thousand people.

The loud-speaking telephone is shown in section, diagrammatically,
in the sketch (Fig. 3), in which A is the chalk cylinder mounted on
a shaft, B. The palladium-faced pen or spring, C, is connected to
diaphragm D. The instrument in its commercial form is shown in Fig. 4.

VI. THE TELEPHONE

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