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281
Heroes of the Telegraph
·
John Munro
71
The
design
of
the
cable
was
a
subject
of
experiment
by
Professor
Morse
and
others.
It
was
known
that
the
conductor
should
be
of
copper,
possessing
a
high
conductivity
for
the
electric
current,
and
that
its
insulating
jacket
of
gutta-percha
should
offer
a
great
resistance
to
the
leakage
of
the
current.
Moreover,
experience
had
shown
that
the
protecting
sheath
or
armour
of
the
core
should
be
light
and
flexible
as
well
as
strong,
in
order
to
resist
external
violence
and
allow
it
to
be
lifted
for
repair.
There
was
another
consideration,
however,
which
at
this
time
was
rather
a
puzzle.
As
early
as
1823
Mr.
(afterwards
Sir)
Francis
Ronalds
had
observed
that
electric
signals
were
retarded
in
passing
through
an
insulated
wire
or
core
laid
under
ground,
and
the
same
effect
was
noticeable
on
cores
immersed
in
water,
and
particularly
on
the
lengthy
cable
between
England
and
the
Hague.
Faraday
showed
that
it
was
caused
by
induction
between
the
electricity
in
the
wire
and
the
earth
or
water
surrounding
it.
A
core,
in
fact,
is
an
attenuated
Leyden
jar;
the
wire
of
the
core,
its
insulating
jacket,
and
the
soil
or
water
around
it
stand
respectively
for
the
inner
tinfoil,
the
glass,
and
the
outer
tinfoil
of
the
jar.
When
the
wire
is
charged
from
a
battery,
the
electricity
induces
an
opposite
charge
in
the
water
as
it
travels
along,
and
as
the
two
charges
attract
each
other,
the
exciting
charge
is
restrained.
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