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.

In fact, it is enough to remark that the iron filings telephone transmitter described in a preceding article (1. c.) is reversible and capable of serving as a receiver - not a very intense one, it is true, but here it is a question of the nature of the phenomena, and not of their intensity. It at once results that in receivers, as in transmitters, the rigidity of the iron diaphragm is in nowise indispensable for telephonic effects, such as the production of continuous series of successive or simultaneous sounds and of articulate speech.

The diaphragm serves but to increase the intensity of these effects, as in the transmitter, by concentrating the lines of force of the field, and by presenting a greater surface to the air - the necessary vehicle of sound. When it is thick, the internal motions that it takes on in consequence of variations in the field, and which are transmitted to the surrounding air and the ear, are solely those of resonance. When it is very thin, the peculiar motions resulting from its geometric form and its structure may become superposed upon the preceding, because it may then happen that the corresponding sounds remain within the limits of the pitch wherein the human voice usually moves (from ut to ut); but then, also, as the harmonics of the voice in nowise coincide with the proper sounds of the diaphragm, the intensity of the effects is obtained at the expense of a good reproduction of the timbre. This is certainly one of the causes of the nasal timbre of most thin-diaphragmed telephones. By diminishing their thickness, we lose in quality what we gain in intensity.

But even in this latter respect there is a maximum for receivers, as I have already pointed out that there is for iron filings transmitters. For a magnetic field of given intensity, there is, all things equal, a diaphragm thickness that gives a maximum telephonic result. Such result, which is analogous to those that occur in other electro-magnetic phenomena, may explain the want of success of many tentatives made somewhat at haphazard, with a view to increasing the intensity of telephonic effects.

[2]Note presented to the Academy of Sciences, November 16, 1885.