DOC.
14
PROOF
OF
AMPERE'S
CURRENTS
189
Published
in
Koninklijke
Akademie
van
Wetenschappen te
Amsterdam. Section of Sciences.
Proceedings 18 (1915-1916):
696-711.
Original
Dutch version submitted
23
April 1915, pub-
lished
14 May
1915.
[1]This is
a
translation of Einstein and De Haas
1915b,
which
is in
Dutch.
Significant
dif-
ferences between the translation and the
original
version
are
annotated.
See
also Einstein and
De
Haas 1915a
(Doc.
13)
for
an
earlier
account
of the
same
work with
more
experimental
details, and
the editorial
note,
"Einstein
on
Ampere's
Molecular
Currents,"
pp.
145-149,
for
general background.
[2]See, e.g.,
Harman
1982,
chap. 2,
for
an
overview of
nineteenth-century
theories of
mag-
netism.
[3]See, e.g.,
Lorentz
1909 for
an
overview of electron
theory.
[4]Curie's
law,
found
empirically
in 1895
(Curie
1895),
states
that for
paramagnetic
sub-
stances
the
susceptibility
is
inversely proportional
to
the
temperature.
In 1905
Paul
Langevin
derived
a
formula for the
susceptibility
of
paramagnetic
substances that reduces
to
Curie's
law
for small
fields (see Langevin 1905).
His
theory
was
based
on
the
hypothesis
that each
atom
carries
a
permanent magnetic
moment,
due
to
intra-atomic electronic motion. Einstein dis-
cussed Curie's law and
Langevin's
theory in
his
course on
the kinetic
theory
of
heat
at
the Uni-
versity
of
Zurich,
summer
semester 1910 (see Vol.
3,
Doc.
4,
[pp.
38-39],
for Einstein's lecture
notes
on
this
topic).
[5]See
the editorial
note,
"Einstein
on
Ampere's
Molecular
Currents,"
p.
146,
for
a
discus-
sion of Einstein's views
on
zero-point energy.
[6]Richardson
1908. Richardson had
unsuccessfully
tried
to measure
the mechanical effect
of
a
sudden
magnetization
of
an
unmagnetized
iron
cylinder.
See
the editorial
note,
"Einstein
on
Ampere's
Molecular Currents,"
p.
149,
for other
contemporary experiments on Ampere's
currents.
[7]The
value of the numerical coefficient
in
eq. (5) corresponds to
a
value of
1.77
x
107
emu/g
for the
specific charge
of the
electron, in
accordance with
recent experimental
results
(see, e.g., Neumann,
G.
1914).
[8]At
the
corresponding point
(p. 155)
in
Einstein and De Haas 1915a
(Doc.
13),
the
satu-
ration
magnetization per
cm3 is
put at
1100. See
also
pp.
706 and 710 of this
paper,
where the
values
1200
and
1300
are
used.
The
density
of iron
is 7.8
g/cm3.
[9]"1/4x"
should be
"1/2tc,"
as
in
Einstein and De Haas 1915b.
[10]A more
detailed
description
of the
apparatus
is
given
in
Einstein and
De
Haas 1915a
(Doc. 13), §4.
[11]See
Weiss
1907,
1908 for Pierre Weiss's
theory
of
ferromagnetism.
Einstein discussed
Weiss's
theory
in
his
course
on
the kinetic
theory
of
heat
at
the
University
of
Zurich,
summer
semester 1910 (see
Vol.
3,
Doc.
4,
[pp.
41-43],
for Einstein's lecture
notes
on
this
topic).
[12]Foucault
currents
are
the
eddy currents
induced
in
a
conductor
moving
in
a
magnetic
field.
[13]The
number 0.0065
corrects
the values 0.0070 and 0.0069
given
in
Einstein and De Haas
1915a
(Doc.
13), pp.
162
and
164, respectively.
[14]In
the determination of the
charge
of the
circulating particles
as
presented
in
Einstein
and De Haas 1915a
(Doc.
13), pp.
165-166,
it
was
erroneously
assumed that the
torque
and
the
angular
displacement
have
the
same
phase.
The
error
was
first
pointed
out by
Lorentz
in
a
telegram
to
Einstein
(see
Einstein
to
H. A.
Lorentz,
28 April 1915, in
which Einstein acknowl-
edges
the
error).
Einstein corrected the mistake in Einstein 1915d
(Doc. 16).
[15]"v"
should be
"2v,"
as
in
Einstein
and
De Haas 1915b.
[16]In
a
letter
to
G. L. de Haas-Lorentz,
Einstein made the
stronger
statement
that the results
for small
displacements
seemed
to be
systematically
wrong (see
Einstein
to
Geertruida
de
Haas-Lorentz,
before
10 April 1915).
[17]The
number 0.124
corrects
the value 0.128
given
in Einstein and
De
Haas 1915a
(Doc.
13), p.
169.
[18]At
the
corresponding point
(p. 169)
in
Einstein and
De
Haas 1915a
(Doc.
13),
the
sat-
urization
magnetization per
cm3 is
put at
1260.
The
use
of
the
value
1300
here
leads
to
slightly
different results for
Is
and
X.
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