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.
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_{2} to ut_{5});
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.
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