_, or _self-heterodyne_ receptor, as this

type is called, and a two-step audio-frequency amplifier you can

clearly hear many of the cableless stations of Europe and others that

send out long waves. For receiving long wave stations, however, you

must have a long aerial--a single wire 200 or more feet in length will

do--and the higher it is the louder will be the signals. Where it is

not possible to put the aerial up a hundred feet or more above the

ground, you can use a lower one and still get messages in

_International Morse_ fairly strong.

The Parts and Connections of an Autodyne, or Self-Heterodyne,

Receiving Set.--For this long wave receiving set you will need: (1)

one _variocoupler_ with the primary coil wound on the stator and the

secondary coil and tickler coil wound on the rotor, or you can use

three honeycomb or other good compact coils of the longest wave you

want to receive, a table of which is given in Chapter XII; (2) two

_.001 mfd. variable condensers_; (3) one _.0005 mfd. variable

condenser_; (4) one _.5 to 2 megohm grid leak resistance_; (5) one

_vacuum tube detector_; (6) one _A battery_; (7) one _rheostat_; (8)

one _B battery_; (9) one _potentiometer_; (10) one _.001 mfd. fixed

condenser_ and (11) one pair of _headphones_. For the two-step

amplifier you must, of course, have besides the above parts the

amplifier tubes, variable condensers, batteries rheostats,

potentiometers and fixed condensers as explained in Chapter IX. The

connections for the autodyne, or self-heterodyne, receiving set are

shown in Fig. 60.

[Illustration: Fig. 60.--Wiring Diagram of Long Wave Antodyne, or

Self-Heterodyne Receptor.]

The Separate Heterodyne Long Wave Receiving Set.--This is a better

long wave receptor than the self heterodyne set described above for

receiving wireless telegraph signals sent out by a continuous long

wave transmitter. The great advantage of using a separate vacuum tube

to generate the heterodyne oscillations is that you can make the

frequency of the oscillations just what you want it to be and hence

you can make it a little higher or a little lower than the

oscillations set up by the received waves.

The Parts and Connections of a Separate Heterodyne Long Wave Receiving

Set.--The parts required for this long wave receiving set are: (1)

four honeycomb or other good _compact inductance_ coils of the longest

wave length that you want to receive; (2) three _.001 mfd. variable

condensers_; (3) one _.0005 mfd. variable condenser_; (4) one _1

megohm grid leak resistance_; (5) one _vacuum tube detector_; (6) one

_A battery_; (7) two rheostats; (8) two _B batteries_, one of which is

supplied with taps; (9) one _potentiometer_; (10) one _vacuum tube

amplifier_, for setting up the heterodyne oscillations; (11) a pair of

_headphones_ and (12) all of the parts for a _two-step amplifier_ as

detailed in Chapter IX, that is if you are going to use amplifiers.

The connections are shown in Fig. 61.

[Illustration: Fig. 61.--Wiring Diagram of Long Wave Separate

Heterodyne Receiving Set.]

In using either of these heterodyne receivers be sure to carefully

adjust the B battery by means of the potentiometer.

[Footnote: The amplifier tube in this case is used as a generator of

oscillations.]

CHAPTER XIV

HEADPHONES AND LOUD SPEAKERS

Wireless Headphones.--A telephone receiver for a wireless receiving

set is made exactly on the same principle as an ordinary Bell

telephone receiver. The only difference between them is that the

former is made flat and compact so that a pair of them can be fastened

together with a band and worn on the head (when it is called a

_headset_), while the latter is long and cylindrical so that it can be

held to the ear. A further difference between them is that the

wireless headphone is made as sensitive as possible so that it will

respond to very feeble currents, while the ordinary telephone receiver

is far from being sensitive and will respond only to comparatively

large currents.

How a Bell Telephone Receiver Is Made.--An ordinary telephone receiver

consists of three chief parts and these are: (1) a hard-rubber, or

composition, shell and cap, (2) a permanent steel bar magnet on one

end of which is wound a coil of fine insulated copper wire, and (3) a

soft iron disk, or _diaphragm_, all of which are shown in the

cross-section in Fig. 62. The bar magnet is securely fixed inside of

the handle so that the outside end comes to within about 1/32 of an

inch of the diaphragm when this is laid on top of the shell and the

cap is screwed on.

[Illustration: Fig. 62.--Cross-section of Bell telephone Receiver.]

[Illustration: original © Underwood and Underwood. Alexander Graham

Bell, Inventor of the Telephone, now an ardent Radio Enthusiast.]

The ends of the coil of wire are connected with two binding posts

which are in the end of the shell, but are shown in the picture at the

sides for the sake of clearness. This coil usually has a resistance of

about 75 ohms and the meaning of the _ohmic resistance_ of a receiver

and its bearing on the sensitiveness of it will be explained a little

farther along. After the disk, or diaphragm, which is generally made

of thin, soft sheet iron that has been tinned or japanned, [Footnote:

A disk of photographic tin-type plate is generally used.] is placed

over the end of the magnet, the cap, which has a small opening in it,

is screwed on and the receiver is ready to use.

How a Wireless Headphone Is Made.--For wireless work a receiver of the

watch-case type is used and nearly always two such receivers are

connected with a headband. It consists of a permanent bar magnet bent

so that it will fit into the shell of the receiver as shown at A in

Fig. 63.

[Illustration: Fig. 63.--Wireless Headphone.]

The ends of this magnet, which are called _poles_, are bent up, and

hence this type is called a _bipolar_ receiver. The magnets are wound

with fine insulated wire as before and the diaphragm is held securely

in place over them by screwing on the cap.

About Resistance, Turns of Wire and Sensitivity of Headphones.--If you

are a beginner in wireless you will hear those who are experienced

speak of a telephone receiver as having a resistance of 75 ohms, 1,000

ohms, 2,000 or 3,000 ohms, as the case may be; from this you will

gather that the higher the resistance of the wire on the magnets the

more sensitive the receiver is. In a sense this is true, but it is not

the resistance of the magnet coils that makes it sensitive, in fact,

it cuts down the current, but it is the _number of turns_ of wire on

them that determines its sensitiveness; it is easy to see that this is

so, for the larger the number of turns the more often will the same

current flow round the cores of the magnet and so magnetize them to a

greater extent.

But to wind a large number of turns of wire close enough to the cores

to be effective the wire must be very small and so, of course, the

higher the resistance will be. Now the wire used for winding good

receivers is usually No. 40, and this has a diameter of .0031 inch;

consequently, when you know the ohmic resistance you get an idea of

the number of turns of wire and from this you gather in a general way

what the sensitivity of the receiver is.

A receiver that is sensitive enough for wireless work should be wound

to not less than 1,000 ohms (this means each ear phone), while those

of a better grade are wound to as high as 3,000 ohms for each one. A

high-grade headset is shown in Fig. 64. Each phone of a headset should

be wound to the same resistance, and these are connected in series as

shown. Where two or more headsets are used with one wireless receiving

set they must all be of the same resistance and connected in series,

that is, the coils of one head set are connected with the coils of the

next head set and so on to form a continuous circuit.

[Illustration: Fig. 64.--Wireless Headphone.]

The Impedance of Headphones.--When a current is flowing through a

circuit the material of which the wire is made not only opposes its

passage--this is called its _ohmic resistance_--but a

_counter-electromotive force_ to the current is set up due to the

inductive effects of the current on itself and this is called

_impedance_. Where a wire is wound in a coil the impedance of the

circuit is increased and where an alternating current is used the

impedance grows greater as the frequency gets higher. The impedance of

the magnet coils of a receiver is so great for high frequency

oscillations that the latter cannot pass through them; in other words,

they are choked off.

How the Headphones Work.--As you will see from the cross-sections in

Figs. 62 and 63 there is no connection, electrical or mechanical,

between the diaphragm and the other parts of the receiver. Now when

either feeble oscillations, which have been rectified by a detector,

or small currents from a B battery, flow through the magnet coils the

permanent steel magnet is energized to a greater extent than when no

current is flowing through it. This added magnetic energy makes the

magnet attract the diaphragm more than it would do by its own force.

If, on the other hand, the current is cut off the pull of the magnet

is lessened and as its attraction for the diaphragm is decreased the

latter springs back to its original position. When varying currents

flow through the coils the diaphragm vibrates accordingly and sends

out sound waves.

About Loud Speakers.--The simplest acoustic instrument ever invented

is the _megaphone_, which latter is a Greek word meaning _great

sound_. It is a very primitive device and our Indians made it out of

birch-bark before Columbus discovered America. In its simplest form it

consists of a cone-shaped horn and as the speaker talks into the small

end the concentrated sound waves pass out of the large end in whatever

direction it is held.

Now a loud speaker of whatever kind consists of two chief parts and

these are: (1) a _telephone receiver_, and (2) a _megaphone_, or

_horn_ as it is called. A loud speaker when connected with a wireless

receiving set makes it possible for a room, or an auditorium, full of

people, or an outdoor crowd, to hear what is being sent out by a

distant station instead of being limited to a few persons listening-in

with headphones. To use a loud speaker you should have a vacuum tube

detector receiving set and this must be provided with a one-step

amplifier at least.

To get really good results you need a two-step amplifier and then

energize the plate of the second vacuum tube amplifier with a 100 volt

B battery; or if you have a three-step amplifier then use the

high voltage on the plate of the third amplifier tube. Amplifying

tubes are made to stand a plate potential of 100 volts and this is the

kind you must use. Now it may seem curious, but when the current flows

through the coils of the telephone receiver in one direction it gives

better results than when it flows through in the other direction; to

find out the way the current gives the best results try it out both

ways and this you can do by simply reversing the connections.

The Simplest Type of Loud Speaker.--This loud speaker, which is

called, the Arkay, [Footnote: Made by the Riley-Klotz Mfg. Co.,

Newark, N. J.] will work on a one- or two-step amplifier. It consists

of a brass horn with a curve in it and in the bottom there is an

adapter, or frame, with a set screw in it so that you can fit in one

of your headphones and this is all there is to it. The construction is

rigid enough to prevent overtones, or distortion of speech or music.

It is shown in Fig. 65.

[Illustration: Fig. 65.--Arkay Loud Speaker.]

Another Simple Kind of Loud Speaker.--Another loud speaker, see Fig.

66, is known as the _Amplitone_ [Footnote: Made by the American

Pattern, Foundry and Machine Co., 82 Church Street, N. Y. C.] and it

likewise makes use of the headphones as the sound producer. This

device has a cast metal horn which improves the quality of the sound,

and all you have to do is to slip the headphones on the inlet tubes of

the horn and it is ready for use. The two headphones not only give a

longer volume of sound than where a single one is used but there is a

certain blended quality which results from one phone smoothing out the

imperfections of the other.

[Illustration: Fig. 66.--Amplitone Loud Speaker.]

A Third Kind of Simple Loud Speaker.--The operation of the

_Amplitron_, [Footnote: Made by the Radio Service Co., 110 W. 40th

Street, N. Y.] as this loud speaker is called, is slightly different

from others used for the same purpose. The sounds set up by the

headphone are conveyed to the apex of an inverted copper cone which is

7 inches long and 10 inches in diameter. Here it is reflected by a

parabolic mirror which greatly amplifies the sounds. The amplification

takes place without distortion, the sounds remaining as clear and

crisp as when projected by the transmitting station. By removing the

cap from the receiver the shell is screwed into a receptacle on the

end of the loud speaker and the instrument is ready for use. It is

pictured in Fig. 67.

[Illustration: Fig. 67.--Amplitron Loud Speaker.]

A Super Loud Speaker.--This loud speaker, which is known as the

_Magnavox Telemegafone_, was the instrument used by Lt. Herbert E.

Metcalf, 3,000 feet in the air, and which startled the City of

Washington on April 2, 1919, by repeating President Wilson's _Victory

Loan Message_ from an airplane in flight so that it was distinctly

heard by 20,000 people below.

This wonderful achievement was accomplished through the installation

of the _Magnavox_ and amplifiers in front of the Treasury Building.

Every word Lt. Metcalf spoke into his wireless telephone transmitter

was caught and swelled in volume by the _Telemegafones_ below and

persons blocks away could hear the message plainly. Two kinds of these

loud speakers are made and these are: (1) a small loud speaker for the

use of operators so that headphones need not be worn, and (2) a large

loud speaker for auditorium and out-door audiences.

[Illustration: original © Underwood and Underwood. World's Largest

Loud Speaker ever made. Installed in Lytle Park, Cincinnati, Ohio, to

permit President Harding's Address at Point Pleasant, Ohio, during the

Grant Centenary Celebration to be heard within a radius of one

square.]

Either kind may be used with a one- or two-step amplifier or with a

cascade of half a dozen amplifiers, according to the degree of

loudness desired. The _Telemegafone_ itself is not an amplifier in the

true sense inasmuch as it contains no elements which will locally

increase the incoming current. It does, however, transform the

variable electric currents of the wireless receiving set into sound

vibrations in a most wonderful manner.

A _telemegafone_ of either kind is formed of: (1) a telephone receiver

of large proportions, (2) a step-down induction coil, and (3) a 6 volt

storage battery that energizes a powerful electromagnet which works

the diaphragm. An electromagnet is used instead of a permanent magnet

and this is energized by a 6-volt storage battery as shown in the

wiring diagram at A in Fig. 68. One end of the core of this magnet is

fixed to the iron case of the speaker and together these form the

equivalent of a horseshoe magnet. A movable coil of wire is supported

from the center of the diaphragm the edge of which is rigidly held

between the case and the small end of the horn. This coil is placed

over the upper end of the magnet and its terminals are connected to

the secondary of the induction coil. Now when the coil is energized by

the current from the amplifiers it and the core act like a solenoid in

that the coil tends to suck the core into it; but since the core is

fixed and the coil is movable the core draws the coil down instead.

The result is that with every variation of the current that flows

through the coil it moves up and down and pulls and pushes the

diaphragm down and up with it. The large amplitude of the vibrations

of the latter set up powerful sound waves which can be heard several

blocks away from the horn. In this way then are the faint incoming

signals, speech and music which are received by the amplifying

receiving set reproduced and magnified enormously. The _Telemegafone_

is shown complete at B.

[Illustration: Fig. 68.--Magnavox Loud Speaker.]

CHAPTER XV

OPERATION OF VACUUM TUBE RECEPTORS

From the foregoing chapters you have seen that the vacuum tube can be

used either as a _detector_ or an _amplifier_ or as a _generator_ of

electric oscillations, as in the case of the heterodyne receiving set.

To understand how a vacuum tube acts as a detector and as an amplifier

you must first know what _electrons_ are. The way in which the vacuum

tube sets up sustained oscillations will be explained in Chapter XVIII

in connection with the _Operation of Vacuum Tube Transmitters_.

What Electrons Are.--Science teaches us that masses of matter are made

up of _molecules_, that each of these is made up of _atoms_, and each

of these, in turn, is made up of a central core of positive particles

of electricity surrounded by negative particles of electricity as

shown in the schematic diagram, Fig. 69. The little black circles

inside the large circle represent _positive particles of electricity_

and the little white circles outside of the large circle represent

_negative particles of electricity_, or _electrons_ as they are

called.

[Illustration: Fig. 69.--Schematic Diagram of an Atom.]

It is the number of positive particles of electricity an atom has that

determines the kind of an element that is formed when enough atoms of

the same kind are joined together to build it up. Thus hydrogen, which

is the lightest known element, has one positive particle for its

nucleus, while uranium, the heaviest element now known, has 92

positive particles. Now before leaving the atom please note that it is

as much smaller than the diagram as the latter is smaller than our

solar system.

What Is Meant by Ionization.--A hydrogen atom is not only lighter but

it is smaller than the atom of any other element while an electron is

more than a thousand times smaller than the atom of which it is a

part. Now as long as all of the electrons remain attached to the

surface of an atom its positive and negative charges are equalized and

it will, therefore, be neither positive nor negative, that is, it will

be perfectly neutral. When, however, one or more of its electrons are

separated from it, and there are several ways by which this can be

done, the atom will show a positive charge and it is then called a

_positive ion_.

In other words a _positive ion_ is an atom that has lost some of its

negative electrons while a _negative ion_ is one that has acquired

some additional negative _electrons_. When a number of electrons are

being constantly given by the atoms of an element, which let us

suppose is a metal, and are being attracted to atoms of another

element, which we will say is also a metal, a flow of electrons takes

place between the two oppositely charged elements and form a current

of negative electricity as represented by the arrows at A in Fig. 70.

[Illustration: Fig. 70.--Action of Two-electrode Vacuum Tube.]

When a stream of electrons is flowing between two metal elements, as a

filament and a plate in a vacuum tube detector, or an amplifier, they

act as _carriers_ for more negative electrons and these are supplied

by a battery as we shall presently explain. It has always been

customary for us to think of a current of electricity as flowing from

the positive pole of a battery to the negative pole of it and hence we

have called this the _direction of the current_. Since the electronic

theory has been evolved it has been shown that the electrons, or

negative charges of electricity, flow from the negative to the

positive pole and that the ionized atoms, which are more positive than

negative, flow in the opposite direction as shown at B.

How Electrons are Separated from Atoms.--The next question that arises

is how to make a metal throw off some of the electrons of the atoms of

which it is formed. There are several ways that this can be done but

in any event each atom must be given a good, hard blow. A simple way

to do this is to heat a metal to incandescence when the atoms will

bombard each other with terrific force and many of the electrons will

be knocked off and thrown out into the surrounding space.

But all, or nearly all, of them will return to the atoms from whence

they came unless a means of some kind is employed to attract them to

the atoms of some other element. This can be done by giving the latter

piece of metal a positive charge. If now these two pieces of metal are

placed in a bulb from which the air has been exhausted and the first

piece of metal is heated to brilliancy while the second piece of metal

is kept positively electrified then a stream of electrons will flow

between them.

Action of the Two Electrode Vacuum Tube.--Now in a vacuum tube

detector a wire filament, like that of an incandescent lamp, is

connected with a battery and this forms the hot element from which the

electrons are thrown off, and a metal plate with a terminal wire

secured to it is connected to the positive or carbon tap of a dry

battery; now connect the negative or zinc tap of this with one end of

a telephone receiver and the other end of this with the terminals of

the filament as shown at A in Fig. 71. If now you heat the filament

and hold the phone to your ear you can hear the current from the B

battery flowing through the circuit.

[Illustration: (A) and (B) Fig. 71.--How a Two Electrode Tube Acts as

a Relay or a Detector.]

[Illustration: (C) Fig. 71.--Only the Positive Part of Oscillations

Goes through the Tube.]

Since the electrons are negative charges of electricity they are not

only thrown off by the hot wire but they are attracted by the positive

charged metal plate and when enough electrons pass, or flow, from the

hot wire to the plate they form a conducting path and so complete the

circuit which includes the filament, the plate and the B or

plate battery, when the current can then flow through it. As the

number of electrons that are thrown off by the filament is not great

and the voltage of the plate is not high the current that flows

between the filament and the plate is always quite small.

How the Two Electrode Tube Acts as a Detector.--As the action of a two

electrode tube as a detector [Footnote: The three electrode vacuum

tube has entirely taken the place of the two electrode type.] is

simpler than that of the three electrode vacuum tube we shall describe

it first. The two electrode vacuum tube was first made by Mr. Edison

when he was working on the incandescent lamp but that it would serve

as a detector of electric waves was discovered by Prof. Fleming, of

Oxford University, London. As a matter of fact, it is not really a

detector of electric waves, but it acts as: (1) a _rectifier_ of the

oscillations that are set up in the receiving circuits, that is, it

changes them into pulsating direct currents so that they will flow

through and affect a telephone receiver, and (2) it acts as a _relay_

and the feeble received oscillating current controls the larger direct

current from the B battery in very much the same way that a telegraph

relay does. This latter relay action will be explained when we come to

its operation as an amplifier.

We have just learned that when the stream of electrons flow from the

hot wire to the cold positive plate in the tube they form a conducting

path through which the battery current can flow. Now when the electric

oscillations surge through the closed oscillation circuit, which

includes the secondary of the tuning coil, the variable condenser, the

filament and the plate as shown at B in Fig. 71 the positive part of

them passes through the tube easily while the negative part cannot get

through, that is, the top, or positive, part of the wave-form remains

intact while the lower, or negative, part is cut off as shown in the

diagram at C. As the received oscillations are either broken up into

wave trains of audio frequency by the telegraph transmitter or are

modulated by a telephone transmitter they carry the larger impulses of

the direct current from the B battery along with them and these flow

through the headphones. This is the reason the vacuum tube amplifies

as well as detects.

How the Three Electrode Tube Acts as a Detector.--The vacuum tube as a

detector has been made very much more sensitive by the use of a third

electrode shown in Fig. 72. In this type of vacuum tube the third

electrode, or _grid_, is placed between the filament and the plate and

this controls the number of electrons flowing from the filament to the

plate; in passing between these two electrodes they have to go through

the holes formed by the grid wires.

[Illustration: (A) and (B) Fig. 72.--How the Positive and Negative

Voltages of Oscillations Act on the Electrons.]

[Illustration: (C) Fig. 72.--How the Three Electrode Tube Acts as a

Detector and Amplifier.]

[Illustration: (D) Fig. 72.--How the Oscillations Control the Flow of

the Battery Current through the Tube.]

If now the grid is charged to a higher _negative_ voltage than the

filament the electrons will be stopped by the latter, see A, though

some of them will go through to the plate because they travel at a

high rate of speed. The higher the negative charge on the grid the

smaller will be the number of electrons that will reach the plate and,

of course, the smaller will be the amount of current that will flow

through the tube and the headphones from the B battery.

On the other hand if the grid is charged _positively_, see B, then

more electrons will strike the plate than when the grid is not used or

when it is negatively charged. But when the three electrode tube is

used as a detector the oscillations set up in the circuits change the

grid alternately from negative to positive as shown at C and hence the

voltage of the B battery current that is allowed to flow through the

detector from the plate to the filament rises and falls in unison with

the voltage of the oscillating currents. The way the positive and

negative voltages of the oscillations which are set up by the incoming

waves, energize the grid; how the oscillator tube clips off the

negative parts of them, and, finally, how these carry the battery

current through the tube are shown graphically by the curves at D.

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

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 space 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.

[Illustration: (A) Fig. 75.--Apparatus for Experimental C. W.

Telegraph Transmitter.]

[Illustration: 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.

[Illustration: 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.

[Illustration: Fig. 76--Experimental C.W. Telegraph Transmitter]