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281
Heroes of the Telegraph
·
John Munro
86
When
there
is
no
current
on
the
instrument,
the
spot
of
light
remains
stationary
at
the
zero
position
on
the
screen;
but
the
instant
a
current
traverses
the
long
wire
of
the
coil,
the
suspended
magnets
twist
themselves
horizontally
out
of
their
former
position,
the
mirror
is
of
course
inclined
with
them,
and
the
beam
of
light
is
deflected
along
the
screen
to
one
side
or
the
other,
according
to
the
nature
of
the
current.
If
a
POSITIVE
current--that
is
to
say,
a
current
from
the
copper
pole
of
the
battery--gives
a
deflection
to
the
RIGHT
of
zero,
a
NEGATIVE
current,
or
a
current
from
the
zinc
pole
of
the
battery,
will
give
a
deflection
to
the
left
of
zero,
and
VICE
VERSA.
The
air
in
the
little
chamber
surrounding
the
mirror
is
compressed
at
will,
so
as
to
act
like
a
cushion,
and
'deaden'
the
movements
of
the
mirror.
The
needle
is
thus
prevented
from
idly
swinging
about
at
each
deflection,
and
the
separate
signals
are
rendered
abrupt
and
'dead
beat,'
as
it
is
called.
At
a
receiving
station
the
current
coming
in
from
the
cable
has
simply
to
be
passed
through
the
coil
of
the
'speaker'
before
it
is
sent
into
the
ground,
and
the
wandering
light
spot
on
the
screen
faithfully
represents
all
its
variations
to
the
clerk,
who,
looking
on,
interprets
these,
and
cries
out
the
message
word
by
word.
The
small
weight
of
the
mirror
and
magnets
which
form
the
moving
part
of
this
instrument,
and
the
range
to
which
the
minute
motions
of
the
mirror
can
be
magnified
on
the
screen
by
the
reflected
beam
of
light,
which
acts
as
a
long
impalpable
hand
or
pointer,
render
the
mirror
galvanometer
marvellously
sensitive
to
the
current,
especially
when
compared
with
other
forms
of
receiving
instruments.
Messages
have
been
sent
from
England
to
America
through
one
Atlantic
cable
and
back
again
to
England
through
another,
and
there
received
on
the
mirror
galvanometer,
the
electric
current
used
being
that
from
a
toy
battery
made
out
of
a
lady's
silver
thimble,
a
grain
of
zinc,
and
a
drop
of
acidulated
water.
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