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ON April 27, 1877, Edison filed in
the United States Patent Office an
application for a patent on a telephone, and on
May 3, 1892, more than fifteen years
afterward, Patent No. 474,230 was
granted thereon. Numerous other patents have
been issued to him for improvements in
telephones, but the one above specified may be
considered as the most important of them, since
it is the one that first discloses the principle
of the carbon transmitter.
This patent embodies but two claims, which are
as follows:
"1. In a speaking-telegraph transmitter,
the combination of a metallic diaphragm and disk
of plumbago or equivalent material, the
contiguous faces of said disk and diaphragm being
in contact, substantially as described.
"2. As a means for effecting a varying
surface contact in the circuit of a
speaking-telegraph transmitter, the combination
of two electrodes, one of plumbago or similar
material, and both having broad surfaces in
vibratory contact with each other, substantially
as described."
The advance that was brought about by Edison's
carbon transmitter will be more apparent if we
glance first at the state of the art of telephony
prior to his invention.
Bell was undoubtedly the first inventor of the
art of transmitting speech over an electric
circuit, but, with his particular form of
telephone, the field was circumscribed.
Bell's telephone is shown in the diagrammatic
sectional sketch (Fig. 1).
In the drawing M is a bar magnet contained in
the rubber case, L. A bobbin, or coil of
wire, B, surrounds one end of the magnet. A
diaphragm of soft iron is shown at D, and E is
the mouthpiece. The wire terminals of the
coil, B, connect with the binding screws, C
C.
The next illustration shows a pair of such
telephones connected for use, the working parts
only being designated by the above reference
letters.
It will be noted that the wire terminals are
here put to their proper uses, two being joined
together to form a line of communication, and
the other two being respectively connected to
"ground."
Now, if we imagine a person at each one of the
instruments (Fig. 2) we shall find that when
one of them speaks the sound vibrations impinge
upon the diaphragm and cause it to act as a
vibrating armature. By reason of its
vibrations, this diaphragm induces very weak
electric impulses in the magnetic coil. These
impulses, according to Bell's theory,
correspond in form to the sound-waves, and,
passing over the line, energize the magnet coil
at the receiving end, thus giving rise to
corresponding variations in magnetism by reason
of which the receiving diaphragm is similarly
vibrated so as to reproduce the sounds. A
single apparatus at each end is therefore
sufficient, performing the double function of
transmitter and receiver. It will be noticed
that in this arrangement no battery is used The
strength of the impulses transmitted is therefore
limited to that of the necessarily weak induction
currents generated by the original sounds minus
any loss arising by reason of resistance in the
line.
Edison's carbon transmitter overcame this vital
or limiting weakness by providing for independent
power on the transmission circuit, and by
introducing the principle of varying the
resistance of that circuit with changes in the
pressure. With Edison's telephone there is
used a closed circuit on which a battery current
constantly flows, and in that circuit is a pair
of electrodes, one or both of which is carbon.
These electrodes are always in contact with a
certain initial pressure, so that current will
be always flowing over the circuit. One of the
electrodes is connected with the diaphragm on
which the sound-waves impinge, and the
vibrations of this diaphragm cause corresponding
variations in pressure between the electrodes,
and thereby effect similar variations in the
current which is passing over the line to the
receiving end. This current, flowing around
the receiving magnet, causes corresponding
impulses therein, which, acting upon its
diaphragm, effect a reproduction of the original
vibrations and hence of the original sounds.
In other words, the essential difference is
that with Bell's telephone the sound-waves
themselves generate the electric impulses, which
are therefore extremely faint. With Edison's
telephone the sound-waves simply actuate an
electric valve, so to speak, and permit
variations in a current of any desired strength.
A second distinction between the two telephones
is this: With the Bell apparatus the very weak
electric impulses generated by the vibration of
the transmitting diaphragm pass over the entire
line to the receiving end, and, in
consequence, the possible length of line is
limited to a few miles, even under ideal
conditions. With Edison's telephone the
battery current does not flow on the main line,
but passes through the primary circuit of an
induction-coil, from the secondary of which
corresponding impulses of enormously higher
potential are sent out on the main line to the
receiving end. In consequence, the line may be
hundreds of miles in length. No modern
telephone system is in use to-day that does not
use these characteristic features: the varying
resistance and the induction-coil. The system
inaugurated by Edison is shown by the diagram
(Fig. 3), in which the car- bon
transmitter, the induction-coil, the line,
and the distant receiver are respectively
indicated.
In Fig. 4 an early form of the Edison carbon
transmitter is represented in sectional view.
The carbon disk is represented by the black
portion, E, near the diaphragm, A, placed
between two platinum plates D and G, which are
connected in the battery circuit, as shown by
the lines. A small piece of rubber tubing,
B, is attached to the centre of the metallic
diaphragm, and presses lightly against an ivory
piece, F, which is placed directly over one of
the platinum plates. Whenever, therefore, any
motion is given to the diaphragm, it is
immediately followed by a corresponding pressure
upon the carbon, and by a change of resistance
in the latter, as described above.
It is interesting to note the position which
Edison occupies in the telephone art from a
legal standpoint. To this end the reader's
attention is called to a few extracts from a
decision of Judge Brown in two suits brought in
the United States Circuit Court, District
of Massachusetts, by the American Bell
Telephone Company against the National
Telephone Manufacturing Company, et al.,
and Century Telephone Company, et al.,
reported in Federal Reporter, 109, page
976, et seq. These suits were brought on
the Berliner patent, which, it was claimed,
covered broadly the electrical transmission of
speech by variations of pressure between opposing
electrodes in constant contact. The Berliner
patent was declared invalid, and in the course
of a long and exhaustive opinion, in which the
state of art and the work of Bell, Edison,
Berliner, and others was fully discussed, the
learned Judge made the following remarks:
"The carbon electrode was the invention of
Edison.... Edison preceded Berliner in the
transmission of speech.... The carbon
transmitter was an experimental invention of a
very high order of merit.... Edison, by
countless experiments, succeeded in advancing
the art. . . . That Edison did produce
speech with solid electrodes before Berliner is
clearly proven.... The use of carbon in a
transmitter is, beyond controversy, the
invention of Edison. Edison was the first to
make apparatus in which carbon was used as one of
the electrodes.... The carbon transmitter
displaced Bell's magnetic transmitter, and,
under several forms of construction, remains the
only commercial instrument.... The advance
in the art was due to the carbon electrode of
Edison.... It is conceded that the Edison
transmitter as apparatus is a very important
invention.... An immense amount of
painstaking and highly ingenious experiment
preceded Edison's successful result. The
discovery of the availability of carbon was
unquestionably invention, and it resulted in the
`first practical success in the art.' "
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