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