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281
Heroes of the Telegraph
·
John Munro
84
But
all
these
instruments
have
one
great
drawback
for
delicate
work,
and,
however
suitable
they
may
be
for
land
lines,
they
are
next
to
useless
for
long
cables.
They
require
a
certain
definite
strength
of
current
to
work
them,
whatever
it
may
be,
and
in
general
it
is
very
considerable.
Most
of
the
moving
parts
of
the
mechanism
are
comparatively
heavy,
and
unless
the
current
is
of
the
proper
strength
to
move
them,
the
instrument
is
dumb,
while
in
Bain's
the
solution
requires
a
certain
power
of
current
to
decompose
it
and
leave
the
stain.
In
overland
lines
the
current
traverses
the
wire
suddenly,
like
a
bullet,
and
at
its
full
strength,
so
that
if
the
current
be
sufficiently
strong
these
instruments
will
be
worked
at
once,
and
no
time
will
be
lost.
But
it
is
quite
different
on
submarine
cables.
There
the
current
is
slow
and
varying.
It
travels
along
the
copper
wire
in
the
form
of
a
wave
or
undulation,
and
is
received
feebly
at
first,
then
gradually
rising
to
its
maximum
strength,
and
finally
dying
away
again
as
slowly
as
it
rose.
In
the
French
Atlantic
cable
no
current
can
be
detected
by
the
most
delicate
galvanoscope
at
America
for
the
first
tenth
of
a
second
after
it
has
been
put
on
at
Brest;
and
it
takes
about
half
a
second
for
the
received
current
to
reach
its
maximum
value.
This
is
owing
to
the
phenomenon
of
induction,
very
important
in
submarine
cables,
but
almost
entirely
absent
in
land
lines.
In
submarine
cables,
as
is
well
known,
the
copper
wire
which
conveys
the
current
is
insulated
from
the
sea-water
by
an
envelope,
usually
of
gutta-percha.
Now
the
electricity
sent
into
this
wire
INDUCES
electricity
of
an
opposite
kind
to
itself
in
the
sea-water
outside,
and
the
attraction
set
up
between
these
two
kinds
'holds
back'
the
current
in
the
wire,
and
retards
its
passage
to
the
receiving
station.
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