In this figure a section of the instrument is given, and its form greatly resembles that of Bell. L L is the vibrating disk; O′ O, the mouthpiece; M, the opening to the mouthpiece; N N N, the case for the instrument, which is, like the mouthpiece, made of ebonite, and below the disk it presents a rather large cavity, and a tubular hole which is scooped in the handle. In its upper part this tube terminates in a cylindrical rim, furnished with a worm on which is screwed a little rod with a ridge on its inner side, and the rheostatic system is placed within this tube. The system consists, first, of a piston E, fitted to the end of a long screw E F, and the turning of the button will move the piston up or down within a certain limit. Above this piston there is fitted a very thin platinum plate A, connected by a flexible chain and a wire with a binding-screw P′. Another plate B, exactly similar, is connected with the binding-screw P, and the carbon disk C is placed between these two plates. This disk is composed of compressed lamp-black and petroleum, and its resistance is one _ohm_, or 110 yards, of telegraphic wire. Finally, an ebonite disk is fastened to the upper platinum plate, and an elastic pad, composed of a piece of caoutchouc tube G, and of a cork disk H, is interposed between the vibrating plate L L and the disk B, in order that the vibrations of the plate may not be checked by the rigid obstacle formed by the whole rheostatic system. When these different parts are in position, the instrument is regulated by the screw F, and this is easily done by screwing or unscrewing it until the receiving telephone gives out its maximum of sound. [Illustration: FIG. 28.] In another model, represented in fig. 28, which has produced the best results in the distinctness with which sounds are transmitted, the vibrating plate L L is supported on the disks of the secondary carbon conductor C by means of a little iron cylinder A, instead of the caoutchouc pad, and the pressure is regulated by a screw placed below _e_. The mouthpiece E of the instrument is more prominent, and its opening is larger. Finally, the instrument, which is cased in nickel silver, is without a handle. The rigid disk _b_, resting on the first platinum plate _p_, is of aluminium instead of ebonite. [Illustration: FIG. 29.]