This apparatus has recently been the object of some experiments which resulted in its being finally adopted in the army. We think that our readers will read a description of it with interest. Its mode of construction is based upon a theoretic conception of the lines of force, which its inventor explains as follows in his Elementary Treatise on Electricity:

"To every position of the disk of a magnetic telephone with respect to the poles of the magnet there corresponds a certain distribution of the lines of force, which latter shift themselves when the disk is vibrating. If the bobbin be met by these lines in motion, there will 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."



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.



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.



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.

Stations wholly magnetic may be established by adding to the transmitter and two receivers a Sieur phonic call, which will actuate them powerfully, and cause them to produce a noise loud enough for a call. It would be interesting to try this telephone on a city line, and to a great distance on those telegraph lines that are provided with the Van Rysselberghe system. Excellent results would certainly be obtained, for, as we have recently been enabled to ascertain, the voice has a remarkable intensity in this telephone, while at the same time perfectly preserving its quality.--La Nature.