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281
Heroes of the Telegraph
·
John Munro
87
The
practical
advantage
of
this
extreme
delicacy
is,
that
the
signal
waves
of
the
current
may
follow
each
other
so
closely
as
almost
entirely
to
coalesce,
leaving
only
a
very
slight
rise
and
fall
of
their
crests,
like
ripples
on
the
surface
of
a
flowing
stream,
and
yet
the
light
spot
will
respond
to
each.
The
main
flow
of
the
current
will
of
course
shift
the
zero
of
the
spot,
but
over
and
above
this
change
of
place
the
spot
will
follow
the
momentary
fluctuations
of
the
current
which
form
the
individual
signals
of
the
message.
What
with
this
shifting
of
the
zero
and
the
very
slight
rise
and
fall
in
the
current
produced
by
rapid
signalling,
the
ordinary
land
line
instruments
are
quite
unserviceable
for
work
upon
long
cables.
The
mirror
instrument
has
this
drawback,
however--it
does
not
'record'
the
message.
There
is
a
great
practical
advantage
in
a
receiving
instrument
which
records
its
messages;
errors
are
avoided
and
time
saved.
It
was
to
supply
such
a
desideratum
for
cable
work
that
Sir
William
Thomson
invented
the
siphon
recorder,
his
second
important
contribution
to
the
province
of
practical
telegraphy.
He
aimed
at
giving
a
GRAPHIC
representation
of
the
varying
strength
of
the
current,
just
as
the
mirror
galvanometer
gives
a
visual
one.
The
difficulty
of
producing
such
a
recorder
was,
as
he
himself
says,
due
to
a
difficulty
in
obtaining
marks
from
a
very
light
body
in
rapid
motion,
without
impeding
that
motion.
The
moving
body
must
be
quite
free
to
follow
the
undulations
of
the
current,
and
at
the
same
time
must
record
its
motions
by
some
indelible
mark.
As
early
as
1859,
Sir
William
sent
out
to
the
Red
Sea
cable
a
piece
of
apparatus
with
this
intent.
The
marker
consisted
of
a
light
platinum
wire,
constantly
emitting
sparks
from
a
Rhumkorff
coil,
so
as
to
perforate
a
line
on
a
strip
of
moving
paper;
and
it
was
so
connected
to
the
movable
needle
of
a
species
of
galvanometer
as
to
imitate
the
motions
of
the
needle.
But
before
it
reached
the
Red
Sea
the
cable
had
broken
down,
and
the
instrument
was
returned
dismantled,
to
be
superseded
at
length
by
the
siphon
recorder,
in
which
the
marking
point
is
a
fine
glass
siphon
emitting
ink,
and
the
moving
body
a
light
coil
of
wire
hung
between
the
poles
of
a
magnet.
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