In 1878, David Edward Hughes discovered that the imperfect contact
formed between two pieces of some such substance as carbon or charcoal
is very sensitive to the slightest changes in pressure, and when
included in an electric circuit with a battery and a telephone receiver,
will transmit sounds. Such an instrument is called a _microphone_. It
has various forms but in most of them one piece of carbon or charcoal is
held loosely between two other pieces in such a manner as to be easily
affected by the slightest vibrations conveyed to it through the air or
any other medium.
[Illustration: Fig. 143.—A Microphone connected to a Telephone Receiver,
and a Battery.]
Figure 143 illustrates a simple form of instrument embodying this
principle. A small pencil of carbon is supported loosely between two
blocks of the same substance glued to a thin wooden sounding-board of
pine. The sounding-board is mounted in an upright position on a wooden
base. The carbon pencil rests loosely in two small indentations in the
carbon blocks. The blocks are connected, by means of a very fine wire or
a strip of tinfoil, with one or two cells of battery and a telephone
receiver. Any vibration or sounds in range of the microphone will cause
the sounding-board to vibrate. This will affect the pressure of the
contact between the carbon pencil and the two blocks. When the pressure
between the two is increased the resistance in the path of the electric
current is decreased and more current immediately flows through the
circuit. On the other hand, when the pressure is decreased, the
resistance is increased and less current flows through the telephone
receiver. The amount of current flowing in the circuit thus keeps step
with the changes in the resistance, and accordingly produces sounds in
the telephone receiver. The vibrations emitted from the receiver are
usually much greater than those of the original sounds, and so the
microphone may be used to magnify weak sounds such as the ticking of
clock-wheels or the footfalls of insects. If a watch is laid on the base
of the microphone, the ticking of the escapement wheel can be heard with
startling loudness. The sounds caused by a fly walking on a microphone
may be made to sound as loud as the tramp of a horse.
[Illustration: Fig. 144.—A Very Sensitive Form of Microphone, with which
the Footsteps of a Fly can be heard.]
The electrical _stethoscopes_ used by physicians to listen to the action
of the heart are in principle only a microphone and telephone receiver
connected to a battery.
The drawing in Figure 144 illustrates a very sensitive microphone that
is quite easy to make. With this instrument it is possible to hear the
tramping of a fly’s feet or the noise of its wings.
The base upon which the apparatus is mounted serves as the
sounding-board and is made in the form of a hollow wooden box. It can be
made from an ordinary cigar-box by removing the paper and taking the box
apart. The piece forming the top of the box must be planed down until it
is only three thirty-seconds of an inch thick. The box should measure
about five inches square and three-quarters of an inch thick when
finished. Do not use any nails or small brads whatsoever in its
construction, but fasten it together with glue. If you use any nails you
will decrease the sensitiveness of the instrument quite appreciably. The
bottom of the box should be left open. The result is a sounding-board of
the same principles as that of the banjo head. Small feet, one-quarter
of an inch square, are glued to the four under corners so as to raise
the bottom clear of the table, or whatever the microphone may be placed
upon. The bottom of each one of the small feet is cushioned with a layer
of felt so that no jars will be transmitted to the instrument by any
object upon which it is resting.
The carbon pencil used on this type of instrument is pivoted in the
center and rests at one end upon a carbon block.
The carbon block is made about one inch long, one-quarter of an inch
thick, and one-half of an inch wide. A small hole is drilled near each
end to receive a screw which fastens the block to the sounding-board. A
fine wire is led from one of these screws to a binding-post mounted at
the side of the box. Another wire leads from a second binding-post to a
standard which is also fastened to the sounding-board with a small
screw.
The standard is made from a sheet of thin brass and is bent into the
shape shown in the illustration.
The pencil is a piece of one-quarter-inch carbon rod, two and
three-quarter inches long. A small hole is drilled one and five-eighths
of an inch from one end with a sewing-needle, and a piece of fine brass
wire, pointed at both ends, pushed in. The wire should be a tight fit in
the hole. It should be about one-half of an inch long, and may be made
from an ordinary pin.
The slide-bar is used to regulate the pressure of the pencil upon the
carbon block and is simply a piece of soft copper wire about one-eighth
of an inch in diameter. It is bent into the shape shown in the
illustration so that it will slide over the carbon pencil. The sides of
the standard should press just tightly enough against the ends of the
pivot which passes through the carbon pencil to hold it in position
without slipping, and at the same time allow it to swing freely up and
down.
The two binding-posts should be connected in series with two dry cells
and a pair of good telephone receivers. Place the receivers against the
ears. Move the slide-bar gently back and forth until the voice of any
one talking in another part of the room can be heard distinctly in the
telephone receivers. In order to hear faint whispers, move the slide-bar
away from the carbon block.
In order to hear a fly walk it is necessary to have the carbons very dry
and clean. The instrument must be very carefully adjusted. Cover the
microphone with a large glass globe and place a fly inside of the globe.
Whenever the fly walks on any part of the microphone you will be able to
hear each footstep in the telephone receivers. When he flies about
inside of the globe, his wings will cause a loud roaring and buzzing
noise to be heard in the receivers.
Telephones
Not many years ago, when the telephone made its first appearance, it was
the wonder of the times just as wireless telegraphy is to-day. Starting
as an exceedingly simple and inexpensive apparatus, it has gradually
developed into a wonderful and complex system, so that at the present
time, instead of experiencing difficulty in telephoning over distances
of fifty or one hundred miles, as at first, it is possible to carry on a
conversation over a line two thousand miles long as easily as it is face
to face.
Like the telegraph, the principle of the telephone is that of a current
of electricity flowing over a line wire into a pair of electro-magnets,
but with many important differences.
When compared with telegraph apparatus, the telephone is found to be
exceedingly sensitive. A telegraph relay requires perhaps about
one-hundredth of an ampere to work it properly. A telegraph sounder will
require about one-tenth of an ampere, but a telephone receiver will
render speech audible with less than a millionth of an ampere, and
therefore may almost be said to be a hundred thousand times more
sensitive than a sounder.
Another difference between the telephone and the telegraph lies in the
fact that the currents flowing over a telegraph line do not usually vary
at a rate greater than twenty or thirty times a second, whereas
telephone currents change their intensity hundreds of times a second.
The telephone is an instrument for the transmission of speech to a
distance by means of electricity, wherein the speaker talks to an
elastic plate of thin sheet-iron which vibrates and sends out a
pulsating current of electricity.
The transmission of the vibrations depends upon well-known principles of
electricity, and does not consist of the actual transmission of sounds,
but of electrical impulses which keep perfect accord or step with the
sound waves produced by the voice in the transmitter. These electrical
currents pass through a pair of small electro-magnets acting upon a
plate or diaphragm, which in turn agitates the air in a manner similar
to the original voice speaking into the transmitter and thus emits
sounds.
That part of the apparatus which takes up the sounds and changes them
into electric currents composes the _transmitter_. When words are spoken
into the mouthpiece they strike a diaphragm, on the back of which is
fastened a small cup-shaped piece of carbon. A second cup is mounted in
a rigid position directly back of the first. The space between them is
filled with small polished granules of carbon. When these granules are
in a perfectly loose state and are undisturbed, their resistance to an
electric current is very great and they allow almost none to flow.² When
slightly compressed their resistance is greatly lowered and they permit
the current to pass. The vibrations of the diaphragm cause the carbon
cup mounted on its back to move and exert a varying pressure upon the
granules with a corresponding variation in their resistance and the
amount of current which will pass through.
[Illustration: Fig. 145.—A Telephone System, consisting of a Receiver,
Transmitter, and a Battery connected in Series. Words spoken into the
Transmitter are reproduced by the Receiver.]
The _receiver_, or that part of the apparatus which transforms the
pulsating current back into sound waves consists of a thin iron disk,
placed very near but not quite touching the end of a small steel bar,
permanently magnetized, and about which is wound a coil of fine
insulated wire.
The transmitter and the receiver are connected together in series with a
battery as in Figure 145. When words are spoken into the transmitter the
little carbon granules are immediately thrown into motion, and being
alternately compressed and released cause corresponding changes in the
current flowing through the receiver from the battery. The magnetism of
the receiver changes with each change in the electric current, and thus
by alternately attracting and repelling the diaphragm causes it to
vibrate and emit sounds. Such is the _principle_ of the telephone. The
telephones in actual service to-day are complicated with bells,
magnetos, induction coils, condensers, relays, and various other
apparatus, which fact renders them more efficient.
The bells and magnetos are for the purpose of calling the central
operator or the person at the other end of the line and drawing
attention to the fact that some one wishes to get into communication
with him. The older styles of telephones used what is known as a
polarized bell and a hand magneto for this purpose. A polarized bell is
a very sensitive piece of apparatus which will operate with very little
current. A magneto is a small hand dynamo which when turned with a crank
will generate a current causing the bell at the other end of the line to
ring. When the telephone receiver is raised off its hook in order to
place it to the ear the bell and magneto are automatically disconnected
from the line and the receiver and the transmitter are connected in
their place. The current necessary to supply the telephone and receiver
is supplied by two or three dry cells placed inside of each telephone.
The latest types of instruments employ what is known as the central
energy system, wherein the current is supplied by a large storage
battery located at the central office and serving as a current supply to
all the telephones connected to that system.
It would be impossible to enter into the history of the telephone far
enough to explain the details of some of the various systems in
every-day use in such a book as this because of the immense amount of
material it would be necessary to present. Such a work would occupy a
volume of its own. Additional information may be readily found in any
reference library. However, the "boy electrician" who wishes to make a
telephone for communicating between the house and barn, or with his chum
down the street, will find the necessary information in the following
pages. If this work is carried out carefully and a home-made telephone
system built and installed it will not only prove a very interesting
undertaking but will also serve to dispel all mystery which may surround
this device in the mind of the young experimenter.
How to Build a Telephone
Telephone receivers are useful for many purposes in electrical work
other than to receive speech. They are used in connection with wireless
instruments, in place of a galvanometer in measuring electrical
circuits, and for testing in various ways.
Telephone receivers are of two types. One of them is long and
cumbersome, and is very similar to the original Bell telephone receiver.
The other is small and flat, and is called a "watch-case" receiver. A
watch-case receiver is shown in Figure 146. It consists of a U-shaped
permanent magnet so mounted as to exert a polarizing influence upon a
pair of little electro-magnets, before the poles of which is placed an
iron diaphragm. For convenience, these parts are assembled in a small
cylindrical casing, usually of hard rubber. The permanent magnet exerts
a continual pull upon the diaphragm, tending to draw it in. When the
telephone currents pass through the little magnets, they will either
strengthen the permanent magnet and assist it in attracting the
diaphragm, or detract from its strength and allow the diaphragm to
recede, depending upon which direction the current flows.
[Illustration: Fig. 146.—A Watch-Case Telephone Receiver.]
Watch-case receivers are usually employed for wireless telegraph work
because they are very light in weight and can easily be attached to a
head-band in order to hold them to the ears and leave the hands free.
Watch-case receivers can be purchased for forty-five to seventy-five
cents at almost any electrical supply house. They are very useful to the
amateur experimenter in many ways.
A telephone receiver capable of giving fair results on a short telephone
line can be very easily made, but of course will not prove as efficient
as one which is purchased ready-made from a reliable electrical
manufacturer.
The first practical telephone receiver was invented by Alexander Graham
Bell and was made somewhat along the same lines as that shown in Figure
147.
Such a receiver may be made from a piece of curtain-pole, three and
three-quarter inches long and about one and one-eighth inches in
diameter. A hole, three-eighths of an inch in diameter, is bored along
the axis throughout its entire length, to receive the permanent magnet.
The shell of the receiver is a cup-shaped piece of hard wood, two and
one-half inches in diameter and one inch deep. It will have to be turned
on a lathe. Its exact shape and dimensions are best understood from the
dimensions shown in the cross section in Figure 147. The shell is firmly
attached to one end of the piece of curtain-pole by gluing.
The permanent magnet is a piece of hard steel, three-eighths of an inch
in diameter and four and five-eighths of an inch in length. The steel
will have to be tempered or hardened before it will make a suitable
magnet, and the best way to accomplish this is to have a blacksmith do
it for you by heating the rod and then plunging it into water when just
at the right temperature.
[Illustration: Fig. 147.—A Simple Form of Telephone Receiver.]
One end of the bar is fitted with two thick fiber washers about
seven-eighths of an inch in diameter and spaced one-quarter of an inch
apart. The bobbin so formed is wound full of No. 36 B. & S. gauge
single-silk-covered magnet wire. The ends of the wire are passed through
two small holes in the fiber washers and then connected to a pair of
heavier wires. The wires are run through two holes in the curtain-pole,
passing lengthwise from end to end, parallel to the hole bored to
receive the bar magnet.
This bar magnet is then pushed through the hole until the end of the rod
on which the spool is fixed is just below the level of the edges of the
shell.
The two wires are connected to binding-posts, _A_ and _B_, mounted on
the end of the receiver. A hook is also provided so that the receiver
may be hung up.
The diaphragm is a circular piece of thin sheet-iron, two and one-half
inches in diameter. It is placed over the shell, and the bar magnet
adjusted until the end almost touches the diaphragm. The magnet should
fit into the hole very tightly, so that it will have to be driven in
order to be moved back and forth.
The diaphragm is held in place by a hard-wood cap, two and three-quarter
inches in diameter and having a hole three-quarters of an inch in
diameter in the center. The cap is held to the shell by means of four
small brass screws.
The receiver is now completed and should give a loud click each time
that a battery is connected or disconnected from the two posts, _A_ and
_B_.
The original Bell telephone apparatus was made up simply of two
receivers without any battery or transmitter. In such a case the current
is generated by "induction." The receiver is used to speak through as
well as to hear through. This method of telephoning is unsatisfactory
over any appreciable distances. The time utilized in making a
transmitter will be well spent.
A simple form of transmitter is shown in Figure 148. The wooden back,
_B_, is three and one-half inches square and three-quarters of an inch
thick. The front face of the block is hollowed out in the center as
shown in the cross-section view.
The face-plate, _A_, is two and one-half inches square and one-half an
inch thick. A hole, seven-eighths of an inch in diameter, is bored
through the center. One side is then hollowed out to a diameter of one
and three-quarter inches, so as to give space for the diaphragm to
vibrate as shown in the cross-sectional drawing.
The carbon buttons are one inch in diameter and three-sixteenths of an
inch thick. A small hole is bored in the center of each to receive a
brass machine screw. The hole is countersunk, so as to bring the head of
the screw down as close to the surface of the carbon as is possible.
Then, using a sharp knife or a three-cornered file, score the surface of
the carbon until it is covered with criss-cross lines.
The diaphragm is a piece of thin sheet-iron cut in the form of a circle
two and one-half inches in diameter. A small hole is bored through the
center of this. One of the carbon buttons is fastened to the center of
the diaphragm with a small screw and a nut.
Cut out a strip of flannel or thin felt, nine-sixteenths of an inch wide
and three and one-half inches long. Around the edge of the carbon button
mounted on the diaphragm, bind this strip with silk thread in such a
manner that the strip forms a cylinder closed at one end with the
button.
Fill the cylinder with polished carbon telephone transmitter granules to
a depth of about one-eighth of an inch. These granules will have to be
purchased from an electrical supply house. They are finely polished
small carbon balls, much like birdshot in appearance.
Slip a long machine-screw through the hole in the second carbon button
and clamp it in place with a nut. Then place the carbon button in the
cylinder so that it closes up the end. The space between the two buttons
should be about three-sixteenths of an inch. Bind the flannel or felt
around the button with a piece of silk thread so that it cannot slip out
of place. The arrangement of the parts should now be the same as that
shown by the cross-sectional drawing in the upper right-hand corner of
Figure 148.
The complete transmitter is assembled as shown in the lower part of
Figure 148.
A small tin funnel is fitted into the hole in the face-plate, _A_, to
act as a mouthpiece.
A screw passes through the back, _B_, and connects to the diaphragm. The
screw is marked "_E_" in the illustration. A binding-post is threaded on
the screw so that a wire may be easily connected. The screw passing
through the back carbon button also passes through a hole in the wooden
back, and is clamped firmly in position with a brass nut so that the
button is held very rigidly and cannot move. The front button, being
attached to the diaphragm, is free to move back and forth with each
vibration of the latter.
[Illustration: Fig. 148.—A Home-made Telephone Transmitter.]
The carbon granules should fill the space between the buttons
three-quarters full. They should lie loosely together, and not be packed
in.
When connected to a battery and a telephone receiver the current passes
from the post, _D_, to the back button, through the mass of carbon
granules into the front button and out at the post, _E_. When the voice
is directed into the mouthpiece, the sound waves strike the diaphragm
and cause it to vibrate. The front button attached to it then also
vibrates and constantly changes the pressure on the carbon granules.
Each change in pressure is accompanied by an immediate change in
resistance and consequently the amount of current flowing.
Figure 149 shows a complete telephone ready for mounting on the wall. It
consists of a receiver, telephone transmitter, bell, hook, and
push-button. The bell is mounted on a flat base-board. The transmitter
is similar to that just described, but is built into the front of a
box-like cabinet. The box is fitted with a push-button at the lower
right-hand corner. A simple method of making a suitable push-button is
shown in the upper left-hand part of the illustration. It consists of
two small brass strips arranged so that pushing a small wooden plug
projecting through the side of the cabinet will bring the two strips
together and make an electrical connection.
The "hook" consists of a strip of brass, pivoted at one end with a
round-headed brass wood screw and provided with a small spring, so that
when the receiver is taken off of the hook it will fly up and make
contact with a screw, marked _C_ in the illustration. When the receiver
is on the hook, its weight will draw the latter down against the screw,
_D_. The hook is mounted on the base-board of the telephone, and
projects through a slot cut in the side of the cabinet.
Four binding-posts are mounted on the lower part of the base-board. The
two marked _B_ and _B_ are for the battery.
[Illustration: Fig. 149.—A Complete Telephone Instrument. Two
Instruments such as this are necessary to form a simple Telephone
System.]
That marked _L_ is for the "line," and _G_ is for the ground connection
or the return wire.
[Illustration: Fig. 150.—Diagram of Connection for the Telephone
Instrument shown in Fig. 149.]
The diagram of the connections is shown in Figure 150. The line-wire
coming from the telephone at the other station enters through the
binding-post marked _L_, and then connects to the hook. The lower
contact on the hook is connected to one terminal of the bell. The other
terminal of the bell leads to the binding-post marked _G_, which is
connected to the ground, or to the second line-wire, where two are used.
The post, _G_, and one post, _B_, are connected together. The other post
marked _B_ connects to one terminal of the transmitter. The other
terminal of the transmitter is connected to the telephone receiver. The
other post of the telephone receiver leads to the upper contact on the
hook marked _C_. The push-button is connected directly across the
terminals of the transmitter and the receiver so that when the button is
pushed it short-circuits the transmitter and the receiver. When the
receiver is on the hook and the latter is down so that it makes contact
with _D_ any current coming over the line-wire will pass through the
bell and down through the ground or the return-wire to the other
station, thus completing the circuit. If the current is strong enough it
will ring the bed. When the receiver is lifted off the hook, the spring
will cause the hook to rise and make contact with the screw marked _C_.
This will connect the receiver, transmitter, and the battery to the line
so that it is possible to talk. If, however, it is desired to ring the
bell on the instrument at the other end of the line, all that it is
necessary to do is to press the push-button. This will short-circuit the
receiver and the transmitter and ring the bell. The battery current is
flowing over the line all the time when the receiver is up, but the
transmitter and the receiver offer so much resistance to its flow that
not enough current can pass to ring the bell until the resistance is cut
out by short-circuiting them with the push-button.
The instruments at both ends of the line should be similar. In
connecting them together care should be taken to see that the batteries
at each end of the line are arranged so that they are in series and do
not oppose each other. One side of the line may be a wire, but the
return may be the ground, as already explained in the chapter on
telegraph apparatus.
A transmitter of the "desk-stand" type may be made according to the
scheme shown in Figure 151. It consists simply of a transmitter mounted
upon an upright, and provided with a base so that it may stand on a desk
or a table.
[Illustration: Fig. 151.—A Desk-Stand Type of Telephone.]
It is also fitted with a hook and a push-button, so that it is a
complete telephone instrument with the exception of the bell and the
battery. The battery and the bell may be located in another place and
connected to the desk-stand by means of a flexible wire or "electrical
cord."
Figure 152 shows what is known as a telephone induction coil. Induction
coils are used in telephone systems whenever it is necessary to work
over a long distance. Such a system is more complicated, and requires
considerable care in making the connections, but is far superior to the
system just described.
[Illustration: Fig. 152.—A Telephone Induction Coil.]
An induction coil consists of two fiber or hard-wood heads, about one
inch square and one-quarter of an inch thick, mounted on the ends of an
iron core composed of a bundle of small iron wires about two and
one-half inches long. The core should be about five-sixteenths of an
inch in diameter.
The core is covered with a layer of paper and then wound with three
layers of No. 22 B. & S. single-cotton-covered wire. These three layers
of wire form the _primary_. The primary is covered with a layer of paper
and then the secondary is wound on. The secondary consists of twelve
layers of No. 36 B. & S. single-silk-covered magnet wire. It is
advisable to place a layer of paper between layers of the secondary
winding, and to give each one a coating of shellac. The two secondary
terminals of the coil are led out through holes in the fiber head and
kept separate from the primary terminals.
[Illustration: Fig. 153.—Diagram of Connection for a Telephone System
employing an Induction Coil at each Station.]
The wiring diagram of a telephone system using an induction coil at each
station is shown in Figure 153. The speech sent over a line using an
induction coil system is much clearer and more easily understood than
that on a line not using such a device.
In building telephone instruments or connecting them up, care and
accuracy will go a long way towards success. Telephony involves some
very delicate and sensitive vibratory mechanical and electrical actions,
and such instruments must be very carefully made.
² A transmitter is really a microphone built especially to receive
the sounds of the human voice, and operates on the same principle.