90

DOCS.

76,

77 APRIL

1915

this

follows,

taking

into account

the

theorem of momentum

[conservation],

that

upon magnetic

commutation

of

a

magnetizable body

the

molecules translate into

one

rotation

moment

or

other,

according

to

the

formula

rotation

moment

=

2

electron

masses

-.

electron

charge

d-dt(magnetization)

_

2ßdM

[e]

dt

The

experiments

were

done

by observing

the

oscil-

lation

motions

of

a

little

iron rod

that

was

hanging

on a glass

fiber within

a

coil

through

which

an

al-

ternating

current

was

flowing

(use

of

resonance.)

glass

fiber

coil

little iron

core

77. To Tullio Levi-Civita

[Berlin,]

20

April

[1915]

Highly

esteemed and dear

Colleague,

Just

now

I

was

studying

your very interesting

letter

of

April

15th.

Admittedly

I

do not

concur-as

is

explained

in

more

detail in

the

following-with

your

proof

that the

selection of

the

ôguv's as

“quasi-constants”

was

unworkable;

but

I

gladly

acknowledge

that

you

have

put

your finger

on

the

weakest

point

of

the

proof,

namely,

on

the

independence

of

the

A(uv)’s.

Here

the

proof

lacks

precision;[1]

the

statement

on

page

1072

of

the

paper

“it

will

thus

be

equivalent

to

an

arbitrary

variation

of

the

6guv's"

dispenses

with

rigorous

substantiation; in

the

special

case

of

a

constant

guv

it

is

even

incorrect.[2]

Yet

I

do cherish

the

firm belief

that

in

general

it

applies,

because

the

number

of

freely

selectable variables

that determine

the

10

Sguv,s

is

10,

and because

both

variations

b1

and

b2

are

of

fundamentally

different

kinds,

in

that

a

b2

variation

generally

is not

a

b1

variation.

Now to

your proof. Right

at

the

outset

you say

that

for small

regions

£

A(uv)

=

ƒ

Ôg^dr

=

J

byg^dr,

thus

that the

ƒ S2g^dT

terms

vanish.

This

already

I

contest,[3]

namely

for

exactly

the

same reason

that

I

regard

your

deduction of

the

vanishing

of

the

J

Sadr’s