By E. MERCADIER.
[Footnote: Note presented to the Academy of Sciences, Oct. 19, 1885.]
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_{5}_, 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_{5}_, 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.
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