develop in its wire a difference of potential that, according to
Faraday's law, will be proportional to their number. All things equal,
then, a telephone transmitter will be so much the more potent in
proportion as the lines set in motion by the vibrations of the disk and
meeting the bobbin wire are greater in number. In like manner, a
receiver will be so much the more potent in proportion as the lines of
force, set in motion by variations in the induced currents that are
traversing the bobbin and meeting the disk, are more numerous. It will
consequently be seen that, generally speaking, it is well to send as
large a number of lines of force as possible through the bobbin."

[Illustration: FIG. 1.--THE COLSON TELEPHONE.]

In order to obtain such a result, the thin tin-plate disk has to be
placed between the two poles of the magnet. The pole that carries the
fine wire bobbin acts at one side and in the center of the disk, while
the other is expanded at the extremity and acts upon the edge and the
other side. This pole is separated from the disk by a copper washer, and
the disk is thus wholly immersed in the magnetic field, and is traversed
by the lines of force radiatingly.

This telephone is being constructed by Mr. De Branville, with the
greatest care, in the form of a transmitter (Fig. 2) and receiver (Fig.
3). At A may be seen the magnet with its central pole, P, and its
eccentric one, P'. This latter traverses the vibrating disk, M, through
a rubber-lined aperture and connects with the soft iron ring, F, that
forms the polar expansion. These pieces are inclosed in a nickelized
copper box provided with a screw cap, C. The resistance of both the
receiver and transmitter bobbin is 200 ohms.