Messrs. Thomson & Bottomley have recently invented a peculiar circuit cutter based upon the use of a metal whose melting point is exceedingly low. Recourse is had to this process for breaking the current within as short a time as possible. In this new device the ends of the conductors are soldered together with the metal in question at one or several points of the circuit. The metal employed is silver or copper of very great conductivity, seeing that the increase of temperature in a conductor, due to a sudden increase of the current, is inversely proportional to the product of the electric resistance by the specific heat of the conductor; that these metals are best adapted for giving constant and definite results; and that the contacts are better than with lead or the other metals of low melting point which are frequently employed in circuit cutters.
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.
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.