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281
Heroes of the Telegraph
·
John Munro
18
He
achieved
renown
by
a
great
experiment--the
measurement
of
the
velocity
of
electricity
in
a
wire.
His
method
was
beautiful
and
ingenious.
He
cut
the
wire
at
the
middle,
to
form
a
gap
which
a
spark
might
leap
across,
and
connected
its
ends
to
the
poles
of
a
Leyden
jar
filled
with
electricity.
Three
sparks
were
thus
produced,
one
at
either
end
of
the
wire,
and
another
at
the
middle.
He
mounted
a
tiny
mirror
on
the
works
of
a
watch,
so
that
it
revolved
at
a
high
velocity,
and
observed
the
reflections
of
his
three
sparks
in
it.
The
points
of
the
wire
were
so
arranged
that
if
the
sparks
were
instantaneous,
their
reflections
would
appear
in
one
straight
line;
but
the
middle
one
was
seen
to
lag
behind
the
others,
because
it
was
an
instant
later.
The
electricity
had
taken
a
certain
time
to
travel
from
the
ends
of
the
wire
to
the
middle.
This
time
was
found
by
measuring
the
amount
of
lag,
and
comparing
it
with
the
known
velocity
of
the
mirror.
Having
got
the
time,
he
had
only
to
compare
that
with
the
length
of
half
the
wire,
and
he
found
that
the
velocity
of
electricity
was
288,000
miles
a
second.
Till
then,
many
people
had
considered
the
electric
discharge
to
be
instantaneous;
but
it
was
afterwards
found
that
its
velocity
depended
on
the
nature
of
the
conductor,
its
resistance,
and
its
electro-static
capacity.
Faraday
showed,
for
example,
that
its
velocity
in
a
submarine
wire,
coated
with
insulator
and
surrounded
with
water,
is
only
144,000
miles
a
second,
or
still
less.
Wheatstone's
device
of
the
revolving
mirror
was
afterwards
employed
by
Foucault
and
Fizeau
to
measure
the
velocity
of
light.
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