Many of the readers of this book are probably "wireless experimenters"
who have constructed their own wireless telegraph apparatus, but not
many have ever built a wireless telephone set. The arrangement described
in the following chapter may be built by almost any boy and will prove a
very interesting and instructive piece of apparatus. It is of no real
practical value as a wireless telephone, owing to the fact that the
distance over which it will transmit intelligible speech is limited to
150 to 300 feet. However if you have a chum who lives next door or
across the street or within the distance named above, it is easily
possible for you to construct a wireless telephone which will enable you
to remain in your own rooms and hold a conversation with each other,
without any connecting wires.
[Illustration: FIG. 179.—When a Bar Magnet is plunged into a Hollow Coil
of Wire, a Momentary Current of Electricity is Generated.]
The instruments depend for operation upon what is known as magnetic
induction. Michael Faraday, who was a famous English scientist,
discovered in 1831 that if a magnet is suddenly plunged into a hollow
coil of wire, a momentary current of electricity is generated in the
coil. You can try this experiment for yourself by connecting a
galvanometer with a hollow coil of wire and then suddenly plunging a bar
magnet into the coil.
If you observe closely you will notice that the needle of the
galvanometer is deflected, this indicating that a current has passed
through the circuit. The galvanometer should of course be far enough
away so that the needle is not affected directly by the magnetism of the
bar magnet itself.
[Illustration: FIG. 180.—Magnetic Phantom showing the Lines of Force
about a Bar Magnet.]
As long as the bar magnet is motionless, it will not induce any current
in the coil. As soon as it is moved however, the currents are set up.
The mechanical work done in moving the magnet is really the source of
electrical energy. The medium which changes the mechanical energy into
electricity is called the magnetic field. The real nature of a magnetic
field is very hard to explain and not easily understood. It is a
peculiar state or condition of the space in the immediate neighborhood
of a magnet. It is possible to show its existence by placing a sheet of
cardboard over a magnet and then sprinkling iron filings over the
surface of the cardboard. If the cardboard is tapped slightly, the
filings will settle down in curving lines, forming a magnetic "phantom."
The curved lines into which the filings form themselves, represent the
paths of the lines of force which make up the magnetic field. The
illustration in Figure 180 shows the magnetic phantom about a bar
magnet.
When the space in the neighborhood of a wire or a coil of wire through
which a current of electricity is passing is examined by means of some
iron filings sprinkled on a sheet of cardboard, it is found that a
similar state of affairs exists there and that it also possesses a
magnetic field.
This can be readily shown by punching a small hole in the centre of a
sheet of cardboard or stiff paper and passing a wire which is carrying a
strong current of electricity through the hole at right angles to the
surface of the cardboard. Sprinkle some iron filings on the cardboard
and they will arrange themselves in circles around the wire, forming a
magnetic phantom giving definite proof to the eyes of the existence of
the magnetic field.
Some of my readers may have begun to wonder what a magnetic field has to
do with the operation of the wireless telephone. It has already been
explained that this particular type of wireless telephone depends upon
magnetic induction for its operation. It is necessary to have some sort
of a conception of the magnetic field before you will be able to
understand magnetic induction.
[Illustration: FIG. 181.—Magnetic Phantom about a Coil of Wire carrying
a current.]
In order that you may gain an idea of how the instruments work, suppose
that two coils of wire, A and B, are connected as shown in Figure 182.
The coil A is connected in series with a set of batteries and a
telegraph key. The other coil B is connected to a telephone receiver.
The coils are placed parallel to each other and only a few inches apart.
When the key is pressed so that the circuit is complete and the battery
current flows through the coil A, it creates a magnetic field in the
vicinity of the coil and lines of force are set up in the immediate
neighborhood.
The lines of force will travel through and about coil B and *induce in
it a current of electricity*. The electric current will cause a sound
like a click in the telephone receiver. When a telephone transmitter is
substituted for the key and words are spoken into the mouthpiece, the
current passing through the coil from the battery will be varied in
accordance with the vibrations of the voice. The changes in the current
will produce changes in the magnetic field set up by the coil and the
changes in the magnetic field will produce a varying current in the coil
B so that the words will be distinctly repeated by the receiver
connected to B.
[Illustration: FIG. 182.—Illustrating the Principle of the Induction
Wireless Telephone.]
This experiment is very easy to perform and may be carried out by almost
any boy with the materials he probably already has around his shop.
Twenty-five to thirty turns of magnet wire of almost any size wound
around one end of a cardboard tube three or four inches in diameter will
serve as the coil A. A similar coil wound around the tube five or six
inches away, an ordinary telephone transmitter and receiver and two or
three dry cells form all the rest of the equipment desired. Words spoken
into the transmitter connected to the coil A will be distinctly heard in
the telephone receiver connected to the coil B, although there is no
connection between the two coils other than the *magnetic field*. Such
an outfit as this is of course only experimental and intended solely for
the purpose of demonstrating the principles of operation of the larger
outfit. If it is desired to make a practical set for talking over a
considerable distance it will be necessary to make coils much larger in
diameter and to build them so they contain a larger number of turns.
[Illustration: FIG. 183.—Showing how the Coils may be formed by winding
around nails set in a circle in the Floor.]
Large coils can be made by drawing a circle about four feet in diameter
on the floor of your shop. Then drive a number of small nails around the
circle about three inches apart. This arrangement will serve as a form
to wind the coil. After the winding is completed you can pull a few of
the nails out and lift it off and you will have a circular coil four
feet in diameter. Two coils will be required. They should be made of No.
20 B. & S. Gauge cotton covered wire. Each coil should contain about
seventy-five turns of wire. About three pounds of wire will be required
for each coil. After winding the coils should be tied with string or
tape about every six inches so that they will not come apart. The nails
can then be pulled out of the floor and the coil is ready for use. If it
is intended to subject the coils to any amount of handling, it is a good
idea to secure two large hoops such as girls use for rolling along the
sidewalk, about four feet in diameter. Then make the coils exactly the
same diameter as the hoops and when they are finished bind them to the
hoops with tape. Two binding posts may be mounted on each of the hoops
and the terminals of the coil connected to them.
Two coils are required in order to establish a complete wireless
telephone system, one at each station. Each coil may be used for both
sending and receiving messages. Each station should be arranged as in
Figure 184. The transmitter should be of the type known as "long
distance." The switch is a two point switch.
[Illustration: FIG. 184.—Circuit Diagram showing how the Coil is
connected so as to serve for either transmitting or receiving.]
It is a good plan to use two telephone receivers mounted on a head band
so that both ears can be used and the hands left free to manipulate the
switch. If a set of telephone receivers wound to a high resistance for
wireless telegraph purposes is used it will greatly increase the range
of the outfit.
Six to eight first class dry cells will be required to form the battery.
A push button is placed in series with the transmitter so that the
battery current will be consumed only when the button is pressed. This
makes it necessary to push the button when sending a message. When the
lever of the two point switch rests on the contact A the circuit is
completed for transmitting. When the lever rests on B the telephone
receivers are connected to the coil and the apparatus is ready for
receiving a message.
If the apparatus is used only for demonstrating purposes, a camera
tripod will be found very convenient for supporting the coils. The
transmitter and switch can be fastened to the top of the tripod and the
batteries placed in a box on the ground. It is absolutely necessary that
the coils be perfectly parallel to each other when messages are being
transmitted back and forth. It is well to start with the coils
comparatively close to each other, say ten or twelve feet at first, and
then gradually increase the distance.
Coils approximately four feet in diameter and containing about
seventy-five turns of wire will transmit and receive messages about one
hundred and fifty feet. The range can be increased slightly by using
more batteries. If too much battery is used, however, the telephone
transmitter will become very hot and is liable to burn out.
It is impossible, of course, to "call up" over a set of wireless phones
such as these. If you succeed in getting your set to work satisfactorily
with your chum across the street and wish to talk to him it will be
necessary for you to arrange some predetermined time.
In order to operate your phone place the switch lever on the contact A.
Hold your finger tightly against the push button so that the circuit is
completed and talk slowly and distinctly into the transmitter. The
transmitter will operate the best when the mouthpiece points on a level
with your mouth. The lips should be about one inch away from the
mouthpiece. Speak in a low even tone. The switch at your friend’s
station should be resting on B. When you finish speaking throw your
switch to B and listen in your telephone receivers. The operator at the
other station should throw his switch to A and push the button. He may
then talk to you.