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281
Heroes of the Telegraph
·
John Munro
210
The
soft
and
springy
button
of
lamp-black
became
no
longer
necessary,
since
it
was
not
so
much
the
resistance
of
the
material
which
varied
as
the
resistance
at
the
contacts
of
its
parts
and
the
platinum
electrodes.
Two
metals,
or
two
pieces
of
hard
carbon,
or
a
piece
of
metal
and
a
piece
of
hard
carbon,
were
found
to
regulate
the
current
in
accordance
with
the
vibrations
of
the
voice.
Edison
therefore
discarded
the
soft
and
fragile
button,
replacing
it
by
contacts
of
hard
carbon
and
metal,
in
short,
by
a
form
of
microphone.
The
carbon,
or
microphone
transmitter,
was
found
superior
to
the
magneto-electric
transmitter
of
Bell;
but
the
latter
was
preferable
as
a
receiver
to
the
louder
but
less
convenient
chemical
receiver
of
Edison,
and
the
most
successful
telephonic
system
of
the
day
is
a
combination
of
the
microphone,
or
new
carbon
transmitter,
with
the
Bell
receiver.
The
'micro-tasimeter,'
a
delicate
thermoscope,
was
constructed
in
1878,
and
is
the
outcome
of
Edison's
experiments
with
the
carbon
button.
Knowing
the
latter
to
be
extremely
sensitive
to
minute
changes
of
pressure,
for
example,
those
of
sonorous
vibrations,
he
conceived
the
idea
of
measuring
radiant
heat
by
causing
it
to
elongate
a
thin
bar
or
strip
of
metal
or
vulcanite,
bearing
at
one
end
on
the
button.
To
indicate
the
effect,
he
included
a
galvanometer
in
the
circuit
of
the
battery
and
the
button.
The
apparatus
consisted
of
a
telephone
button
placed
between
two
discs
of
platinum
and
connected
in
circuit
with
the
battery
and
a
sensitive
galvanometer.
The
strip
was
supported
so
that
one
end
bore
upon
the
button
with
a
pressure
which
could
be
regulated
by
an
adjustable
screw
at
the
other.
The
strip
expanded
or
contracted
when
exposed
to
heat
or
cold,
and
thrust
itself
upon
the
button
more
or
less,
thereby
varying
the
electric
current
and
deflecting
the
needle
of
the
galvanometer
to
one
side
or
the
other.
The
instrument
was
said
to
indicate
a
change
of
temperature
equivalent
to
one-millionth
of
a
degree
Fahrenheit.
It
was
tested
by
Edison
on
the
sun's
corona
during
the
eclipse
observations
of
July
29,
1875,
at
Rawlings,
in
the
territory
of
Wyoming.
The
trial
was
not
satisfactory,
however,
for
the
apparatus
was
mounted
on
a
hen-house,
which
trembled
to
the
gale,
and
before
he
could
get
it
properly
adjusted
the
eclipse
was
over.
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