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1
378
The Story of the Atlantic Telegraph
·
Henry M. Field
373
The
following,
taken
in
substance
from
an
English
scientific
review,
will
indicate
briefly,
but
with
sufficient
clearness,
the
problem
to
be
solved
in
signalling
to
great
distances
under
the
sea,
and
the
instruments
by
which
this
is
accomplished:--
The
speed
of
signalling
through
a
submarine
cable
depends
upon
its
electrostatic
capacity,
which,
unless
it
be
very
small,
gives
rise
to
"retardation."
In
the
Proceedings
of
the
Royal
Society
for
1855,
Sir
William
Thomson
showed
how
the
effect
at
the
distant
end
of
a
cable,
caused
by
the
application
of
a
battery
at
one
end,
could
be
calculated
and
represented
graphically
in
what
is
called
the
"curve
of
arrival."
After
contact
is
first
made
at
the
sending
end
between
the
cable
and
one
pole
of
the
battery
(the
other
pole
being
to
earth),
a
certain
interval
of
time
elapses
before
any
effect
is
felt
at
the
distant
end.
This
interval
of
time
is
denoted
by
the
letter
a.
After
the
interval
of
time
a
has
passed,
a
current
begins
to
issue
from
the
cable
at
the
receiving
end,
and
increases
in
strength
very
rapidly.
After
a
further
interval
of
4a
or
after
a
period
of
5a
from
the
first
application
of
the
battery,
it
attains
about
half
its
maximum
strength,
and
there
is
very
little
sensible
increase
in
strength
after
a
time
equal
to
10a
has
elapsed.
The
curve
of
arrival
is
drawn
by
taking
distances
along
O
X
to
represent
intervals
of
time,
and
distances
along
O
Y
to
represent
strengths
of
current.
Curve
No.
I.
shows
the
gradual
increase
in
strength
of
the
received
current
at
one
end
of
a
cable
when
the
battery
is
applied
to
and
kept
in
contact
with
the
other
end.
For
a
distance
corresponding
to
the
interval
of
time
a,
the
curve
does
not
sensibly
deviate
from
the
straight
line
O
X;
in
other
words,
no
effect
is
observable
at
the
receiving
end
during
this
time.
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