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<h2> VI. THE TELEPHONE </h2>
<p>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.</p>
<p>This patent embodies but two claims, which are as follows:</p>
<p>"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.</p>
<p>"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."</p>
<p>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.</p>
<p>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).</p>
<p>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.</p>
<p>The next illustration shows a pair of such telephones connected for use,
the working parts only being designated by the above reference letters.</p>
<p>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."</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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 carbon transmitter, the induction-coil, the line,
and the distant receiver are respectively indicated.</p>
<p>In Fig. 4 an early form of the Edison carbon transmitter is represented in
sectional view.</p>
<p>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.</p>
<p>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.'"</p>
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