138
DOC.
28
SIMPLE EXPERIMENT
[p.
173]
Doc.
28
A
Simple Experiment
to Demonstrate
Ampere's
Molecular Currents
by
A. Einstein
(Presented
in the session of
February 25,
1915)
[p.
174]
[2]
The
following
describes
a simple experiment
which
can serve
to demonstrate
Ampere's
molecular
currents
in lectures.
It is
a
variation
of
an experiment
which
I
did
together
with
Mr.
De Haas.1
We
want to
demonstrate the
(apparent) angular
momentum
that
a
tiny
iron rod
exhibits under
a
reversion of its
magnetization,
so
that the orbits
of
the electrons
change
their orientation.
A
major difficulty
for
a simple
demonstration
of
this effect
lies in the fact that the
purely magnetic
forces,
exerted
onto
the rod
by
the
magnetizing
field,
are
very large compared
to
the forces
to
be measured. In order
to
minimize these
difficulties,
the rod is
not
permanently exposed
to
the
magnetic
field,
but
only during
times
so
short
(about 1/1000
of
a
second)
that the field
just
suffices
to
reverse
the
remanent
magnetization
of the rod.
The
arrangement
that
was ultimately
used
can
be
seen (in
a
vertical axial
cut-
away)
in the sketch
(fig.
1)
below. In the middle
of
the
tiny
iron rod
S
(diameter
1.4
millimeters,
length
about
10
centimeters)
that is to be
investigated
is
a
small
mirror,
and the rod is
suspended
by
a
thread
of
quartz
of several centimeters in
length
and
about 10
[i
in
diameter. At the
top,
this thread of
quartz
is
glued
to
a
copper spindle
with
a
turning knob,
the latter bored
through
the center of
a
cork that has
friction. The cork
is
pressed
into
a
wooded extension R'
of
a
pressure-coil
pipe R;
the latter
is
mounted
on a
board H with
balancing screws.
Around the
pipe
R
are
two
(e.g., serially connected)
coils
£
which
together
have about 4000
windings,
and there is
space
left between the coils for
a
glass
window
(not
shown in the
sketch)
through
which
a light-pointer can
be
(objectively)
observed
when the rod executes oscillations.
In
series
with the
two coils is
a
capacitor
of
2
microfarads. The
open
circuit
is
connected to
a
120-volt direct current
supply
via
a
commutator of 500 to 1000 ohms
Fig. 1.
[1]
1Verh. d. D.
Phys.
Ges.
17 (1915),
p.
152.
In the meantime
Barnett
[Phys.
Rev. (2) 6
(1915),
p.
171],
succeeded in also
showing
the
reverse
of
the
effect that is described above.