ones. 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