mount thereon a light disk of vulcanite, wood, ivory, gutta-percha, or any other substance which it is capable of vibrating. When using this telephone receiver, the disk is pressed to the ear in such a manner that its surface covers the aperture of the ear. When these telephone receivers are used on a line of some considerable length, the patentees prefer to magnetize the electro-magnet by a constant current from a local battery, and to effect the variation of this constant magnetization inductively and not directly. The electro-magnet is, then, not inserted in the line at all, but in the primary circuit of an induction coil, and connected with a local battery. The line is connected to the secondry circuit of the induction coil. This device possesses the advantage that the electro-magnet can be powerfully magnetized with very little battery power, no matter how long the line may be, and that steel magnets are entirely dispensed with. It is not necessary to have a separate battery for this purpose, as the microphone battery may also be used for the telephone receiver. The shape of the vibrating electro-magnets is immaterial, as they may be made of a variety of forms.--_Eng. Mechanic_. [Illustration: FIG. 1. FIG. 2] * * * * * ON AN ELECTRIC POWER HAMMER. By MARCEL DEPREZ. [Footnote: _La Lumiére Electrique_.] In a lecture delivered by me on the 15th of last June in the amphitheater of the Conservatoire des Arts et Metiers, on the application of electricity to the production, transmission, and division of power, I operated for the first time an electric power hammer that I shall here describe. Its essential part is a sectional solenoid that I have likewise made an application of in an electric motor which I presented in July, 1830, to the Societé de Physique. Let us suppose we superpose, one on the other, a hundred flat bobbins of a centimeter in thickness in such a way as to form a single solenoid one meter in height, and that the incoming and outgoing wires of each of them be connected with the contiguous bobbins exactly in the same way as they are in the consecutive sections or a dynamo-electric machine ring. Finally, let us complete the resemblance by causing each junction of the wire of one of the bobbins with the wire of its neighbor to end in a metallic plate set into an insulating piece containing as many plates as there are bobbins, plus one. Over this species of collector, which maybe rectilinear or wound around a cylinder, let us pass two brushes fixed to an insulating piece that may be moved by hand. Now, if we place these two brushes at a distance such that the number of the plates of the collector included between them be, for example, equal to ten, and we give them any degree of displacement whatever, after rendering them interdependent, the current entering through one of these brushes and making its exit through the other will always traverse 10 bobbins.