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1
378
The Story of the Atlantic Telegraph
·
Henry M. Field
374
If
now,
instead
of
being
continuously
applied
to
the
battery
at
the
sending
end,
the
cable
had
been
applied
to
it
during
a
short
interval
of
time,
and
then
disconnected
from
the
battery
and
connected
to
earth,
the
curve
of
arrival
would
be
of
the
form
shown
by
curve
No.
II.
Curve
No.
II.
shows
the
effect
of
applying
the
battery
during
a
length
of
time
equal
to
4a,
and
then
putting
the
cable
to
earth.
It
will
be
seen
that
a
current
gradually
diminishing
in
strength
continues
to
flow
out
of
the
cable
at
the
distant
end
for
a
considerable
time
after
the
battery
has
been
disconnected.
This
continued
discharge
is
what
gives
rise
to
the
difficulty
experienced
in
reading
the
signals
sent
through
long
cables.
The
instrument
first
used
for
receiving
signals
through
a
long
submarine
cable
(the
short-lived
1858
Atlantic
cable)
was
the
Mirror
Galvanometer,
which
consisted
of
a
small
mirror
with
four
light
magnets
attached
to
its
back
(weighing,
in
all,
less
than
half-a-grain),
suspended
by
means
of
a
single
silk
fibre,
in
a
proper
position
within
the
hollow
of
a
bobbin
of
fine
wire:
a
suitable
controlling
magnet
being
placed
adjacent
to
the
apparatus.
The
action
of
this
instrument
is
as
follows.
On
the
passage
of
a
current
of
electricity
through
the
fine
wire
coil,
the
suspended
magnets
with
the
mirror
attached,
tend
to
take
up
a
position
at
right
angles
to
the
plane
of
the
coil,
and
are
deflected
to
one
side
or
the
other
according
as
the
current
is
in
one
direction
or
the
other.
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