[Illustration: FIG. 2.--TRANSMITTER TAKEN APART.]

The transmitter is 3½ in. in diameter, and is provided with a
re-enforcing mouthpiece. It is regulated by means of a screw which is
fixed in the bottom of the box, and which permits of varying the
distance between the disk and the core that forms the central pole of
the magnet. The regulation, when once effected, lasts indefinitely. The
regulation of the receiver, which is but 2¼ in. in diameter, is
performed once for all by the manufacturer. One of the advantages of
this telephone is that its regulation is permanent. Besides this, it
possesses remarkable power and clearness, and is accompanied with no
snuffling sounds, a fact doubtless owing to all the molecules of the
disk being immersed in the magnetic field, and to the actions of the two
poles occurring concentrically with the disk. As we have above said,
this apparatus is beginning to be appreciated, and has already been the
object of several applications in the army. The transmitter is used by
the artillery service in the organization of observatories from which to
watch firing, and the receiver is added to the apparatus pertaining to
military telegraphy. The two small receivers are held to the lens of the
operator by the latter's hat strap, while the transmitter is suspended
in a case supported by straps, with the mouthpieces near the face (Fig.
1).

In the figure, the case is represented as open, so as to show the
transmitter. The empty compartment below is designed for the reception
and carriage of the receivers, straps, and flexible cords. This
arrangement permits of calling without the aid of special apparatus, and
it has also the advantage of giving entire freedom to the man on
observation, this being something that is indispensable in a large
number of cases.