_Telephones with a battery and voltaic arcs._--In order to obtain variations of resistance of still greater sensitiveness than is the case with liquids or pulverised substances, the idea has been suggested of employing conductors of heated gas, and several arrangements of battery telephones have been made in which the circuit was completed by a stratum of air, separating the vibrating disk from a platinum point, which serves to excite an electric discharge of high tension. Under these conditions, the stratum of air becomes the conductor, and the intensity of the current which traverses it corresponds to its thickness. This problem has been solved, either by means of voltaic currents of high tension, or by a Ruhmkorff coil. The former system was arranged by M. Trouvé, and he writes as follows on the subject in the journal ‘La Nature’ of April 6, 1878: ‘A metallic vibrating membrane forms one of the poles of a high tension battery; the other pole is fastened before the disk by a micrometer screw which can be adjusted so as to vary the distance from the disk according to the tension of the battery, but without ever coming in contact with it. The distance must not in any case exceed that to which the discharge of the battery can extend. Under these conditions, the membrane which vibrates under the influence of the waves of sound has the effect of constantly modifying the distance between the two poles, and thus of continually varying the intensity of the current: consequently the receiving instrument (a Bell telephone, or telephone with an electro-magnet) is subjected to magnetic variations, corresponding to the variations of the current which affect it, and this has the effect of making the receiving instrument vibrate at the same moment. This kind of telephonic instrument relies, therefore, on the possibility of varying within wide limits the resistance of the outer circuit of a high-tension battery, in which the poles are not in contact. In order to vary the conditions of this resistance, it is also possible to interpose some vapour or other medium, such as air, or gas of greater or less rarity.’ M. Trouvé thinks that he was successful with his battery of small disks, moistened with sulphate of copper and sulphate of zinc, arranging these elements, to the number of five or six hundred, in glass tubes of small diameter. It is well known that it is unnecessary for the elements to be of large size in order to obtain tension currents.