DOCUMENT 180 OCTOBER 1909 211
[4]Einstein assumed
a
position
as
Extraordinary
Professor
at
the
University
of Zurich
on
15
October
(see
Doc.
154,
note
2).
[5]At
the
Salzburg meeting
Max Born had
presented
a
paper
on
rigid
motion
in
special
rel-
ativity
(Born 1909b),
on
which Sommerfeld had commented
in
the discussion
following
the
paper.
Einstein and Born had discussed the
subject
and had discovered that
setting
a
rigid
disk
into rotation would
give
rise
to
a
paradox:
the rim becomes
Lorentz-contracted,
whereas the
radius remains invariant
(see
Born
1910, p.
233).
The existence of
this paradox
was
first
point-
ed
out
in
print
by
Paul Ehrenfest
(1880-1933)
in
a
paper
that
was
received
on
the date of
this
letter
(see Ehrenfest 1909). See
also
Stachel 1980
for
a
historical discussion of Einstein's
treat-
ment
of the
rigidly rotating
disk.
[6]See
Einstein
1907j (Vol. 2,
Doc.
47), §§17-20,
where Einstein introduces the
equivalence
principle
and
explores
some
of
its
consequences.
[7]See
Einstein 1905i
(Vol. 2,
Doc.
14),
in
which Einstein shows that the
entropy
of
mono-
chromatic radiation of small
density depends
on
its
volume,
as
if the radiation consists of
in-
dependent
energy quanta.
180.
To
Lucien Chavan
[Zurich, 19
October
1909]
Lieber Herr Chavan!
Ich sende
Ihnen mein Dienstbüchlein
&
die
Niederlassungs-Bewilligung
mit der
Bitte,
mich bei Polizei und Kreiskommando und Polizei abzumel-
den.[1]
Es ist hier schon
losgegangen.[2]
Meine
neue
Situation
gefällt
mir sehr
gut.
Aber
es
ist
anstrengend.[3]
Mit herzlichen Grüssen Ihr
A.
Einstein.
[.
.
.][4]
AKS
(Sz,
Archiv der
Einstein-Gesellschaft).
[72
277].
The
verso
is
addressed "Herrn
L.
Cha-
van
Beundenfeld-str.
5
Bern.,"
and
postmarked
"Zürich
8
(Fluntern)
19.X.09.-5."
[1]On moving
from Bern
to
Zurich-on
14
October
(see
Mileva Einstein-Maric
to
Helene
Savic,
3
September
1909,
Milan
Popovic, Belgrade)-Einstein,
a
Swiss
national,
was
required
to notify
the Bern
military
command of his
new
address and
to
register
his
change
of address
with the
police.
Einstein's
departure
is noted
in
his
military
service book but
no
date
is
given
(see
Dienstbüchlein,
13
March
1901).
The
police registration is
dated
22
October
1909 (see
Einwohnerregister
Berner und
Schweizer
(Einsassen), SzBe-Ar,
E 2.2.1.0.
054).
[2]Winter semester
1909/1910
at
the
University
of Zurich
began 18
October (set Zürich
Ver-
zeichnis 1909b,
title
page).
[3]Einstein
lectured
on
introductory
mechanics
(four
hours
a
week
to eighteen
students)
and
on
thermodynamics (two
hours
a
week
to
fifteen students
and
three
auditors),
as
well
as con-
ducting
a
physics
seminar
one
hour
a
week for twelve students
(see
Kontrollbücher über die
Honorargebühren, W.S. 1909/10, SzZU, Kassa-Archiv,
and Zürich
Verzeichnis 1909b,
p.
22).
The lecture
notes
on
mechanics
are
presented
as
Vol.
3,
Doc.
1.
[4]A postscript by
Mileva Einstein-Maric
to
Jeanne Chavan-Perrin
is
omitted.
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