We thus see that the advantage of the iron diaphragm over filings is truly reduced to the presentation of a much larger number of magnetic molecules to the action of the field and to external actions, within the same volume. It increases the _intensity_ of the telephonic effects, although for _the production_ of the latter with all their variety, fineness, and perfection it is nowise indispensable. It suffices, after a manner, to materialize the lines of force with iron filings, and to act mechanically upon them, and consequently upon the field itself. * * * * * ON THE THEORY OF THE RECEIVER OF THE ELECTRO-MAGNETIC TELEPHONE. By E. MERCADIER. [Footnote: Note presented to the Academy of Sciences, November 16, 1885.] On a former occasion I described some experiments that had led me to a theory of the telephone transmitter; a few words will suffice to expose that of the receiver. Such theory gave rise during the first years succeeding the invention of the telephone to a considerable number of investigations, the principal results of which may be summed up in the two following points: 1. All the parts of a telephone receiver--core, helix, disk, handle, etc.--vibrate simultaneously (Boudet, Laborde, Breguet, Ader, Du Moncel, and others). But there is no doubt that by far the most energetic effects are those of the disk. It has been possible to put the vibrations of the core and helix beyond a doubt only by employing very energetic transmitter currents, or very simplified and special arrangements of the receiver (Ader, Du Moncel, and others). 2. In telephone receivers we may employ disks or diaphragms of any thickness up to six inches (Bell, Breguet, and others). From the first point it had already resulted that the diaphragm was no more indispensable in the receiver than it was in the transmitter, as I have already shown (_Comptes Rendus_, t. ci., p. 944); and, from the second point, that there were other effects in a receiver than those that could result from the transverse vibrations corresponding to the fundamental sound and to the harmonics of the diaphragm. So Du Moncel, basing a theory upon these two categories of facts, asserted that the effects of the telephone receiver were principally due to the molecular vibrations of the core of the electro-magnet (analogous to those that had been studied by Page, De la Rive, Wetheim, Reis, and others), super-excited and re-enforced by the iron diaphragm operating as an armature. This theory has certainly truth for a basis; but it is incomplete, in that the molecular vibrations of the core are but a very feeble accessory phenomenon, and not a prominent one. At all events, I believe that we can, in a few words, and very simply, present the theory of the telephone receiver by going back to the facts that served me as a basis for the theory of the transmitter, and that result from studies made with telephones of ordinary forms.