The first point to be studied in this theory is the role performed by the iron or steel diaphragm of the telephone, both as regards the nature of the movements that it effects through elasticity and the conversion of mechanical into magnetic energy as a result of its motions.

I. When we produce simple or complex vibratory motions in the air in front of the diaphragm, like those that result from articulate speech, either the fundamental and harmonic sounds of the diaphragm are not produced, or else they play but a secondary role.

(1.) In fact, diaphragms are never set in vibration, as is supposed, when we desire to determine the series of harmonics and nodal lines, since we do not leave them to themselves until they have been set in motion, and we do not allow a free play to the action of elastic forces; in a word, the vibrations that they are capable of effecting are constantly forced ones.

(2.) When a disk is set into a groove, and its edges are fixed, theory indicates that the first harmonics of the free disk should only rise a little. Let us take steel disks 4 inches in diameter and but 0.08 inch in thickness, and of which the fundamental sound in a free state is about ut, and which the setting only further increases. It is impossible to see how this fundamental and the harmonics can be set in play when a continuous series of sounds or accords below ut, are produced before the disk; and yet these sounds are produced perfectly (with feeble intensity, it is true, in an ordinary telephone) with their pitch and quality. They produce, then, in the transmitting diaphragm other motions than those of the fundamental sound and of its peculiar harmonics.

(3.) It is true that in practice the edges of the telephone diaphragm are in nowise fixed, but merely set into a groove, or rather clamped between wooden or metallic rings, whose mass is comparable to their own; and they are, therefore, as regards elasticity, in an ill ascertained state. Yet a diaphragm of the usual diameter (from 2 to 4 inches), and very thin (from 0.001 to 0.02 inch), clamped in this way by its edges, is capable of vibrating when a continuous series of sounds are produced near it, by means, for example, of a series of organ pipes. But the series of sounds that it clearly re-enforces, in exhibiting a kind of complex nodal lines, is plainly discontinuous; and how, therefore, would the existence of such series suffice to explain the production of a continuous scale of isolated or superposed sounds, the chief property of the telephone?

(4.) The interposition of a plate of any substance whatever between the diaphragm and the source of the vibratory motions in nowise alters the telephonic qualities of the diaphragm, and consequently the nature of the motions that it effects - a fact that would be very astonishing if the motions were those that corresponded to the peculiar sounds of the diaphragm. This fact is already known, and I have verified it with mica, glass, zinc, copper, cork, wood, paper, cotton, a feather, soft wax, sand, and water, even in taking thicknesses of from 5 to 8 inches of these substances.

(5.) We can put a diaphragm manifestly out of condition to effect its peculiar scale of harmonics by placing small, unequal, and irregularly distributed bodies upon its surface, by cutting it out in the form of a wheel, and by punching a sufficient number of holes in it to reduce it half in bulk. None of these modifications removes its telephonic qualities.

(6.) We can go still further, and employ diaphragms of scarcely any stiffness and elasticity without altering their essential telephonic properties, the reproduction of a continuous series of sounds, accords, and timbres. Such is the case with a sheet iron diaphragm. It is very difficult, then, to imagine a fundamental sound and its harmonics.

The conclusion from all this appears to me to be that the mechanism by virtue of which telephone diaphragms perform their motions is at least analogous to, if not identical with, that through which solid bodies of any form whatever (a wall, for example) transmit to all of their surfaces all the simple or complex successive or simultaneous vibratory motions, of periods varying in a continuous or discontinuous manner, that are produced in the air in contact with the other surface. In a word, we have here a phenomenon of resonance. In diaphragms of sufficient thickness this kind of motion would exist alone. In thin diaphragms the motions that correspond to their special sounds might become superposed upon the preceding, and this would be prejudicial rather than useful, since, in such a case, if there resulted a re-enforcement of the effects produced, it would be at the expense of the reproduction of the timbre, the harmonics of the diaphragm being capable of coinciding only through the merest accident with those of the sounds that were setting in play the fundamental sound of the diaphragm.

This is what experiment clearly demonstrates.

II. Let us now pass to the magnetic role of the telephone diaphragm. Such role can be clearly enough defined by the following facts:

(1.) The presence of the magnetic field of the telephone in nowise changes the preceding conclusions.

(2.) Upon farther and farther diminishing the stiffness and elasticity of the diaphragm, I have succeeded in suppressing it entirely. In fact, it is only necessary to substitute for it, in any telephone whatever, a few grains of iron filings, thrown upon the pole of the magnet, covered with a bit of paper or cardboard, in order to render it possible to reproduce all sounds, and articulate speech with its characteristic quality, although, it is true, with very feeble intensity.

(3.) In order to increase the intensity of the effect produced, it suffices to substitute for the iron diaphragm a thin disk of any sort of slightly flexible substance, metallic or otherwise, cardboard, for example, and through the aperture of the usual cover of the instrument to scatter over it from 1½ to 3 grains of iron filings. In this way we obtain an iron filings telephone. By properly increasing the intensity of the magnetic field, I have been able to form telephones of this kind that produced in an ordinary receiver as intense effects as those given by the usual transmitters with stiff disks, and which, too, were reversible. But for a field of given intensity, there is a weight of iron filings that produces a maximum of effect.

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.

[1]Note presented to the Academy of Sciences, Oct. 19, 1885.