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281
Heroes of the Telegraph
·
John Munro
275
In
1857,
he
investigated
the
electro-static
induction
and
retardation
of
currents
in
insulated
wires,
a
phenomenon
which
he
had
observed
in
1850,
and
communicated
an
account
of
it
to
the
French
Academy
of
Sciences.
'In
these
researches
he
developed
mathematically
Faraday's
theory
of
molecular
induction,
and
thereby
paved
the
way
in
great
measure
for
its
general
acceptance.'
His
ozone
apparatus,
his
telegraph
instrument
working
with
alternate
currents,
and
his
instrument
for
translating
on
and
automatically
discharging
submarine
cables
also
belong
to
the
year
1857.
The
latter
instruments
were
applied
to
the
Sardinia,
Malta,
and
Corfu
cable.
In
1859,
he
constructed
an
electric
log;
he
discovered
that
a
dielectric
is
heated
by
induction;
he
introduced
the
well
known
Siemens'
mercury
unit,
and
many
improvements
in
the
manufacture
of
resistance
coils.
He
also
investigated
the
law
of
change
of
resistance
in
wires
by
heating;
and
published
several
formulae
and
methods
for
testing
resistances
and
determining
'faults'
by
measuring
resistances.
These
methods
were
adopted
by
the
electricians
of
the
Government
service
in
Prussia,
and
by
Messrs.
Siemens
Brothers
in
London,
during
the
manufacture
of
the
Malta
to
Alexandria
cable,
which,
was,
we
believe,
the
first
long
cable
subjected
to
a
system
of
continuous
tests.
'In
1861,
he
showed
that
the
electrical
resistance
of
molten
alloys
is
equal
to
the
sum
of
the
resistances
of
the
separate
metals,
and
that
latent
heat
increases
the
specific
resistance
of
metals
in
a
greater
degree
than
free
heat.'
In
1864
he
made
researches
on
the
heating
of
the
sides
of
a
Leyden
jar
by
the
electrical
discharge.
In
1866
he
published
the
general
theory
of
dynamo-electric
machines,
and
the
principle
of
accumulating
the
magnetic
effect,
a
principle
which,
however,
had
been
contemporaneously
discovered
by
Mr.
S.
A.
Varley,
and
described
in
a
patent
some
years
before
by
Mr.
Soren
Hjorth,
a
Danish
inventor.
Hjorth's
patent
is
to
be
found
in
the
British
Patent
Office
Library,
and
until
lately
it
was
thought
that
he
was
the
first
and
true
inventor
of
the
'dynamo'
proper,
but
we
understand
there
is
a
prior
inventor
still,
though
we
have
not
seen
the
evidence
in
support
of
the
statement.
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