DOCUMENT
606
AUGUST 1918 863
[4]In
Ehrenhaft’s
experiments,
the electrical
charge
of
small
particles was
determined from the
speed
with which
they
moved
upward or
downward under the combined influence
of
gravity
and
an
electric field
between
the two
plates
of
a
horizontal condenser.
See,
e.g., Konstantinowsky
1918 for
a
general
review
of
Ehrenhaft’s work.
[5]In
the other
transcription,
there
is
a closing
parenthesis
at
this
point.
[6]A
is the
square
root
of
the
mean square displacement,
which
is
proportional
to
,ft;
thus,
the
mean square
displacement per
unit
of
time
A2/t
is
independent
of
time. Einstein’s
theory
of
Brown-
ian motion
(see
Einstein 1905k
[Vol.
2,
Doc.
16])
provides a
relation between
A2/t
and the
mobility
of
the
particles
(the
proportionality
constant between force and
velocity);
from the
mobility,
the size
of
the
particles can
be determined
(see
Konstantinowsky
1915,
p.
277).
As
was pointed
out
by
Ehren-
haft,
this method led
to
results that
disagreed
with other determinations
of
the
particle
size
(see
Ehren-
haft 1918a,
p. 71).
[7]The
other
transcription
has
"Zahlen"
instead
of
"Zeiten."
[8]In
Ehrenhaft 1918b,
the author
reports on a
series
of
experiments
in which
falling
test
particles
are
exposed
to
a
beam
of
light.
It
appeared
that
some
types
of
particles begin
to
move
in
the direction
of
the
light
source,
instead of
away
from it.
Ehrenhaft
dubbed this
phenomenon "light-negative
photophoresis" ("lichtnegative Photophorese").
See also Doc.
396,
in
particular,
note
5,
for
more on
this
phenomenon
and for Einstein’s comments
on an
earlier
publication
of
Ehrenhaft that describes it.
[9]Karl
Schwarzschild had calculated the radiation
pressure on
small
perfectly reflecting spheres
(see
Schwarzschild
1901).
He found that for radiation
of
given
constant
energy density
w,
the
pressure
on
a sphere
of
radius
a,
divided
by
na2w
(the
radiation
energy reaching
the
particle),
first increases
with
increasing a,
then reaches
a
maximum and
finally
decreases to
a
constant value
of
1.
The value
of
a
at
which the maximum is reached
depends on
the
wavelength
of
the radiation. Peter
Debye
later
generalized
this work to
spheres
of
arbitrary composition
(see
Debye
1909).
From his
experiments,
Ehrenhaft determined
the ratio
of
pressure
and
incoming
radiation
energy
for the various
types
of
par-
ticles that he used. He found
a
maximum, and,
using
Schwarzschild’s
formula,
used its
position to
determine the size
of
the
particles, finding good agreement
with
other,
independent
determinations.
[10]See
Konstantinowsky
1918. Kurt
Konstantinowsky
(1892-?)
had obtained his doctorate under
Ehrenhaft in 1916.
[11]See
Franck
and
Hertz
1914b;
see
also Doc.
11
for
an
earlier
comment
by
Einstein
on
their
experiment.
[12]In
the
other
transcription, e
is
replaced by
X2,
the
mean square
displacement
of
a
Brownian
particle
per
unit
of
time. The
most
plausible reading
is
e/m,
a
well-established constant
quantity
for
electrons. One
of
Ehrenhaft’s
points
of
criticism
was
that the
constancy
of
this ratio did not
imply
the
constancy
of
both factors.
[13]The
comparison
is made in
Konstantinowsky
1918,
pp.
475-476.
[14]See
Ehrenhaft
1918b, p. 9. B
is
the
mobility.
606.
To
Hermann Anschutz-Kaempfe
Ahrenshoop,
den
22.
VIII. 1918.
Sehr
geehrter
Herr
Dr.
Anschütz!
Ich habe Ihre
Ausführungen
zu
dem Buche
von
Usener mit Interesse
gelesen.[1]
Nun weiss ich
doch,
warum
mir
Usener
vor
einiger
Zeit
das Büchlein zusandte![2]
Ich zweifle nicht
an
der
unredlichen
Absicht des
Autors,
der mit vielem Raffine-
ment
arbeitet,
und kann
es
Ihnen
nachfühlen,
das
so
eine
Handlungsweise
verlet-
zend wirkt.
Aber
jeder
weiss,
dass
Sie
es
gewesen
sind,
der
den
Kreiselkompass