_Experiments on the Effects produced by Voltaic and Induced Currents._--The comparative study of the effects produced in the telephone by voltaic and induced currents was one of the first and most important. In 1873, as we have seen, Mr. Elisha Gray converted the voltaic currents, which he employed to cause the vibrations of his transmitting plate, into induced currents by means of an induction coil, such as Ruhmkorff’s. The voltaic currents then traversed the primary helix of the coil, and the induced currents reacted on the receiving instrument, producing on its electro-magnetic system the vibrations excited at the sending station. When Mr. Edison designed his battery telephone, he had recourse to the same means to work his receiving telephone, since he had ascertained that induced currents were superior to voltaic currents. But this peculiarity of Mr. Edison’s arrangement was not clearly understood from the descriptions which reached Europe, so that several persons believed that they had invented this arrangement--among others, Colonel Navez and MM. Pollard and Garnier. Colonel Navez, in an interesting paper on the new telephonic system, presented to the Belgian Royal Academy, February 2, 1878, only suggests this arrangement as a mode of reproducing speech at a great distance; but he quotes no experiment which distinctly shows the advantages of this combination. Twenty days later, MM. Pollard and Garnier, unacquainted with Colonel Navez’s researches, sent to me the results they had obtained by similar means, and these results appeared to me so interesting that I communicated them to the Académie des Sciences, February 25, 1878. In order that the importance of these results may be clearly understood, I will repeat the text of M. Pollard’s letter, addressed to me on February 20, 1878: ‘With the object of increasing the variations of electric intensity in the Edison system, we induce a current in the circuit of a small Ruhmkorff coil, and we fix the receiving telephone to the extremities of the induced wire. The current received has the same intensity as that of the inducing current, and consequently the variations produced in the current which works the telephone have a much wider range. The intensity of the transmitted sounds is strongly increased, and the value of this increase depends upon the relative number of spirals in the inducing and induced circuits. Our attempts to determine the best proportions have been laborious, since it is necessary to make a coil for each experiment; we have hitherto obtained excellent results with a small Ruhmkorff coil reduced to its simplest form, that is, without condenser or contact-breaker. The inducing wire is No. 16, and is wound in five layers; the induced wire is No. 32, and in twenty layers. The length of the coil is seven centimètres.