2.00 120 2.22 160
3.00 158 2.93 215
4.00 194 3.58 265
5.00 228 4.22 310
8.00 324 6.00 450
10.00 384 7.10 530
12.00 450 8.30 625
The coil which you wound at the beginning of your experiment had only 75
turns and was tapped so that you could, by manipulating the two switches
of Fig. 112, get small variations in inductance. In Table III is given
the values of the inductance which is controlled by the switches of that
figure, the corresponding number of turns, and the wave-length to which
the antenna should then be tuned. I am giving this for two values of
antenna capacity, as I have done before. By the aid of these three
tables you should have small difficulty in taking care of matters of
tuning for all wave-lengths below about 3000 meters. If you want to get
longer waves than that you had better buy a few banked-wound coils.
These are coils in which the turns are wound over each other but in such
a way as to avoid in large part the "capacity effects" which usually
accompany such winding. You can try winding them for yourself but I
doubt if the experience has much value until you have gone farther in
the study of the mathematical theory of radio than this series of
letters will carry you.
TABLE III
Circuit of Fig. 112
Number Inductance in Wave length with antenna of
of turns. mil-henries. 0.0001 mf. 0.0002 mf.
14 0.04 120 170
20 0.07 160 220
28 0.12 210 290
36 0.18 250 360
44 0.25 300 420
56 0.38 370 520
75 0.60 460 650
In the secondary circuit there is only one capacity, that of the
variable condenser. If it has a range of values from about 0.00005 mf.
to 0.0005 mf. your coil of 60 turns and 0.42 mf. permits a range of
wave-lengths from 270 to 860 m. Using half the coil the range is 150 to
480 m. With the larger condenser the ranges are respectively 270 to 1220
and 270 to 670. For longer wave-lengths load with inductance. Four times
the inductance will tune to double these wave-lengths.
[Footnote 11: If you can afford to buy, or if you can borrow, ammeters
and voltmeters of the proper range you should take the characteristic
yourself.]
LETTER 22
HIGH-POWERED RADIO-TELEPHONE TRANSMITTERS
MY DEAR EXPERIMENTER:
This letter is to summarize the operations which must be performed in
radio-telephone transmission and reception; and also to describe the
circuit of an important commercial system.
To transmit speech by radio three operations are necessary. First, there
must be generated a high-frequency alternating current; second, this
current must be modulated, that is, varied in intensity in accordance
with the human voice; and third, the modulated current must be supplied
to an antenna. For efficient operation, of course, the antenna must be
tuned to the frequency which is to be transmitted. There is also a
fourth operation which is usually performed and that is amplification.
Wherever the electrical effect is smaller than desired, or required for
satisfactory transmission, vacuum tubes are used as amplifiers. Of this
I shall give you an illustration later.
Three operations are also essential in receiving. First, an antenna must
be so arranged and tuned as to receive energy from the distant
transmitting station. There is then in the receiving antenna a current
similar in wave form to that in the transmitting antenna. Second, the
speech significance of this current must be detected, that is, the
modulated current must be demodulated. A current is then obtained which
has the same wave form as the human voice which was the cause of the
modulation at the distant station. The third operation is performed by a
telephone receiver which makes the molecules of air in its neighborhood
move back and forth in accordance with the detected current. As you
already know a fourth operation may be carried on by amplifiers which
give on their output sides currents of greater strength but of the same
forms as they receive at their input terminals.
In transmitting and in receiving equipment two or more of these
operations may be performed by the same vacuum tube as you will remember
from our discussion of the regenerative circuit for receiving. For
example, also, in any receiving set the vacuum tube which detects is
usually amplifying. In the regenerative circuit for receiving continuous
waves by the heterodyne method the vacuum tube functions as a generator
of high-frequency current and as a detector of the variations in current
which occur because the locally-generated current does not keep in step
with that generated at the transmitting station.
Another example of a vacuum tube performing simultaneously two different
functions is illustrated in Fig. 120 which shows a simple
radio-telephone transmitter. The single tube performs in itself both the
generation of the radio-frequency current and its modulation in
accordance with the output of the carbon-button transmitter. This audion
is in a feed-back circuit, the oscillation frequency of which depends
upon the condenser _C_ and the inductance _L_. The voice
drives the diaphragm of the transmitter and thus varies the resistance
of the carbon button. This varies the current from the battery,
_B_{a}_, through the primary, _T_{1}_, of the transformer
_T_. The result is a varying voltage applied to the grid by the
secondary _T_{2}_. The oscillating current in the plate circuit of
the audion varies accordingly because it is dependent upon the grid
voltage. The condenser _C_{r}_ offers a low impedance to the
radio-frequency current to which the winding _T_{2}_ of
audio-frequency transformer offers too much.
[Illustration: Fig 120]
In this case the tube is both generator and "modulator." In some cases
these operations are separately performed by different tubes. This was
true of the transmitting set used in 1915 when the engineers of the Bell
Telephone System talked by radio from Arlington, near Washington, D. C.,
to Paris and Honolulu. I shall not draw out completely the circuit of
their apparatus but I shall describe it by using little squares to
represent the parts responsible for each of the several operations.
First there was a vacuum tube oscillator which generated a small current
of the desired frequency. Then there was a telephone transmitter which
made variations in a direct-current flowing through the primary of a
transformer. The e. m. f. from the secondary of this transformer and the
e. m. f. from the radio-frequency oscillator were both impressed upon
the grid of an audion which acted as a modulator. The output of this
audion was a radio-frequency current modulated by the voice. The output
was amplified by a two-stage audion amplifier and supplied through a
coupling coil to the large antenna of the U. S. Navy Station at
Arlington. Fig. 121 shows the system.
[Illustration: Fig 121]
The audion amplifiers each consisted of a number of tubes operating in
parallel. When tubes are operated in parallel they are connected as
shown in Fig. 122 so that the same e. m. f. is impressed on all the
grids and the same plate-battery voltage on all the plates. As the grids
vary in voltage there is a corresponding variation of current in the
plate circuit of each tube. The total change of the current in the
plate-battery circuit is, then, the sum of the changes in all the
plate-filament circuits of the tubes. This scheme of connections gives a
result equivalent to that of a single tube with a correspondingly larger
plate and filament.
[Illustration: Fig 122]
Parallel connection is necessary because a single tube would be
overheated in delivering to the antenna the desired amount of power. You
remember that when the audion is operated as an amplifier the resistance
to which it supplies current is made equal to its own internal
resistance of _R_{p}_. That means that there is in the plate
circuit just as much resistance inside the tube as outside. Hence there
is the same amount of work done each second in forcing the current
through the tube as through the antenna circuit, if that is what the
tube supplies. "Work per second" is power; the plate battery is spending
energy in the tube at the same rate as it is supplying it to the antenna
where it is useful for radiation.
[Illustration: Pl. XI.--Broadcasting Equipment, Developed by the American
Telephone and Telegraph Company and the Western Electric Company.]
All the energy expended in the tube appears as heat. It is due to the
blows which the electrons strike against the plate when they are drawn
across from the filament. These impacts set into more rapid motion the
molecules of the plate; and the temperature of the tube rises. There is
a limit to the amount the temperature can rise without destroying the
tube. For that reason the heat produced inside it must not exceed a
certain limit depending upon the design of the tube and the method of
cooling it as it is operated. In the Arlington experiments, which I
mentioned a moment ago, the tubes were cooled by blowing air on them
from fans.
We can find the power expended in the plate circuit of a tube by
multiplying the number of volts in its battery by the number of amperes
which flows. Suppose the battery is 250 volts and the current 0.02
amperes, then the power is 5 watts. The "watt" is the unit for measuring
power. Tubes are rated by the number of watts which can be safely
expended in them. You might ask, when you buy an audion, what is a safe
rating for it. The question will not be an important one, however,
unless you are to set up a transmitting set since a detector is usually
operated with such small plate-voltage as not to have expended in it an
amount of power dangerous to its life.
In recent transmitting sets the tubes are used in parallel for the
reasons I have just told, but a different method of modulation is used.
The generation of the radio-frequency current is by large-powered tubes
which are operated with high voltages in their plate circuits. The
output of these oscillators is supplied to the antenna. The intensity of
the oscillations of the current in these tubes is controlled by changing
the voltage applied in their plate circuits. You can see from Fig. 123
that if the plate voltage is changed the strength of the alternating
current is changed accordingly. It is the method used in changing the
voltage which is particularly interesting.
[Illustration: Fig 123]
The high voltages which are used in the plate circuits of these
high-powered audions are obtained from generators instead of batteries.
You remember from Letter 20 that an e. m. f. is induced in a coil when
the coil and a magnet are suddenly changed in their positions, one being
turned with reference to the other. A generator is a machine for turning
a coil so that a magnet is always inducing an e. m. f. in it. It is
formed by an armature carrying coils and by strong electromagnets. The
machine can be driven by a steam or gas engine, by a water wheel, or by
an electric motor. Generators are designed either to give steady streams
of electrons, that is for d-c currents, or to act as alternators.
[Illustration: Fig 124]
Suppose we have, as shown in Fig. 124, a d-c generator supplying
current to a vacuum tube oscillator. The current from the generator
passes through an iron-cored choke coil, marked _L_{a}_ in the figure.
Between this coil and the plate circuit we connect across the line a
telephone transmitter. To make a system which will work efficiently we
shall have to suppose that this transmitter has a high resistance, say
about the same as the internal resistance, _R_{p}_, of the tube and
also that it can carry as large a current.
Of the current which comes from the generator about one-half goes to
the tube and the rest to the transmitter. If the resistance of the
transmitter is increased it can't take as much current. The coil,
_L_{a}_, however, because of its inductance, tends to keep the same
amount of current flowing through itself. For just an instant then the
current in _L_{a}_ keeps steady even though the transmitter doesn't
take its share. The result is more current for the oscillating tube. On
the other hand if the transmitter takes more current, because its
resistance is decreased, the choke coil, _L_{a}_, will momentarily tend
to keep the current steady so that what the transmitter takes must be
at the expense of the oscillating tube.
That's one way of looking at what happens. We know, however, from Fig.
123 that to get an increase in the amplitude of the current in the
oscillating tube we must apply an increased voltage to its plate
circuit. That is what really happens when the transmitter increases in
resistance and so doesn't take its full share of the current. The
reason is this: When the transmitter resistance is increased the
current in the transmitter decreases. Just for a moment it looks as
though the current in _L_{a}_ is going to decrease. That's the way it
looks to the electrons; and you know what electrons do in an inductive
circuit when they think they shall have to stop. They induce each other
to keep on for a moment. For a moment they act just as if there was
some extra e. m. f. which was acting to keep them going. We say,
therefore, that there is an extra e. m. f., and we call this an e. m.
f. of self-induction. All this time there has been active on the plate
circuit of the tube the e. m. f. of the generator. To this there is
added at the instant when the transmitter resistance increases, the e.
m. f. of self-induction in the coil, _L_{a}_ and so the total e. m. f.
applied to the tube is momentarily increased. This increased e. m. f.,
of course, results in an increased amplitude for the alternating
current which the oscillator is supplying to the transmitting antenna.
When the transmitter resistance is decreased, and a larger current
should flow through the choke coil, the electrons are asked to speed up
in going through the coil. At first they object and during that instant
they express their objection by an e. m. f. of self-induction which
opposes the generator voltage. For an instant, then, the voltage of the
oscillating tube is lowered and its alternating-current output is
smaller.
[Illustration: Fig 125]
For the purpose of bringing about such threatened changes in current,
and hence such e. m. f.'s of self-induction, the carbon transmitter is
not suitable because it has too small a resistance and too small a
current carrying ability. The plate circuit of a vacuum tube will serve
admirably. You know from the audion characteristic that without changing
the plate voltage we can, by applying a voltage to the grid, change the
current through the plate circuit. Now if it was a wire resistance with
which we were dealing and we should be able to obtain a change in
current without changing the voltage acting on this wire we would say
that we had changed the resistance. We can say, therefore, that the
internal resistance of the plate circuit of a vacuum tube can be changed
by what we do to the grid.
In Fig. 125 I have substituted the plate circuit of an audion for the
transmitter of Fig. 124 and arranged to vary its resistance by changing
the potential of the grid. This we do by impressing upon the grid the e.
m. f. developed in the secondary of a transformer, to the primary of
which is connected a battery and a carbon transmitter. The current
through the primary varies in accordance with the sounds spoken into the
transmitter. And for all the reasons which we have just finished
studying there are similar variations in the output current of the
oscillating tube in the transmitting set of Fig. 125.
In this latter figure you will notice a small air-core coil,
_L_{R}_, between the oscillator and the modulator tube. This coil
has a small inductance but it is enough to offer a large impedance to
radio-frequency currents. The result is, it does not let the alternating
currents of the oscillating tube flow into the modulator. These currents
are confined to their own circuit, where they are useful in establishing
similar currents in the antenna. On the other hand, the coil _L_{R}_
doesn't seriously impede low-frequency currents and therefore it does
not prevent variations in the current which are at audio-frequency. It
does not interfere with the changes in current which accompany the
variations in the resistance of the plate circuit of the modulator.
That is, it has too little impedance to act like _L_{a}_ and so it
permits the modulator to vary the output of the oscillator.
[Illustration: Fig 126]
The oscillating circuit of Fig. 125 includes part of the antenna. It
differs also from the others I have shown in the manner in which grid
and plate circuits are coupled. I'll explain by Fig. 126.
The transmitting set which I have just described involves many of the
principles of the most modern sets. If you understand its operation you
can probably reason out for yourself any of the other sets of which you
will hear from time to time.
LETTER 23
AMPLIFICATION AT INTERMEDIATE FREQUENCIES
DEAR SON:
In the matter of receiving I have already covered all the important
principles. There is one more system, however, which you will need to
know. This is spoken of either as the "super-heterodyne" or as the
"intermediate-frequency amplification" method of reception.
The system has two important advantages. First, it permits sharper
tuning and so reduces interference from other radio signals. Second, it
permits more amplification of the incoming signal than is usually
practicable.
First as to amplification: We have seen that amplification can be
accomplished either by amplifying the radio-frequency current before
detection or by amplifying the audio-frequency current which results
from detection. There are practical limitations to the amount of
amplification which can be obtained in either case. An efficient
multi-stage amplifier for radio-frequencies is difficult to build
because of what we call "capacity effects."
Consider for example the portion of circuit shown in Fig. 127. The wires
_a_ and _b_ act like small plates of condensers. What we
really have, is a lot of tiny condensers which I have shown in the
figure by the light dotted-lines. If the wires are transmitting
high-frequency currents these condensers offer tiny waiting-rooms where
the electrons can run in and out without having to go on to the grid of
the next tube. There are other difficulties in high-frequency
amplifiers. This one of capacity effects between parallel wires is
enough for the present. It is perhaps the most interesting because it is
always more or less troublesome whenever a pair of wires is used to
transmit an alternating current.
[Illustration: Fig 127]
In the case of a multi-stage amplifier of audio-frequency current there
is always the possibility of the amplification of any small variations
in current which may naturally occur in the action of the batteries.
There are always small variations in the currents from batteries, due to
impurities in the materials of the plates, air bubbles, and other
causes. Ordinarily we don't observe these changes because they are too
small to make an audible sound in the telephone receivers. Suppose,
however, that they take place in the battery of the first tube of a
series of amplifiers. Any tiny change of current is amplified many times
and results in a troublesome noise in the telephone receiver which is
connected to the last tube.
In both types of amplifiers there is, of course, always the chance that
the output circuit of one tube may be coupled to and induce some effect
in the input circuit of one of the earlier tubes of the series. This
will be amplified and result in a greater induction. In other words, in
a circuit where there is large amplification, there is always the
difficulty of avoiding a feed-back of energy from one tube to another so
that the entire group acts like an oscillating circuit, that is
"regeneratively." Much of this difficulty can be avoided after
experience.
If a multi-stage amplifier is to be built for a current which does not
have too high a frequency the "capacity effects" and the other
difficulties due to high-frequency need not be seriously troublesome. If
the frequency is not too high, but is still well above the audible
limit, the noises due to variations in battery currents need not bother
for they are of quite low frequency. Currents from 20,000 to 60,000
cycles a second are, therefore, the most satisfactory to amplify.
Suppose, however, one wishes to amplify the signals from a
radio-broadcasting station. The wave-length is 360 meters and the
frequency is about 834,000 cycles a second. The system of
intermediate-frequency amplification solves the difficulty and
we shall see how it does so.
[Illustration: Fig 128]
At the receiving station a local oscillator is used. This generates a
frequency which is about 30,000 cycles less than that of the incoming
signal. Both currents are impressed on the grid of a detector. The
result is, in the output of the detector, a current which has a
frequency of 30,000 cycles a second. The intensity of this detected
current depends upon the intensity of the incoming signal. The "beat
note" current of 30,000 cycles varies, therefore, in accordance with the
voice which is modulating at the distant sending station. The speech
significance is now hidden in a current of a frequency intermediate
between radio and audio. This current may be amplified many times and
then supplied to the grid of a detector which obtains from it a current
of audio-frequency which has a speech significance. In Fig. 128 I have
indicated the several operations.
We can now see why this method permits sharper tuning. The whole idea
of tuning, of course, is to arrange that the incoming signal shall cause
the largest possible current and at the same time to provide that any
signals at other wave-lengths shall cause only negligible currents. What
we want a receiving set to do is to distinguish between two signals
which differ slightly in wave-length and to respond to only one of them.
Suppose we set up a tuned circuit formed by a coil and a condenser and
try it out for various frequencies of signals. You know how it will
respond from our discussion in connection with the tuning curve of Fig.
51 of Letter 13. We might find from a number of such tests that the best
we can expect any tuned circuit to do is to discriminate between signals
which differ about ten percent in frequency, that is, to receive well
the desired signal and to fail practically entirely to receive a signal
of a frequency either ten percent higher or the same amount lower.
For example, if the signal is at 30,000 cycles a tuned circuit might be
expected to discriminate against an interfering signal of 33,000. If the
signal is at 300,000 cycles a tuned circuit might discriminate against
an interfering signal of 330,000 cycles, but an interference at 303,000
cycles would be very troublesome indeed. It couldn't be "tuned out" at
all.
Now suppose that the desired signal is at 300,000 cycles and that there
is interference at 303,000 cycles. We provide a local oscillator of
270,000 cycles a second, receive by this "super-heterodyne" method which
I have just described, and so obtain an intermediate frequency. In the
output of the first detector we have then a current of 300,000--270,000
or 30,000 cycles due to the desired signal and also a current of
303,000--270,000 or 33,000 cycles due to the interference. Both these
currents we can supply to another tuned circuit which is tuned for
30,000 cycles a second. It can receive the desired signal but it can
discriminate against the interference because now the latter is ten
percent "off the tune" of the signal.
You see the question is not one of how far apart two signals are in
number of cycles per second. The question always is: How large in
percent is the difference between the two frequencies? The matter of
separating two effects of different frequencies is a question of the
"interval" between the frequencies. To find the interval between two
frequencies we divide one by the other. You can see that if the quotient
is larger than 1.1 or smaller than 0.9 the frequencies differ by ten
percent or more. The higher the frequency the larger the number of
cycles which is represented by a given size of interval.
While I am writing of frequency intervals I want to tell you one thing
more of importance. You remember that in human speech there may enter,
and be necessary, any frequency between about 200 and 2000 cycles a
second. That we might call the range of the necessary notes in the
voice. Whenever we want a good reproduction of the voice we must
reproduce all the frequencies in this range.
Suppose we have a radio-current of 100,000 cycles modulated by the
frequencies in the voice range. We find in the output of our
transmitting set not only a current of 100,000 cycles but currents in
two other ranges of frequencies. One of these is above the signal
frequency and extends from 100,200 to 102,000 cycles. The other is the
same amount below and extends from 98,000 to 99,800 cycles. We say there
is an upper and a lower "band of frequencies."
All these currents are in the complex wave which comes from the
radio-transmitter. For this statement you will have to take my word
until you can handle the form of mathematics known as "trigonometry."
When we receive at the distant station we receive not only currents of
the signal frequency but also currents whose frequencies lie in these
"side-bands."
No matter what radio-frequency we may use we must transmit and receive
side-bands of this range if we use the apparatus I have described in
the past letters. You can see what that means. Suppose we transmit at a
radio-frequency of 50,000 cycles and modulate that with speech. We
shall really need all the range from 48,000 cycles to 52,000 cycles for
one telephone message. On the other hand if we modulated a 500,000
cycle wave by speech the side-bands are from 498,000 to 499,800 and
500,200 to 502,000 cycles. If we transmit at 50,000 cycles, that is, at
6000 meters, we really need all the range between 5770 meters and 6250
meters, as you can see by the frequencies of the side-bands. At 100,000
cycles we need only the range of wave-lengths between 2940 m. and 3060
m. If the radio-frequency is 500,000 cycles we need a still smaller
range of wave-lengths to transmit the necessary side-bands. Then the
range is from 598 m. to 603 m.
In the case of the transmission of speech by radio we are interested in
having no interference from other signals which are within 2000 cycles
of the frequency of our radio-current no matter what their wave-lengths
may be. The part of the wave-length range which must be kept clear from
interfering signals becomes smaller the higher the frequency which is
being modulated.
You can see that very few telephone messages can be sent in the
long-wave-length part of the radio range and many more, although not
very many after all, in the short wave-length part of the radio range.
You can also see why it is desirable to keep amateurs in the short
wave-length part of the range where more of them can transmit
simultaneously without interfering with each other or with commercial
radio stations.
There is another reason, too, for keeping amateurs to the shortest
wave-lengths. Transmission of radio signals over short distances is best
accomplished by short wave-lengths but over long distances by the longer
wave-lengths. For trans-oceanic work the very longest wave-lengths are
best. The "long-haul" stations, therefore, work in the frequency range
immediately above 10,000 cycles a second and transmit with wave lengths
of 30,000 m. and shorter.
[Illustration: Pl. XII.--Broadcasting Station of the American Telephone
and Telegraph Company on the Roof of the Walker-Lispenard Bldg. in New
York City Where the Long-distance Telephone Lines Terminate.]
LETTER 24
BY WIRE AND BY RADIO
DEAR BOY:
The simplest wire telephone-circuit is formed by a transmitter, a
receiver, a battery, and the connecting wire. If two persons are to
carry on a conversation each must have this amount of equipment. The
apparatus might be arranged as in Fig. 129. This set-up, however,
requires four wires between the two stations and you know the telephone
company uses only two wires. Let us find the principle upon which its
system operates because it is the solution of many different problems
including that of wire-to-radio connections.
[Illustration: Fig 129]
Imagine four wire resistances connected together to form a square as in
Fig. 130. Suppose there are two pairs of equal resistances, namely
_R_{1}_ and _R_{2}_, and _Z_{1}_ and _Z_{2}_. If we connect a
generator, _G_, between the junctions _a_ and _b_ there will be two
separate streams of electrons, one through the R-side and the other
through the Z-side of the circuit. These streams, of course, will not
be of the same size for the larger stream will flow through the side
which offers the smaller resistance.
[Illustration: Fig 130]
Half the e. m. f. between _a_ and _b_ is used up in sending the
stream half the distance. Half is used between _a_ and the points _c_
and _d_, and the other half between _c_ and _d_ and the other end. It
doesn't make any difference whether we follow the stream from _a_ to
_c_ or from _a_ to _d_, it takes half the e. m. f. to keep this
stream going. Points _c_ and _d_, therefore, are in the same condition
of being "half-way electrically" from _a_ to _b_. The result is that
there can be no current through any wire which we connect between
_c_ and _d_.
Suppose, therefore, that we connect a telephone receiver between
_c_ and _d_. No current flows in it and no sound is emitted by
it. Now suppose the resistance of _Z_{2}_ is that of a telephone
line which stretches from one telephone station to another. Suppose also
that _Z_{1}_ is a telephone line exactly like _Z_{2}_ except
that it doesn't go anywhere at all because it is all shut up in a little
box. We'll call _Z_{1}_ an artificial telephone line. We ought to
call it, as little children would say, a "make-believe" telephone line.
It doesn't fool us but it does fool the electrons for they can't tell
the difference between the real line _Z_{2}_ and the artificial
line _Z_{1}_. We can make a very good artificial line by using a
condenser and a resistance. The condenser introduces something of the
capacity effects which I told you were always present in a circuit
formed by a pair of wires.
[Illustration: Fig 131]
At the other telephone station let us duplicate this apparatus, using
the same real line in both cases. Instead of just any generator of an
alternating e. m. f. let us use a telephone transmitter. We connect the
transmitter through a transformer. The system then looks like that of
Fig. 131. When some one talks at station 1 there is no current through
his receiver because it is connected to _c_ and _d_, while the
e. m. f. of the transmitter is applied to _a_ and _b_. The transmitter
sets up two electron streams between _a_ and _b_, and the stream which
flows through the Z-side of the square goes out to station 2. At this
station the electrons have three paths between _d_ and _b_. I have
marked these by arrows and you see that one of them is through the
receiver. The current which is started by the transmitter at station 1
will therefore operate the receiver at station 2 but not at its own
station. Of course station 2 can talk to 1 in the same way.
The actual set-up used by the telephone company is a little different
from that which I have shown because it uses a single common battery at
a central office between two subscribers. The general principle,
however, is the same.
[Illustration: Fig 132]
It won't make any difference if we use equal inductance coils, instead
of the R-resistances, and connect the transmitter to them inductively as
shown in Fig. 132. So far as that is concerned we can also use a
transformer between the receiver and the points _c_ and _d_,
as shown in the same figure.
[Illustration: Fig 133]
We are now ready to put in radio equipment at station 2. In place of the
telephone receiver at station 2 we connect a radio transmitter. Then
whatever a person at station 1 says goes by wire to 2 and on out by
radio. In place of the telephone transmitter at station 2 we connect a
radio receiver. Whatever that receives by radio is detected and goes by
wire to the listener at station 1. In Fig. 133 I have shown the
equipment of station 2. There you have the connections for wire to radio
and vice versa.
One of the most interesting developments of recent years is that of
"wired wireless" or "carrier-current telephony" over wires. Suppose that
instead of broadcasting from the antenna at station 2 we arrange to have
its radio transmitter supply current to a wire circuit. We use this same
pair of wires for receiving from the distant station. We can do this if
we treat the radio transmitter and receiver exactly like the telephone
instruments of Fig. 132 and connect them to a square of resistances. One
of these resistances is, of course, the line between the stations. I
have shown the general arrangement in Fig. 134.
You see what the square of resistances, or "bridge" really does for us.
It lets us use a single pair of wires for messages whether they are
coming or going. It does that because it lets us connect a transmitter
and also a receiver to a single pair of wires in such a way that the
transmitter can't affect the receiver. Whatever the transmitter sends
out goes along the wires to the distant receiver but doesn't affect the
receiver at the sending station. This bridge permits this whether the
transmitter and receiver are radio instruments or are the ordinary
telephone instruments.
[Illustration: Fig 134]
By its aid we may send a modulated high-frequency current over a pair of
wires and receive from the same pair of wires the high-frequency current
which is generated and modulated at the distant end of the line. It lets
us send and receive over the same pair of wires the same sort of a
modulated current as we would supply to an antenna in radio-telephone
transmitting. It is the same sort of a current but it need not be
anywhere near as large because we aren't broadcasting; we are sending
directly to the station of the other party to our conversation.
If we duplicate the apparatus we can use the same pair of wires for
another telephone conversation without interfering with the first. Of
course, we have to use a different frequency of alternating current for
each of the two conversations. We can send these two different modulated
high-frequency currents over the same pair of wires and separate them by
tuning at the distant end just as well as we do in radio. I won't sketch
out for you the tuned circuits by which this separation is made. It's
enough to give you the idea.
In that way, a single pair of wires can be used for transmitting,
simultaneously and without any interference, several different telephone
conversations. It takes very much less power than would radio
transmission and the conversations are secret. The ordinary telephone
conversation can go on at the same time without any interference with
those which are being carried by the modulations in high-frequency
currents. A total of five conversations over the same pair of wires is
the present practice.
This method is used between many of the large cities of the U. S.
because it lets one pair of wires do the work of five. That means a
saving, for copper wire costs money. Of course, all the special
apparatus also costs money. You can see, therefore, that this method
wouldn't be economical between cities very close together because all
that is saved by not having to buy so much wire is spent in building
special apparatus and in taking care of it afterwards. For long lines,
however, by not having to buy five times as much wire, the Bell Company
saves more than it costs to build and maintain the extra special
apparatus.
I implied a moment ago why this system is called a "carrier-current"
system; it is because "the high-frequency currents carry in their
modulations the speech significance." Sometimes it is called a system of
"multiplex" telephony because it permits more than one message at a
time.
This same general principle is also applied to the making of a multiplex
system of telegraphy. In the multiplex telephone system we pictured
transmitting and receiving sets very much like radio-telephone sets. If
instead of transmitting speech each transmitter was operated as a C-W
transmitter then it would transmit telegraph messages. In the same
frequency range there can be more telegraph systems operated
simultaneously without interfering with each other, for you remember how
many cycles each radio-telephone message requires. For that reason the
multiplex telegraph system which operates by carrier-currents permits as
many as ten different telegraph messages simultaneously.
You remember that I told you how capacity effects rob the distant end of
a pair of wires of the alternating current which is being sent to them.
That is always true but the effect is not very great unless the
frequency of the alternating current is high. It's enough, however, so
that every few hundred miles it is necessary to connect into the circuit
an audion amplifier. This is true of carrier currents especially, but
also true of the voice-frequency currents of ordinary telephony. The
latter, however, are not weakened, that is, "attenuated," as much and
consequently do not need to be amplified as much to give good
intelligibility at the distant receiver.
[Illustration: Fig 135]
In a telephone circuit over such a long distance as from New York City
to San Francisco it is usual to insert amplifiers at about a dozen
points along the route. Of course, these amplifiers must work for
transmission in either direction, amplifying speech on its way to San
Francisco or in the opposite direction. At each of the amplifying
stations, or "repeater stations," as they are usually called, two vacuum
tube amplifiers are used, one for each direction. To connect these with
the line so that each may work in the right direction there are used two
of the bridges or resistance squares. You can see from the sketch of
Fig. 135 how an alternating current from the east will be amplified and
sent on to the west, or vice versa.
[Illustration: Fig 136]
There are a large number of such repeater stations in the United States
along the important telephone routes. In Fig. 136 I am showing you the
location of those along the route of the famous "transcontinental
telephone-circuit." This shows also a radio-telephone connection between
the coast of California and Catalina Island. Conversations have been
held between this island and a ship in the Atlantic Ocean, as shown in
the sketch. The conversation was made possible by the use of the vacuum
tube and the bridge circuit. Part of the way it was by wire and part by
radio. Wire and radio tie nicely together because both operate on the
same general principles and use much of the same apparatus.
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INDEX
A-battery for tubes, 42
Accumulator, 29
Acid, action of hydrogen in, 7
Air, constitution of, 10
Ammeter, alternating current, 206;
calibration of, 53;
construction of, 205
Ampere, 49, 54
Amplification, 182; one stage of, 185
Amplitude of vibration, 155
Antenna current variation, 141
Arlington tests, 233
Artificial telephone line, 252
Atom, conception of, 6;
nucleus of, 10;
neutral, 34
Atomic number, 13
Atoms, difference between, 12;
kinds of, 6, 10;
motion of, 35
Attenuation of current in wires, 259
Audibility meter, 218
Audio-frequency amplifier, 185;
limitations of, 185
Audion, 35, 40, 42
Audion, amplifier, 182;
detector, theory of, 126;
modulator, 232;
oscillator, theory of, 89;
frequency control of, 99
B-battery for tubes, 43;
effect upon characteristic, 128
Banked wound coils, 228
Battery, construction of gravity, 16;
dry, 27;
reversible or storage, 29
Band of frequencies, 249
Beat note, detection of, 221, 245
Bell system, Arlington transmitter, 249
Blocking of tube, reason for, 171
Blue vitriol, 16
Bridge circuit, 255
Bureau of Standards, 50
C-battery for tubes, 46, 166;
variation of, 75;
for detection, 66
Calibration of a receiver, 214
Capacity, effect upon frequency, 100;
measurement of, 104;
unit of, 104;
variable, 107
Capacity effects, 243;
elimination of, 228
Carrier current, modulation of, 146;
telephony, 255
Characteristic, of vacuum tube, 68, 74;
effect of B-battery upon, 128;
how to plot a, 70
Characteristic curve of transformer, 64
Chemistry, 8
Choke coils, 210, 221
Circuit, A, B, C, 187;
coupled, 115;
defined, 43;
oscillating, 113;
plate, 45;
short, 30;
tune of a, 117
Condenser, defined, 77;
charging current of, 78;
discharge current of, 80;
impedance of, 135;
theory of, 78;
tuning, 224
Common battery system, 254
Connection for wire to radio, 254
Continuous waves, 86
Copper, atomic number of, 13
Copper sulphate, in solution, 21
Crystals, atomic structure, 147
Crystal detectors, 146;
characteristic of, 148;
circuit of, 150;
theory of, 147
Current, transient, 114;
radio, 144
Cycle, 94, 97
Damped oscillations, 114
Demodulation, 231
Detection, explained, 146
Detectors, audion, 126;
crystal, 146
Direct currents, 205
Dissociation, 22
Distortion, of wave form, 163
Dry battery, 27
Earth, atomic constitution, 11
Effective value, of ampere, 207;
of volt, 207
Efficiency, of regenerative circuit, 182
Electrical charge, 22
Electricity, current of, 15, 16
Electrodes, of vacuum tube, 41;
definition of, 41
Electrolyte, definition of, 34
Electrons, properties of, 4;
planetary, 10, 12;
rate of flow, 48;
vapor of, 39;
wandering of, 14
Electron streams, laws of attraction, 200
E. M. F., 59;
alternating, 76;
of self-induction, 238
Energy, expended in tube, 235;
of electrons, 113;
radiation of, 125
Ether, 88
Feed-back circuit, 182
Frequency, 98, 158;
effect upon pitch, 133;
interval, 247;
natural, 117;
of voice, 163
Fundamental note, of string, 157
Gravity battery, theory of, 23
Grid, action of, 47;
condenser, 169;
current, 173;
leak, 171;
leak, construction, 172, 216;
of audion, 41
Harmonics, 160
Helium, properties of, 9
Henry, 83
Heterodyne, 181
Hot-wire ammeter, 51
Human voice, mechanism of, 152
Hydrogen, action of in acid, 7;
atom of, 7
Impedance, of coil, 136;
of condenser, 136;
of transformer, 195;
effect of iron core upon, 207;
matching of, 196
Intermediate-frequency amplification, 242
Inductance, defined, 83;
effect upon frequency, 100;
impedance of, 135;
mutual, 109;
of coils, 101;
self, 83;
table of values, 227;
unit of, 83;
variable, 108
Induction, principle of, 208
Inducto-meter, 109
Input circuit, 187
Interference, 249
Internal resistance, 191
Ion, definition of, 19;
positive and negative, 20, 21
Ionization, 20
Larynx, 153
Laws of attraction, 204
Loading coil, 224
Loop antenna, 198
Magnet, pole of, 203;
of soft iron, 205;
of steel, 205
Magnetism, 202
Matter, constitution of, 5
Megohm, 172
Microfarad, 104
Mil-ampere, 71
Mil-henry, 83
Modulation, 145, 230, 237, 239
Molecule, kinds of, 6;
motion of, 35
[Greek: mu], 190
Multiplex telegraphy, 258;
telephony, 258
Mutual inductance, 109;
variation of, 110
Natural frequency, 161
Nitrogen, 10
Nucleus of atom, 10, 12
Ohm, defined, 64
Organ pipe, 160
Oscillations, 87;
damped, 114;
to start, 114;
intensity of, 236;
natural frequency of, 117
Output circuit, 187
Overtones, 159
Oxygen, percentage in air, 10
Phase, 180
Plate, of an audion, 41
Plunger type of instrument, 205
Polarity of a coil, 204
Power, defined, 234;
electrical unit of, 235
Proton, properties of, 4
Radio current, modulation of, 145
Radio-frequency amplification, 243;
limitations, 243
Radio-frequency amplifier, 186, 198
Radio station connected to land line, 254
Rating of tubes, 235
Reception, essential operations in, 235
Regenerative circuit, 176;
frequency of, 179
Repeater stations, 261
Resistance, measurement of, 64;
non-inductive, 103;
square, 251
Resonance, 161
Resonance curve, 117
Retard coils, 210
Salt, atomic construction of, 17;
crystal structure, 147;
molecule in solution, 19;
percentage in sea water, 11
Saturation, 38
Sea water, atomic constitution of, 11
Self-inductance, 83;
unit of, 83
Side bands, 248;
relation to wave lengths, 249
Silicon, percentage in earth, 11
Sodium chloride, in solution, 19
Sound, production of, 152
Speech, to transmit by radio, 230
Speed of light, 122
Standard cell, 58
Storage battery, 28, 30
Sulphuric acid, 22
Super-heterodyne, 242;
advantages of, 242
Telephone receiver, 130;
theory of, 131
Telephone transmitter, 142
Telephony, by wire, 253
Tickler coil, 182
Transcontinental telephone line, 261
Transmission, essential operations in, 230
Transmitter, Arlington, 233;
continuous wave, 94, 119;
for high power, 233
Transformer, 185;
step-up, 193
Tubes, connected in parallel, 234
Tuning, curve, 117;
sharp, 214;
with series condenser, 224
Undamped waves (see continuous waves), 86
Vacuum tube, 35, 40;
characteristics of, 67;
construction of, 205;
modulator, 239;
three-electrode, 41;
two-electrode, 42
Variometer, 108
Vibrating string, study of, 154
Vocal cords, 153
Voice frequencies, 163
Volt, definition of, 57;
measurement of, 61
Voltmeter, calibration of, 62;
construction of, 205
Watt, 235
Wave form, 182
Wave length, relation to frequency, 98, 122;
defined, 122
Wire, inductance of, 104
Wire, movement of electrons in, 14;
emission of electrons from, 37
Wire telephony, 253
Wired wireless, 255;
advantages of, 257
X-rays, 147
Zero coupling, 177
Zinc, electrode for battery, 23