DOC.
6
PONDEROMOTIVE FORCES
209
Doc.
6
On the Ponderomotive
Forces
Acting on
Ferromagnetic
Conductors
Carrying
a
Current in
a
Magnetic
Field
by
A. Einstein
[Archives
des sciences
physiques
et
naturelles 30
(1910):
323-324]
A
current-carrying
conductor
in
a magnetic
field
H
is subjected
to
a
ponderomotive
force,
the formula
for
which
is
F
=
[iH],
(1)
where
i is
the
vector
of the
current
density,
and
the
expression
in
the brackets
is
the
vector
product.
This
formula
is
applicable,
in
particular,
in
the
case
where the
body
conducting
the
current is
not
magnetizable,
i.e.,
where the
magnetic
induction
B
is
equal
to
the
intensity
of the
magnetic
field
H. If the conductor of the
current
is
magnetizable
and
its
magnetic
state
is consequently
characterized
by
two vectors
H
and B
that
are
different
from
each
other,
then
one
oneself
which
of these
two vectors
gives
rise to
the
ponderomotive
force
sought.
Till
now
this
role
has
been attributed
to
B,
and
it has
been
accepted
that
F
=
[iB].
(2)
But
we
will show in
a simple special case
that,
even
in
the
case
of
a
magnetic
conductor,
formula
(1)
is
the
correct
one.
Let
D be
a
metal
disk
through
which
a
current flows from
its
center to
its
periphery.
This
current
is
provided
by
a battery P;
the other
lines in
the
diagram
complete
the
circuit.
By
virtue of
the
principle
of
equality
of
action and reaction,
regardless
of
the
material
of
which
the
disk
is
the resultant of
all
the
electrodynamic
forces
acting
on
the
different
parts
of the
system
is
zero.
This must
be
especially so
if
the disk D
is