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281
Heroes of the Telegraph
·
John Munro
206
Still
he
deserves
the
full
credit
of
utilising
it
in
ways
which
were
highly
ingenious
and
bold.
The
'pressure-relay,'
produced
in
1877,
was
the
first
relay
in
which
the
strength
of
the
local
current
working
the
local
telegraph
instrument
was
caused
to
vary
in
proportion
to
the
variation;
of
the
current
in
the
main
line.
It
consisted
of
an
electro-magnet
with
double
poles
and
an
armature
which
pressed
upon
a
disc
or
discs
of
plumbago,
through
which
the
local
current
Passed.
The
electro-magnet
was
excited
by
the
main
line
current
and
the
armature
attracted
to
its
poles
at
every
signal,
thus
pressing
on
the
plumbago,
and
by
reducing
its
resistance
varying
the
current
in
the
local
circuit.
According
as
the
main
line
current
was
strong
or
weak,
the
pressure
on
the
plumbago
was
more
or
less,
and
the
current
in
the
local
circuit
strong
or
weak.
Hence
the
signals
of
the
local
receiver
were
in
accordance
with
the
currents
in
the
main
line.
Edison
found
that
the
same
property
might
be
applied
to
regulate
the
strength
of
a
current
in
conformity
with
the
vibrations
of
the
voice,
and
after
a
great
number
of
experiments
produced
his
'carbon
transmitter.'
Plumbago
in
powder,
in
sticks,
or
rubbed
on
fibres
and
sheets
of
silk,
were
tried
as
the
sensitive
material,
but
finally
abandoned
in
favour
of
a
small
cake
or
wafer
of
compressed
lamp-black,
obtained
from
the
smoke
of
burning
oil,
such
as
benzolene
or
rigolene.
This
was
the
celebrated
'carbon
button,'
which
on
being
placed
between
two
platinum
discs
by
way
of
contact,
and
traversed
by
the
electric
current,
was
found
to
vary
in
resistance
under
the
pressure
of
the
sound
waves.
The
voice
was
concentrated
upon
it
by
means
of
a
mouthpiece
and
a
diaphragm.
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