6. The induction of a vibrating magnet upon an
electric circuit.
7. The origination of electric waves in a circuit
by a sounding magnet.
8. The discovery of the action of the air in a
sounding organ-pipe.
9. Two or three methods for studying the
vibrations of membranes.
10. Lissajous forks for enlarged projections of
sound vibrations.
As soon, therefore, as I gave attention to the subject of telephony, I
was able, with a few preliminary experiments, to determine the proper
conditions for the transmission of speech in an electric circuit; and,
without the slightest knowledge of the mechanism which Prof. Bell had
used, I devised the following arrangement for a speaking-telephone.
[Illustration: FIG. 14.--MY FIRST SPEAKING TELEPHONE.]
[Illustration: FIG. 14.--END VIEW.]
My first speaking-telephone, Fig. 14, consisted of a magnet made out of
half-inch round steel bent into a U form, having the poles about two
inches apart. Over these were slipped two bobbins taken from an old
telegraph register, and were already fitted to a half-inch core. These
bobbins, two inches and a half long, were wound with cotton-covered
copper wire, No. 23, each bobbin containing about 150 feet. This magnet,
with the bobbins slipped upon its poles, was made fast to a post two or
three inches high. The steel was made as strongly magnetic as was
possible, and would hold up three or four times its own weight. In
front of the poles, a sheet of thin steel, one-fiftieth of an inch
thick, was made fast to an upright board having a hole cut through it
three and a half inches in diameter (Fig. 14, end view); the plate was
screwed tightly to this board, so as to cover the hole; and the middle
of the hole was at the same height as the two poles of the magnet. The
wires from the two bobbins were connected, as if to make an
electro-magnet; while the two free terminals were to be connected with
the line-wires. Of course there were two of these instruments, both
alike; and talking and singing were reproduced with these.
A very great number of experiments have been made to determine the best
conditions for each of the essential parts,--the size and strength of
the magnet, the size of the bobbins, as to length and fineness of wire,
the best thickness for the plate for absorbing the vibrations, &c.; and
it is really surprising, how little is the difference between very wide
limits. The following directions will enable any one to construct a
speaking-telephone with which good results may be obtained. The
specifications will be for only one instrument; though of course two
instruments made alike will be necessary for any purposes of speaking or
other signals.
[Illustration: FIG. 15.]
[Illustration: FIG. 16.]
Procure three common horse-shoe magnets about six inches long, all of
the same size; these retail in the market at about a dollar apiece. They
should be strong enough to hold up several times their own weight each.
Next, have turned out of good hard wood,--such as maple or boxwood,--two
spools not over half an inch long and an inch and a half broad, the
sides cut square both inside and out, as shown at S, Fig. 15; a hole
the third of an inch in diameter is to be made through the spool. Into
this hole is to be fitted a short rod of soft iron, I, about an inch
long, which should be a little rounded at the outer end. The bobbins may
be wound with as much insulated copper wire as they will hold. The wire
may be from the one-fortieth to the one-fiftieth of an inch in diameter,
as is most convenient to obtain, the latter size being preferable. The
resistance of such bobbins will probably be from two to three ohms each.
The soft-iron core I must project backwards far enough to be clamped
between the two outer magnets 1 and 3, while the inner one, 2, is drawn
back. When the bobbins are in their places, and are clamped between the
upper and lower magnets, they will stand as shown in Fig. 16, where the
view is from above; the magnets being buttoned down to the block they
rest on (see Fig. 17), which at the same time holds the soft-iron rods
with the bobbins upon them. The wires on these coils must be connected
in the same way they would be in order to make opposite poles of their
outer ends, if a current of electricity were to be sent through the
coils. An upright board B (Fig. 17) six or seven inches square, having a
round hole four inches in diameter cut out from the middle of it, must
be fixed near the end of the base-board; and over this hole is to be
screwed _tightly_ a piece of thin sheet iron or steel; it may be from
the one-twentieth to the one-fiftieth of an inch in thickness. It does
not seem to make much difference about the thickness of this plate. I
have generally got the best results from a plate one-fiftieth of an inch
thick. The upright board carrying this plate must be very rigid,
otherwise the plate will be kept tight to the magnets all the time; and
one of the conditions of success in working is, that this plate shall be
as close as possible to the magnet-ends, but not to touch: therefore fix
the board tight, and adjust the magnets by means of the button shown on
top of them in the perspective figure.
[Illustration: FIG. 17.]
The sounds to be transmitted, of whatever sort they may be, are to be
made on the side P, Fig. 16; and likewise, when the instrument is used
as a receiver, the ear is to be applied at the same place. A tube about
two inches in diameter may be made fast to the front of the board, in a
line with the centre of the plate; this will aid somewhat in hearing.
When two or three persons are to sing, it will be best to have each one
supplied with a tube to sing through; one end of the tube to be placed
close to the front of the plate. The sound of musical instruments, such
as the flute and the cornet, will be reproduced much louder, if the
front of such instrument be allowed to rest upon the rim of the hole in
the board, just in front of the plate.
It is noticeable that low talking can be heard more distinctly than when
a great effort is made; but the sounds though distinct are not strong at
any time, and other sounds seriously interfere with hearing. It is
probable that some way will hereafter be devised for increasing the
usefulness of the invention by increasing the volume of sound. On
account of the weakness of the sound it becomes necessary to provide a
call to attract the attention of one in the room. This may be
accomplished by having a small electric bell worked by a one or two cell
battery. Another way which I have found to be quite as efficient is to
have a rod of iron or steel about a foot long, and half an inch in
diameter, bent into a U form. When this is held by the bend, and struck
upon the floor or with a stick, it vibrates powerfully; and if one of
its prongs be permitted to strike against the plate P, Fig. 16, the
sound will be reproduced loud enough to hear over a large room. I have
never failed to call with this when any one was in the same room with
the telephone.
Wherever a telephone circuit has been made upon telegraph poles having
other wires upon them, the inductive actions of the currents upon the
other wires has been found to seriously interfere with the action of the
telephones, inasmuch as the latter reproduce every other message. One
skilled in reading by sound in the ordinary way can read through the
telephone what message is travelling in a neighboring wire. Messages may
be thus read upon wires as far distant as ten feet from the telephone
circuit. It there fore seems to be essential that each telephone circuit
should be isolated from every other one, else there can be no secrecy in
messages.
A very interesting effect was noticed one night when there was a bright
aurora display. There was a continuous current through the wires,
accompanied with sounds which increased in intensity as the bright
streamers passed by. This will probably lead to some important results
in science.
In all probability the telephone is as much in its infancy as was
ordinary telegraphy in 1840. Since that time the sciences of electricity
and magnetism have had the most of their growth, and telegraphy has kept
pace with the advancing knowledge until its commercial importance is
second to no other agency. Very many important principles that are
invaluable in telegraphy to-day were wholly unknown in 1840; but it may
here be noted that in the telephone, as it now is, there is not a single
principle that was not well enough known in 1840. This will be apparent
to one who follows out the phenomena from the sender to the receiver.
First, the sound in air causing a corresponding movement in a solid
body, iron. This iron, acting inductively upon a magnet, originates
magneto-electric currents in a wire helix about it; and these travel to
another helix, and, re-acting upon the magnet in it, have
electro-magnetic effects, and increase and decrease the strength of the
magnet; and this variable magnetism affects the plate of iron in front
of that magnet, and makes it to vibrate in a corresponding manner, and
thus to restore to the air in one place the vibrations absorbed from the
air in another place. To some it may seem strange that a simple thing as
the telephone is, involving nothing but principles familiar enough to
every one interested in physical science, should have waited nearly
forty years to be invented. The reason is probably this: Men of science,
as a rule, do not feel called upon to apply the principles which they
may discover. They are content to be _discovering_, not _inventing_.
Now, the schools of the country ought to make the youth quite familiar
with the general principles of physical science, that the inventive
ones--and there are many such--may apply them intelligently. Mechanism
is all that stands between us and aerial navigation; all that is
necessary to reproduce human speech in writing; and all that is needed
to realize completely the prophetic picture of the "Graphic," of the
orator who shall at the same instant address an audience in every city
in the world.
* * * * *
Transcriber's Notes:
The musical flat symbol is represented in the text by [flat].
Page 17, "propererties" changed to "properties" (there are other
properties)