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281
Heroes of the Telegraph
·
John Munro
205
In
the
telephone
of
Bell,
the
voice
of
the
speaker
is
the
motive
power
which
generates
the
current
in
the
line.
The
vibrations
of
the
sound
may
be
said
to
transform
themselves
into
electrical
undulations.
Hence
the
current
is
very
weak,
and
the
reproduction
of
the
voice
is
relatively
faint.
Edison
adopted
the
principle
of
making
the
vibrations
of
the
voice
control
the
intensity
of
a
current
which
was
independently
supplied
to
the
line
by
a
voltaic
battery.
The
plan
of
Bell,
in
short,
may
be
compared
to
a
man
who
employs
his
strength
to
pump
a
quantity
of
water
into
a
pipe,
and
that
of
Edison
to
one
who
uses
his
to
open
a
sluice,
through
which
a
stream
of
water
flows
from
a
capacious
dam
into
the
pipe.
Edison
was
acquainted
with
two
experimental
facts
on
which
to
base
the
invention.
In
1873,
or
thereabout,
he
claimed
to
have
observed,
while
constructing
rheostats,
or
electrical
resistances
for
making
an
artificial
telegraph
line,
that
powdered
plumbago
and
carbon
has
the
property
of
varying
in
its
resistance
to
the
passage
of
the
current
when
under
pressure.
The
variation
seemed
in
a
manner
proportional
to
the
pressure.
As
a
matter
of
fact,
powdered
carbon
and
plumbago
had
been
used
in
making
small
adjustable
rheostats
by
M.
Clerac,
in
France,
and
probably
also
in
Germany,
as
early
as
1865
or
1866.
Clerac's
device
consisted
of
a
small
wooden
tube
containing
the
material,
and
fitted
with
contacts
for
the
current,
which
appear
to
have
adjusted
the
pressure.
Moreover,
the
Count
Du
Moncel,
as
far
back
as
1856,
had
clearly
discovered
that
when
powdered
carbon
was
subjected
to
pressure,
its
electrical
resistance
altered,
and
had
made
a
number
of
experiments
on
the
phenomenon.
Edison
may
have
independently
observed
the
fact,
but
it
is
certain
he
was
not
the
first,
and
his
claim
to
priority
has
fallen
to
the
ground.
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