[Illustration: FIG. 1.]
Fig. 1 represents one form of the new device. Here, a is the copper or silver wire, and b is a soldering made with a very fusible metal and securing a continuity of the circuit. Each extremity of the wire, a, is connected with a heavy ring, c, of copper or other good conducting metal. The hook, d, with which the upper ring, c, is in contact, communicates metallically with one of the extremities of the conductor at the place where the latter is interrupted for the insertion of the circuit cutter. The hook, e, with which the lower ring, c, is in contact, tends constantly to descend under the action of a spiral spring, f, which is connected metallically with the other extremity of the principal conductor. The hooks, d and e, are arranged approximately in the same vertical plane, and have a slightly rounded upper and lower surface, designed to prevent the rings, c, of the fusible wire, a, from escaping from the hooks. In Fig. 1 the position of the arm, e, when there is no fusible wire in circuit, is shown by dotted lines. When this arm occupies the position shown by entire lines, it exerts a certain traction upon the soldering, b, and separates the two halves of the wire, a, as soon as the intensity of circulation exceeds its normal value. The mode of putting the wire with fusible soldering into circuit is clearly shown in the engraving.
[Illustration: FIG. 2.]
Fig. 2 shows a different mode of mounting the wire. The wire, q, is soldered in the center, and is bent into the shape of a U, and kept in place by the pieces, r and s. In this way the two ends of it tend constantly to separate from each other. Messrs. Thomson & Bottomley likewise employ weights, simply, for submitting the wire to a constant stress. The apparatus is inclosed in a box provided with a glass cover.—La Lumiere Electrique.
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Despite the simplicity of their parts, and the slight value of the materials employed, the existing micro-telephonic apparatus keep at relatively high prices, and the use of them is often rejected, to the benefit of speaking tubes, when the distance between stations is not too great. We propose to describe a new style of apparatus that are in no wise inferior to those in general use, and the price of which is relatively low.
The microphone transmitter may have several forms. The most elementary of these consists of two pieces of carbon, from one to one and a quarter inches in length by one-half inch in width, between which are fixed two nails, about two inches in length, whose extremities, filed to a point, enter small conical apertures in the carbons. Fig. 1 gives an idea of the arrangement.