The ease with which membranes are thrown into vibrations corresponding
in period to that of the sounding body has already been alluded to on p.
80; and several attempts have been made, at different times, to make
membranes available in telephony. The first of these attempts was made
by Philip Reiss of Friedrichsdorf, Germany, in 1861.
His apparatus consisted of a hollow box, with two apertures: one in
front, in which was inserted a short tube for producing the sound in,
and indicated by the arrow in the cut, Fig. 12; the other on the top.
This was covered with the membrane _m_,--a piece of bladder stretched
tight over it. Upon the middle of the membrane, a thin piece of platinum
was glued; and this piece of platinum was connected by a wire to a
screw-cup from which another wire went to a battery.
[Illustration: FIG. 12.]
A platinum finger, S, rested upon the strip of platinum, but was made
fast at one end to the screw-cup that connected with the other wire from
the battery. Now, when a sound is made in the box, the membrane is made
to vibrate powerfully: this makes the platinum strip to strike as often
upon the platinum finger, and as often to bound away from it, thus
making and breaking the current the same number of times per second. If,
then, a person sings into this box while it is in circuit with the
afore-mentioned click-rod and box, the latter will evidently change its
pitch as often as it is changed by the voice. In this apparatus we have
a telephone with which a melody may be reproduced at a distance with
distinctness. But the sounds are not loud, and they have a tin-trumpet
quality. If one reflects upon the possibilities of such a mechanism, and
upon the conditions necessary to produce a sound of any given quality,
as that of the voice or of a musical instrument as described in
preceding pages, he will understand that it can reproduce only pitch. It
might here be inferred that something more than a single pitch is
transmitted if the sound is like that of a tin trumpet as stated: but
the reason of this is that, whenever a current is passing between two
surfaces that can move only slightly on each other, there is always an
irregularity in the conduction, so as to produce a kind of scratching
sound; and it is this, combined with the other, the true pitch, that
gives the character to the sound of this instrument.
Dr. Wright found that a sound of considerable intensity could be
obtained by passing the interrupted current through the primary wire of
a small induction coil, and placing a conductor made of two sheets of
silvered paper placed back to back in the secondary circuit. The
silvered paper becomes rapidly charged and discharged, making a sound
that can be heard over a large hall, and having the same pitch as the
sending instrument.