bodies of any form, a wall for instance, transmit to one of their surfaces
all the vibratory movements of any kind which are produced in the air in
contact with the other surface. It is a phenomenon or resonance. Movements
corresponding to particular sounds may be superposed in slender diaphragms,
but this superposition must necessarily be disturbing under all but
exceptional circumstances. In proof of this view, it is cited that
diaphragms much too rigid, or charged with irregularly distributed masses
over the surface, or pierced with holes, or otherwise evidently unfitted
for the purpose, are available for transmission. They will likewise serve
when feathers, wool, wood, metals, mica, and other substances to the
thickness of four inches are placed between the diaphragm and the source of
vibratory movement. The magnetic field does not alter these relations in
any way. The real diaphragm may be removed altogether. It is sufficient to
replace it by a few grains of iron filings thrown on the pole covered with
a piece of pasteboard or paper. Such a telephone works distinctly although
feebly; but any slender flexible disk, metallic or not, spread over across
the opening of the cover of the instrument, with one or two tenths of a
gramme (three grains) of iron filings, will yield results of increased and
even ordinary intensity. This is the iron filing telephone, which is
reversible; for a given magnetic field there is a certain weight of iron
filings for maximum intensity. It appears thus that the advantage of the
iron diaphragm over iron filings reduces itself to presenting in a certain
volume a much more considerable number of magnetic molecules to the action
of the field. The iron diaphragm increases the telephonic intensity, but it
is by no means indispensable.

* * * * *