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281
Heroes of the Telegraph
·
John Munro
88
The
principle
of
the
siphon
recorder
is
exactly
the
inverse
of
the
mirror
galvanometer.
In
the
latter
we
have
a
small
magnet
suspended
in
the
centre
of
a
large
coil
of
wire--the
wire
enclosing
the
magnet,
which
is
free
to
rotate
round
its
own
axis.
In
the
former
we
have
a
small
coil
suspended
between
the
poles
of
a
large
magnet--the
magnet
enclosing
the
coil,
which
is
also
free
to
rotate
round
its
own
axis.
When
a
current
passes
through
this
coil,
so
suspended
in
the
highly
magnetic
space
between
the
poles
of
the
magnet,
the
coil
itself
experiences
a
mechanical
force,
causing
it
to
take
up
a
particular
position,
which
varies
with
the
nature
of
the
current,
and
the
siphon
which
is
attached
to
it
faithfully
figures
its
motion
on
the
running
paper.
The
point
of
the
siphon
does
not
touch
the
paper,
although
it
is
very
close.
It
would
impede
the
motion
of
the
coil
if
it
did.
But
the
'capillary
attraction'
of
so
fine
a
tube
will
not
permit
the
ink
to
flow
freely
of
itself,
so
the
inventor,
true
to
his
instincts,
again
called
in
the
aid
of
electricity,
and
electrified
the
ink.
The
siphon
and
reservoir
are
together
supported
by
an
EBONITE
bracket,
separate
from
the
rest
of
the
instrument,
and
INSULATED
from
it;
that
is
to
say,
electricity
cannot
escape
from
them
to
the
instrument.
The
ink
may,
therefore,
be
electrified
to
an
exalted
state,
or
high
POTENTIAL
as
it
is
called,
while
the
body
of
the
instrument,
including
the
paper
and
metal
writing-tablet,
are
in
connection
with
the
earth,
and
at
low
potential,
or
none
at
all,
for
the
potential
of
the
earth
is
in
general
taken
as
zero.
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