AMPÈRE'S
MOLECULAR CURRENTS
147
thors
were aware
of the
problem,
but neither offered
a
solution
to
it
nor
made
a con-
nection to
the work of Niels
Bohr,
who had faced the
problem
two
years
earlier
in
his
first
papers on
atomic
theory.[9]
Later in 1915
Bohr made the connection when
he
cit-
ed Einstein and
De
Haas's results
as
support
for the existence of
nonradiating
elec-
tronic
motions.[10]
The actual
experiment
did
not take
long
to
perform. Though
the work
was
still
in
a
preparatory
stage at
the
beginning
of
January
1915,[11]
Einstein could
write
to
Lorentz before
the
end
of that
month
that the
experiment
had led
to
a
positive
re-
sult,[12]
and
a
few weeks later
he
claimed
that
he had demonstrated the
existence of
molecular
currents,
adding
that the work would
soon
be
completed.[13]
The
first
re-
sults
were
presented
in
a
lecture
to
the Deutsche
Physikalische
Gesellschaft
on
19
February
1915. A
revised and
expanded manuscript
of the lecture
was
submitted
on
10
April.
The theoretical
approach
to
the
problem
was
based
on
the
assumption
that the
mo-
lecular
currents
were
in
fact due
to
rotating
intra-atomic electrons of
mass
m
and
charge
e.
Assuming
that
a
magnetized body
contains
many
such
rotating
atoms,
the
following
relation between its
magnetic
moment
J and its
angular
momentum M
could be derived:
with
M
= Yj,
(1)
A
=
2u/E.
(2)
A sudden reversal of
the magnetization
of the
body
would
thus
produce
a
torque
that
could in
principle
be measured. In Einstein
and De
Haas's
experiment,
an
iron
cylin-
der
was
suspended
inside
a
solenoidal
current
coil
in
such
a
way
that it
was
magne-
tized
axially. Reversing
the
current in
the coil and thus the
magnetic
moment
of the
body
would
set
the
body
in rotation.
By using
an
alternating current
and
tuning
its
frequency
to
the
proper frequency
of the
cylinder,
a
measurable deflection
was
pro-
duced.
[9]See
Bohr 1913.
[10]See
Bohr
1915.
[11]See
Einstein
to Edgar Meyer,
2
January 1915,
in
which Einstein mentions that
he
and
De Haas
are
preparing
the
experiment.
[12]Einstein
to
H. A. Lorentz, 23
January
1915.
Two
days
later,
waxing
enthusiastic in
a
let-
ter
to
his elder
son,
Einstein
speaks
of
a
"beautiful and
important
experiment
...
on
magnets"
("ein
wundervolles und
wichtiges
Experiment
...
über die
Magnete."
Einstein
to
Hans Albert
Einstein, 25
January
1915).
[13]See
Einstein
to
Michele
Besso, 12
February
1915. Two
months later Einstein
announced
to
another friend that De Haas and he "had
experimentally provided
certain
proof
of
the
existence
of
Ampère's
molecular
currents (explanation
of
para-
and
ferromagnetism)" ("Wir
haben
experimentell
den sichern Nachweis der Existenz der
Ampère'schen
Molekularströme
geliefert (Erklärung
des
Para- und
Ferromagnetismus)."
Einstein
to
Heinrich
Zangger,
ca.
10
April 1915).