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281
Heroes of the Telegraph
·
John Munro
208
This
instrument
was
called
the
'electromotograph,'
and
it
occurred
to
Edison
that
in
a
similar
way
the
undulatory
currents
from
his
carbon
transmitter
might,
by
varying
the
friction
between
a
metal
stylus
and
the
prepared
surface,
put
a
tympanum
in
vibration,
and
reproduce
the
original
sounds.
Wonderful
as
it
may
appear,
he
succeeded
in
doing
so
by
the
aid
of
a
piece
of
chalk,
a
brass
pin,
and
a
thin
sheet
or
disc
of
mica.
He
attached
the
pin
or
stylus
to
the
centre
of
the
mica,
and
brought
its
point
to
bear
on
a
cylindrical
surface
of
prepared
chalk.
The
undulatory
current
from
the
line
was
passed
through
the
stylus
and
the
chalk,
while
the
latter
was
moved
by
turning
a
handle;
and
at
every
pulse
of
the
electricity
the
friction
between
the
pin
and
chalk
was
diminished,
so
that
the
stylus
slipped
upon
its
surface.
The
consequence
was
a
vibration
of
the
mica
diaphragm
to
which
the
stylus
was
attached.
Thus
the
undulatory
current
was
able
to
establish
vibrations
of
the
disc,
which
communicated
themselves
to
the
air
and
reproduced
the
original
sounds.
The
replica
was
loud
enough
to
be
heard
by
a
large
audience,
and
by
reducing
the
strength
of
the
current
it
could
be
lowered
to
a
feeble
murmur.
The
combined
transmitter
and
receiver
took
the
form
of
a
small
case
with
a
mouthpiece
to
speak
into,
an
car-piece
on
a
hinged
bracket
for
listening
to
it,
press-keys
for
manipulating
the
call-bell
and
battery,
and
a
small
handle
by
which
to
revolve
the
little
chalk
cylinder.
This
last
feature
was
a
practical
drawback
to
the
system,
which
was
patented
in
1877.
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