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281
Heroes of the Telegraph
·
John Munro
247
The
commonest
and
at
the
same
time
one
of
the
most
sensitive
forms
of
the
instrument
is
called
the
'pencil
microphone,'
from
the
pencil
or
crayon
of
carbon
which
forms
the
principal
part
of
it.
This
pencil
may
be
of
mercurialised
charcoal,
but
the
ordinary
gas-carbon,
which
incrusts
the
interior
of
the
retorts
in
gas-works,
is
usually
employed.
The
crayon
is
supported
in
an
upright
position
by
two
little
brackets
of
carbon,
hollowed
out
so
as
to
receive
the
pointed
ends
in
shallow
cups.
The
weight
of
the
crayon
suffices
to
give
the
required
pressure
on
the
contacts,
both
upper
and
lower,
for
the
upper
end
of
the
Pencil
should
lean
against
the
inner
wall
of
the
cup
in
the
upper
bracket.
The
brackets
are
fixed
to
an
upright
board
of
light,
dry,
resonant
pine-wood,
let
into
a
solid
base
of
the
same
timber.
The
baseboard
is
with
advantage
borne
by
four
rounded
india-rubber
feet,
which
insulate
it
from
the
table
on
which
it
may
be
placed.
To
connect
the
microphone
up
for
use,
a
small
voltaic
battery,
say
three
cells
(though
a
single
cell
will
give
surprising
results),
and
a
Bell
speaking
telephone
are
necessary.
A
wire
is
led
from
one
of
the
carbon
brackets
to
one
pole
of
the
battery,
and
another
wire
is
led
from
the
other
bracket
to
one
terminal
screw
of
the
telephone,
and
the
circuit
is
completed
by
a
wire
from
the
other
terminal
of
the
telephone
to
the
other
pole
of
the
battery.
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