[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.

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

The transmitter is 31/2 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 21/4 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.

[Illustration:  FIG. 3.—­RECEIVER TAKEN APART.]

In certain applications, of course, the receivers may be combined with a microphone; yet on an aerial as well as on a subterranean line the transmitter produces effects which, as regards intensity and clearness, are comparable with those of a pile transmitter.