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378
The Story of the Atlantic Telegraph
·
Henry M. Field
253
The
method
used
by
the
electricians
to
discover
a
fault
is
one
of
such
delicacy
and
beauty
as
shows
the
marvellous
perfection
of
the
instruments
which
science
employs
to
learn
the
secrets
of
nature.
The
galvanometer
is
an
invention
of
Professor
Thomson,
by
which
"a
ray
of
light
reflected
from
a
tiny
mirror
suspended
to
a
magnet
travels
along
a
scale,
and
indicates
the
resistance
to
the
passage
of
the
current
through
the
cable
by
the
deflection
of
the
magnet,
which
is
marked
by
the
course
of
this
speck
of
light.
If
the
light
of
the
mirror
travels
beyond
the
index,
or
out
of
bounds,
an
escape
of
the
current
is
taking
place,
and
what
is
technically
called
a
fault
has
occurred."
Such
was
the
discovery
on
Monday
morning.
At
a
quarter
past
three
o'clock
the
electrician
on
duty
saw
the
light
suddenly
glide
to
the
end
of
the
scale
and
vanish.
Fortunately
it
was
not
a
fatal
injury.
It
did
not
prevent
signalling
through
the
cable,
and
a
message
was
at
once
sent
back
to
the
shore,
giving
notice
of
the
check
that
had
been
received.
But
the
electric
current
did
not
flow
freely.
There
was
a
leak
at
some
point
of
the
line
which
it
would
not
be
prudent
to
pass
over.
They
were
now
seventy-three
miles
from
shore,
having
run
out
eighty-four
miles
of
cable.
The
tests
of
the
electricians
indicated
the
fault
to
be
ten
or
a
dozen
miles
from
the
stern
of
the
ship.
The
only
safe
course
was
to
go
back
and
get
this
on
board,
and
cut
out
the
defective
portion.
It
was
a
most
ungrateful
operation
thus
to
be
undoing
their
own
work,
but
there
was
no
help
for
it.
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