DOCS.
629,
630
SEPTEMBER-OCTOBER
1918 661
of
the
rods
equipped
with
contacts;[4] may
I
make
use
of
this,
whereupon
I would
refer
to
your forthcoming
work?[5]
I
hope you
have received
my
letter,
which
was
sent
out
yesterday.[6]
With
best
regards, yours,
Einstein.
630. From Felix Ehrenhaft
Vienna,
3
October
1918
Dear
Colleague,
Upon returning
to Vienna
a fortnight ago
from
an
excursion
to
Switzerland,
I
discovered
your
kind
letter
postmarked
20
August[1]
as
well
as
your
postal
card
stamped 23
August
1918.
I
beg your
pardon
for
having neglected
for
so
long
to
answer
the
questions posed
in
your
letter,
but
I
was
very
busy.
The fact that the
air in
the
condenser
is
not
brought
into motion
by
the
electrical
field is
elucidated
by
the
circumstance
that
the
velocity
of fall of
un-
charged
test
specimens
remains
precisely
the
same
when
a
random
voltage
is
applied
to the condenser
plates or
when
the
test
specimen
is allowed to fall
with
the
condenser short-circuited.
We
performed
such
experiments already
in
1910
in
Vienna. Hence not
the
slightest
motion in
the
gas
itself arises from
the
electrical
field, so
the
conclusions
about
the
rationality
or
irrationality
are
not obscured
by
it in
any
way.
Thus
your hypothetically proposed gas
motions
from the
electrical
field
play absolutely
no
role,
because
they
do not
occur.
As
far
as
the
nonagreement you
mentioned between
theory
and
experiment
for
Brownian motion
is
concerned,
we on our
part
have made clear
(:Konstantinowsky,
1915
Annalen:) that the
Vt-law is
confirmed
by
the
experiments.[2] By
contrast,
we
have
had
no success
until
now
with
the
mobility
calculation from Brownian
motion,
as
you can
gather
from
my
recent
papers
of 1918
as
well
as
from
the
paper by
Dr. Parankiewicz
(:Annalen
53,
issue
15,
p.
564:).[3]
That
is
why
I
also
wrote
on p.
71
of
my paper
“Über
die
Teilbarkeit”
[“On
the
Divisibility”],
“this
course
will
require
further
investigation.”[4]
This
investigation
is
now
in
progress.
We
have
succeeded,
in
particular,
in
subjecting one
and the
same
test
specimen
to
a
series
of measurements at various
gas pressures. You
write in
your
letter,
“The Brownian motion
appears
‘smoothed out’
to
the observer.
It
appears
to
me
that it
is
not
the
momentary position
of
the
particle
that
is
being registered,
but
only a
certain
temporal
mean
value.”
I
do not
quite
understand this
concept
of
smoothing
out,
since
the
theoretical formula
is,
of
course,
valid
only
for
the
visible Brownian motion.
Perhaps your
time
will
permit you
to inform
me
of this