THE TELEPHONE
VIBRATIONS.--Every manifestation in nature is by way of vibration. The
beating of the heart, the action of the legs in walking, the winking of
the eyelid; the impulses from the sun, which we call light; sound, taste
and color appeal to our senses by vibratory means, and, as we have
hereinbefore stated, the manifestations of electricity and magnetism are
merely vibrations of different wave lengths.
THE ACOUSTIC TELEPHONE.--That sound is merely a product of vibrations
may be proven in many ways. One of the earliest forms of telephones was
simply a "sound" telephone, called the _Acoustic Telephone_. The
principle of this may be illustrated as follows:
Take two cups (A, B), as in Fig. 78, punch a small hole through the
bottom of each, and run a string or wire (C) from the hole of one cup to
that of the other, and secure it at both ends so it may be drawn taut.
Now, by talking into the cup (A) the bottom of it will vibrate to and
fro, as shown by the dotted lines and thereby cause the bottom of the
other cup (B) to vibrate in like manner, and in so vibrating it will
receive not only the same amplitude, but also the same character of
vibrations as the cup (A) gave forth.
[Illustration: _Fig. 78._ ACOUSTIC TELEPHONE]
[Illustration: _Fig. 79._ ILLUSTRATING VIBRATIONS]
SOUND WAVES.--Sound waves are long and short; the long waves giving
sounds which are low in the musical scale, and the short waves high
musical tones. You may easily determine this by the following
experiment:
Stretch a wire, as at B (Fig. 79), fairly tight, and then vibrate it.
The amplitude of the vibration will be as indicated by dotted line A.
Now, stretch it very tight, as at C, so that the amplitude of vibration
will be as shown at E. By putting your ear close to the string you will
find that while A has a low pitch, C is very much higher. This is the
principle on which stringed instruments are built. You will note that
the wave length, which represents the distance between the dotted lines
A is much greater than E.
HEARING ELECTRICITY.--In electricity, mechanism has been made to enable
man to note the action of the current. By means of the armature,
vibrating in front of a magnet, we can see its manifestations. It is now
but a step to devise some means whereby we may hear it. In this, as in
everything else electrically, the magnet comes into play.
[Illustration: _Fig. 80._ THE MAGNETIC FIELD]
In the chapter on magnetism, it was stated that the magnetic field
extended out beyond the magnet, so that if we were able to see the
magnetism, the end of a magnet would appear to us something like a
moving field, represented by the dotted lines in Fig. 80.
The magnetic field is shown in Fig. 80 at only one end, but its
manifestations are alike at both ends. It will be seen that the magnetic
field extends out to a considerable distance and has quite a radius of
influence.
THE DIAPHRAGM IN A MAGNETIC FIELD.--If, now, we put a diaphragm (A) in
this magnetic field, close up to the end of the magnet, but not so close
as to touch it, and then push it in and out, or talk into it so that the
sound waves strike it, the movement or the vibration of the diaphragm
(A) will disturb the magnetic field emanating from the magnet, and this
disturbance of the magnetic field at one end of the magnet also affects
the magnetic field at the other end in the same way, so that the
disturbance there will be of the same amplitude. It will also display
the same characteristics as did the magnetic field when the diaphragm
(A) disturbed it.
A SIMPLE TELEPHONE CIRCUIT.--From this simple fact grew the telephone.
If two magnets are connected up in the same circuit, so that the
magnetic fields of the two magnets have the same source of electric
power, the disturbance of one diaphragm will affect the other similarly,
just the same as the two magnetic fields of the single magnet are
disturbed in unison.
HOW TO MAKE A TELEPHONE.--For experimental and testing purposes two of
these telephones should be made at the same time. The case or holder
(A) may be made either of hard wood or hard rubber, so that it is of
insulating material. The core (B) is of soft iron, 3/8 inch in diameter
and 5 inches long, bored and threaded at one end to receive a screw (C)
which passes through the end of the case (A).
The enlarged end of the case should be, exteriorly, 2-1/4 inches in
diameter, and the body of the case 1 inch in diameter.
[Illustration: _Fig. 81._ SECTION OF TELEPHONE RECEIVER]
Interiorly, the large end of the case is provided with a circular recess
1-3/4 inches in diameter and adapted to receive therein a spool which
is, diametrically, a little smaller than the recess. The spool fits
fairly tight upon the end of the core, and when in position rests
against an annular shoulder in the recess. A hollow space (F) is thus
provided behind the spool (D), so the two wires from the magnet may
have room where they emerge from the spool.
The spool is a little shorter than the distance between the shoulder (E)
and the end of the casing, at G, and the core projects only a short
distance beyond the end of the spool, so that when the diaphragm (H) is
put upon the end of the case, and held there by screws (I) it will not
touch the end of the core. A wooden or rubber mouthpiece (J) is then
turned up to fit over the end of the case.
[Illustration: _Fig. 82._ THE MAGNET AND RECEIVER HEAD]
The spool (D) is made of hard rubber, and is wound with No. 24
silk-covered wire, the windings to be well insulated from each other.
The two ends of the wire are brought out, and threaded through holes (K)
drilled longitudinally through the walls of the case, and affixed to the
end by means of screws (L), so that the two wires may be brought
together and connected with a duplex wire (M).
As the screw (C), which holds the core in place, has its head hidden
within a recess, which can be closed up by wax, the two terminals of the
wires are well separated so that short-circuiting cannot take place.
TELEPHONE CONNECTIONS.--The simplest form of telephone connection is
shown in Fig. 83. This has merely the two telephones (A and B), with a
single battery (C) to supply electricity for both. One line wire (D)
connects the two telephones directly, while the other line (E) has the
battery in its circuit.
[Illustration: _Fig. 83._ SIMPLE TELEPHONE CONNECTION]
COMPLETE INSTALLATION.--To install a more complete system requires, at
each end, a switch, a battery and an electro-magneto bell. You may use,
for this purpose, a bell, made as shown in the chapter on bells.
Fig. 84 shows such a circuit. We now dispense with one of the line
wires, because it has been found that the ground between the two
stations serves as a conductor, so that only one line wire (A) is
necessary to connect directly with the telephones of the two stations.
The telephones (B, B', respectively) have wires (C, C') running to the
pivots of double-throw switches (D, D'), one terminal of the switches
having wires (E, E'), which go to electric bells (F, F'), and from the
bells are other wires (G, G'), which go to the ground. The ground wires
also have wires (H, H'), which go to the other terminals of the switch
(D, D'). The double-throw switch (D, D'), in the two stations, is thrown
over so the current, if any should pass through, will go through the
bell to the ground, through the wires (E, G or E', G').
[Illustration: _Fig. 84._ TELEPHONE STATIONS IN CIRCUIT]
Now, supposing the switch (D'), in station 2, should be thrown over so
it contacts with the wire (H'). It is obvious that the current will then
flow from the battery (I') through wires (H', C') and line (A) to
station 1; then through wire C, switch D, wire E to the bell F, to the
ground through wire G. From wire G the current returns through the
ground to station 2, where it flows up wire G' to the battery, thereby
completing the circuit.
[Illustration: _Fig. 85._ ILLUSTRATING LIGHT CONTACT POINTS]
The operator at station 2, having given the signal, again throws his
switch (D') back to the position shown in Fig. 84, and the operator at
station 1 throws on his switch (D), so as to ring the bell in station 2,
thereby answering the signal, which means that both switches are again
to be thrown over so they contact with the battery wires (H and H'),
respectively. When both are thus thrown over, the bells (G, G') are cut
out of the circuit, and the batteries are both thrown in, so that the
telephones are now ready for talking purposes.
MICROPHONE.--Originally this form of telephone system was generally
employed, but it was found that for long distances a more sensitive
instrument was necessary.
LIGHT CONTACT POINTS.--In 1877 Professor Hughes discovered,
accidentally, that a light contact point in an electric circuit
augmented the sound in a telephone circuit. If, for instance, a light
pin, or a nail (A, Fig. 85) should be used to connect the severed ends
of a wire (B), the sounds in the telephone not only would be louder, but
they would be more distinct, and the first instrument made practically,
to demonstrate this, is shown in Fig. 86.
[Illustration: _Fig. 86._ MICROPHONE]
[Illustration: _Fig. 87._ TRANSMITTER]
HOW TO MAKE A MICROPHONE.--This instrument has simply a base (A) of
wood, and near one end is a perpendicular sounding-board (B) of wood, to
one side of which is attached, by wax or otherwise, a pair of carbon
blocks (C, D). The lower carbon block (C) has a cup-shaped depression in
its upper side, and the upper block has a similar depression in its
lower side. A carbon pencil (E) is lightly held within these cups, so
that the lightest contact of the upper end of the pencil with the
carbon block, makes the instrument so sensitive that a fly, walking upon
the sounding-board, may be distinctly heard through the telephone which
is in the circuit.
MICROPHONE THE FATHER OF THE TRANSMITTER.--This instrument has been
greatly modified, and is now used as a transmitter, the latter thereby
taking the place of the pin (A), shown in Fig. 85.
AUTOMATIC CUT-OUTS FOR TELEPHONES.--In the operation of the telephone,
the great drawback originally was in inducing users of the lines to
replace or adjust their instruments carefully. When switches were used,
they would forget to throw them back, and all sorts of trouble resulted.
It was found necessary to provide an automatic means for throwing in and
cutting out an instrument, this being done by hanging the telephone on
the hook, so that the act merely of leaving the telephone made it
necessary, in replacing the instrument, to cut out the apparatus.
Before describing the circuiting required for these improvements, we
show, in Fig. 87, a section of a transmitter.
A cup-shaped case (A) is provided, made of some insulating material,
which has a diaphragm (B) secured at its open side. This diaphragm
carries the carbon pencil (C) on one side and from the blocks which
support the carbon pencil the wires run to binding posts on the case.
Of course the carbon supporting posts must be insulated from each other,
so the current will go through the carbon pencil (C).
COMPLETE CIRCUITING WITH TRANSMITTER.--In showing the circuiting (Fig.
88) it will not be possible to illustrate the boxes, or casings, which
receive the various instruments. For instance, the hook which carries
the telephone or the receiver, is hinged within the transmitter box. The
circuiting is all that it is intended to show.
[Illustration: _Fig. 88._ COMPLETE TELEPHONIC CIRCUIT]
The batteries of the two stations are connected up by a wire (A), unless
a ground circuit is used. The other side of each battery has a wire
connection (B, B') with one terminal of the transmitter, and the other
terminal of the transmitter has a wire (C, C') which goes to the
receiver. From the other terminal of the receiver is a wire (D, D')
which leads to the upper stop contact (E, E') of the telephone hook. A
wire (F, F') from the lower stop contact (G, G') of the hook goes to one
terminal of the bell, and from the other terminal of the bell is a wire
(H, H') which makes connection with the line wire (A). In order to make
a complete circuit between the two stations, a line wire (I) is run from
the pivot of the hook in station 1 to the pivot of the hook in station
2.
In the diagram, it is assumed that the receivers are on the hooks, and
that both hooks are, therefore, in circuit with the lower contacts (G,
G'), so that the transmitter and receiver are both out of circuit with
the batteries, and the bell in circuit; but the moment the receiver, for
instance, in station 1 is taken off the hook, the latter springs up so
that it contacts with the stop (E), thus establishing a circuit through
the line wire (I) to the hook of station 2, and from the hook through
line (F') to the bell. From the bell, the line (A) carries the current
back to the battery of station (A), thence through the wire (B) to the
transmitter wire (C) to receiver and wire (D) to the post (E), thereby
completing the circuit.
When, at station 2, the receiver is taken off the hook, and the latter
contacts with the post (E'), the transmitter and receiver of both
stations are in circuit with each other, but both bells are cut out.