A membrane of gold-beater's skin was tightly stretched over the end of a
speaking-tube or funnel; on the middle of this membrane a piece of iron,
N S, Fig. 13, was glued. In front of this piece of iron an
electro-magnet M is so situated that its poles are opposite to it, but
not quite touching it. One of the terminal wires of the electro-magnet
goes to the battery B; the other goes to the receiving instrument R,
which consists of a tubular electro-magnet, the coil being enclosed in a
short tube of soft iron; the wire thence goes to the plate E', which is
sunk in the earth. On the top of R, at P, is a rather loose, thin disk
of iron, which acts as an armature to the electro-magnet below it.
[Illustration: FIG. 13.]
Supposing that all the parts are thus properly connected, the current of
electricity from the battery makes both M and R magnetic; the
electro-magnet M will inductively make the piece of iron N S, a magnet,
with its poles unlike those of the inducing electro-magnet; and the two
will mutually attract each other. If now this piece of iron N S be made
to move toward M, a current of electricity will be induced in the coils,
which will traverse the whole circuit. This induced electricity will
consist of a single wave or pulse, and its force will depend upon the
velocity of the approach of N S to M. A like pulse of electricity will
be induced in the coils when N S is made to move away from M; but this
current will move through the circuit in the opposite direction, so that
whether the pulsation goes from M to R, or from R to M, depends simply
upon the direction of the motion of N S.
The electricity thus generated in the wire by such vibratory movements
varies in strength proportional to the movement of the armature;
therefore the line wire between two places will be filled with
electrical pulsation exactly like the aerial pulsations in structure.
Fig. 10, p. 98, may be used to illustrate the condition of the wire
through which the currents pass. The dark part may represent the
strongest part of the wave, while the lighter part would show the
weaker part of the wave. The chief difference would be, that electricity
travels so fast, that what is there represented as one wave in air with
a length of two feet would, in an electric wave, be more than fifty
miles long.
These induced electric currents are but very transient (see p. 31); and
their effect upon the receiver R is to either increase or decrease the
power of the magnet there, as they are in one direction or the other,
and consequently to vary the attractive power exercised upon the iron
plate armature.
Let a simple sound be now made in the tube, consisting of 256 vibrations
per second: the membrane carrying the iron will vibrate as many times,
and so many pulses of induced electricity will be _imposed_ upon the
constant current, which will each act upon the receiver, and cause so
many vibrations of the armature upon it; and an ear held at P will hear
the sound with the same pitch as that at the sending instrument. If two
or more sound-waves act simultaneously upon the membrane, its motions
must correspond with such combined motions; that is, its motions will
be the resultant of all the sound-waves, and the corresponding
pulsations in the current must reproduce at R the same effect. Now, when
a person speaks in the tube, the membrane is thrown into vibrations more
complex in structure than those just mentioned, differing only in number
and intensity. The magnet will cause responses from even the minutest
motion; and therefore an ear at R will hear what is said at the tube.
This was the instrument exhibited at the Centennial Exposition at
Philadelphia, and concerning which Sir William Thompson said on his
return to England, "This is the greatest by far of all the marvels of
the electric telegraph."
The popular impression has been, concerning the telephone, that the
_sound_ was in some way conveyed over the wire. It will be obvious to
every one who may read this, that such is very far from being the case.
The fact is, it is a beautiful example of the convertibility of forces
from one form to another. There is first the initial vibratory
mechanical motion of the air, which is imparted to the membrane carrying
the iron. This motion is converted into electricity in the coil of wire
surrounding the electro-magnet, and at the receiving-end is first
effective as magnetism, which is again converted into vibratory motion
of the iron armature, which motion is imparted to the air, and so
becomes again a sound-wave in air like the original one.
This was the first speaking-telephone that was ever constructed, so far
as the writer is aware, but it was not a practicable instrument. Many
sounds were not reproduced at all, and, according to the report of the
judges at the Philadelphia Exposition, one needed to shout himself
hoarse in order that he might be heard at all.