I am not surprised to learn that Colonel Navez has found a limit to the number of carbon disks, for the reproduction of speech in this system is due both to the greatness of the differences of resistance in the circuit, and to the intensity of the transmitted current. If therefore the instrument’s sensitiveness to articulate sounds is increased by increasing the number of imperfect contacts in the circuit, the intensity of the transmitted sounds is diminished, and thus sounds lose their power. There is consequently a limit to be observed in the number of carbon disks placed upon each other; and it depends on the nature of the imperfect contacts which are employed, and on the tension of the electric generator. In order to stop the unpleasant musical vibrations which accompany telephonic transmissions, Colonel Navez employs for the vibrating plate of the sender a silver-plated copper disk, and for the vibrating plate of the receiver an iron disk lined with brass and soldered together. He also employs caoutchouc tubes with mouthpieces and ear-tubes for the transmission and reception of sound, and these instruments are placed level on a table. For this purpose the magnetised bar of the receiving telephone is replaced by two horizontal magnets, acting through a pole of the same nature on a little iron core which carries the coil, and which is placed vertically between the two magnets. He necessarily makes use of a small Ruhmkorff coil to transform the electricity of the battery into induced electricity. [Illustration: FIG. 30.] [Illustration: FIG. 31.] Figs. 30 and 31 represent the two parts of this telephonic system. The carbon battery is in C (fig. 30), the vibrating disk in L L, and the mouthpiece E, fitted to a caoutchouc tube T E, corresponds at the lower end to the vibrating disk. The carbon battery is placed in metallic contact with the circuit by a platinum rod E C, and the vibrating disk also communicates with the circuit through a binding-screw. In the receiving telephone (fig. 31) the upper part is arranged much as in the ordinary telephones, except that, instead of a mouthpiece, the instrument is fitted with an ear-tube T O. The two horseshoe magnets, A, A, which communicate a uniform polarity to the iron core N, support the induction coil B. The two terminals of this receiver are connected with the supplementary wire of the induction coil, and the two terminals of the sender are connected with the two ends of the primary of this coil, and with the battery which is inserted in the circuit near this instrument. _The Pollard and Garnier Telephones._--The battery telephone made by MM. Pollard and Garnier differs from the foregoing in this particular: it simply employs two points of graphite, mounted in metallic porte-crayons, and these points are directly applied against the vibrating plate with a pressure which must be regulated. Fig. 32 represents the arrangement adopted, which, however, may be infinitely varied.