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1
378
The Story of the Atlantic Telegraph
·
Henry M. Field
375
Of
various
other
forms
of
receiving
instruments
devised
by
Sir
William
Thomson,
the
next
to
be
noticed
is
the
Spark
Recorder,
both
on
account
of
the
principles
involved
in
its
construction,
and
because
it
in
some
respects
foreshadowed
the
more
perfect
instrument,
the
Siphon
Recorder,
which
he
introduced
some
years
later.
The
action
of
the
Spark
Recorder
was
as
follows.
An
indicator,
suitably
supported,
was
caused
to
take
a
to-and-fro
motion,
by
means
of
the
electro-magnetic
action
due
to
the
electric
currents
constituting
the
signals.
This
indicator
was
connected
to
a
Ruhmkorff
coil
or
other
equivalent
apparatus,
designed
to
cause
a
continual
succession
of
sparks
to
pass
between
the
indicator,
and
a
metal
plate
situated
beneath
it
and
having
a
plane
surface
parallel
to
its
line
of
motion.
Over
the
surface
of
this
plate
and
between
it
and
the
indicator,
there
was
passed,
at
a
regularly
uniform
speed
in
a
direction
perpendicular
to
the
line
of
motion
of
the
indicator,
a
material
capable
of
being
acted
on
physically
by
the
sparks,
either
through
their
chemical
action,
their
heat,
or
their
perforating
force.
The
record
of
the
signals
given
by
this
instrument
was
an
undulating
line
of
fine
perforations
or
spots,
and
the
character
and
succession
of
the
undulations
were
used
to
interpret
the
signals
desired
to
be
sent.
The
latest
form
of
receiving
instrument
for
long
submarine
cables,
is
that
of
the
Siphon
Recorder,
for
which
Sir
William
Thomson
obtained
his
first
patent
in
1867.
Within
the
three
succeeding
years
he
effected
great
improvements
on
it,
and
the
instrument
has,
since
that
date,
been
exclusively
employed
in
working
most
of
the
more
important
submarine
cables
of
the
world--indeed
all
except
those
on
which
the
Mirror-Galvanometer
method
is
still
in
use.
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