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281
Heroes of the Telegraph
·
John Munro
85
It
follows,
that
with
a
receiving
instrument
set
to
indicate
a
particular
strength
of
current,
the
rate
of
signalling
would
be
very
slow
on
long
cables
compared
to
land
lines;
and
that
a
different
form
of
instrument
is
required
for
cable
work.
This
fact
stood
greatly
in
the
way
of
early
cable
enterprise.
Sir
William
(then
Professor)
Thomson
first
solved
the
difficulty
by
his
invention
of
the
'mirror
galvanometer,'
and
rendered
at
the
same
time
the
first
Atlantic
cable
company
a
commercial
success.
The
merit
of
this
receiving
instrument
is,
that
it
indicates
with
extreme
sensibility
all
the
variations
of
the
current
in
the
cable,
so
that,
instead
of
having
to
wait
until
each
signal
wave
sent
into
the
cable
has
travelled
to
the
receiving
end
before
sending
another,
a
series
of
waves
may
be
sent
after
each
other
in
rapid
succession.
These
waves,
encroaching
upon
each
other,
will
coalesce
at
their
bases;
but
if
the
crests
remain
separate,
the
delicate
decipherer
at
the
other
end
will
take
cognisance
of
them
and
make
them
known
to
the
eye
as
the
distinct
signals
of
the
message.
The
mirror
galvanometer
is
at
once
beautifully
simple
and
exquisitely
scientific.
It
consists
of
a
very
long
fine
coil
of
silk-covered
copper
wire,
and
in
the
heart
of
the
coil,
within
a
little
air-chamber,
a
small
round
mirror,
having
four
tiny
magnets
cemented
to
its
back,
is
hung,
by
a
single
fibre
of
floss
silk
no
thicker
than
a
spider's
line.
The
mirror
is
of
film
glass
silvered,
the
magnets
of
hair-spring,
and
both
together
sometimes
weigh
only
one-tenth
of
a
grain.
A
beam
of
light
is
thrown
from
a
lamp
upon
the
mirror,
and
reflected
by
it
upon
a
white
screen
or
scale
a
few
feet
distant,
where
it
forms
a
bright
spot
of
light.
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