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281
Heroes of the Telegraph
·
John Munro
268
Next
year,
on
December
31st,
1844,
he
projected
a
mode
of
measuring
the
speed
of
ships
by
vanes
revolving
in
the
water
and
indicating
their
speed
on
deck
by
means
of
the
current.
In
the
same
specification
he
described
a
way
of
sounding
the
sea
by
an
electric
circuit
of
wires,
and
of
giving
an
alarm
when
the
temperature
of
a
ship's
hold
reached
a
certain
degree.
The
last
device
is
the
well-known
fire-alarm
in
which
the
mercury
of
a
thermometer
completes
an
electric
circuit,
when
it
rises
to
a
particular
point
of
the
tube,
and
thus
actuates
an
electric
bell
or
other
alarm.
On
December
12,
1846,
Bain,
who
was
staying
in
Edinburgh
at
that
time,
patented
his
greatest
invention,
the
chemical
telegraph,
which
bears
his
name.
He
recognised
that
the
Morse
and
other
telegraph
instruments
in
use
were
comparatively
slow
in
speed,
owing
to
the
mechanical
inertia
of
the
parts;
and
he
saw
that
if
the
signal
currents
were
made
to
pass
through
a
band
of
travelling
paper
soaked
in
a
solution
which
would
decompose
under
their
action,
and
leave
a
legible
mark,
a
very
high
speed
could
be
obtained.
The
chemical
he
employed
to
saturate
the
paper
was
a
solution
of
nitrate
of
ammonia
and
prussiate
of
potash,
which
left
a
blue
stain
on
being
decomposed
by
the
current
from
an
iron
contact
or
stylus.
The
signals
were
the
short
and
long,
or
'dots'
and
'dashes'
of
the
Morse
code.
The
speed
of
marking
was
so
great
that
hand
signalling
could
not
keep
up
with
it,
and
Bain
devised
a
plan
of
automatic
signalling
by
means
of
a
running
band
of
paper
on
which
the
signals
of
the
message
were
represented
by
holes
punched
through
it.
Obviously
if
this
tape
were
passed
between
the
contact
of
a
signalling
key
the
current
would
merely
flow
when
the
perforations
allowed
the
contacts
of
the
key
to
touch.
This
principle
was
afterwards
applied
by
Wheatstone
in
the
construction
of
his
automatic
sender.
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