398

DOC.

11

LECTURE

ON

ELECTRICITY

&

MAGNETISM

potential

P1

and

charge

e

induces

a

charge

-e on a

grounded

conductor,

which

is

then

brought

into contact with

a

second conductor

with

which

it

shares its

charge

(see

the

figure).

The second

conductor then takes the

potential P.

By

repeating

this

process

a

stationary state is

reached

on

the second

conductor,

which

will

then have

a potential

that

is higher

than

P1.

See

also

Vol.

5,

the editorial

note,

"Einstein's 'Maschinchen'

for

the Measurement of Small

Quantities

of

Elec-

tricity."

[29]Thomson's

drop

multiplier

(Thomson 1867a) serves

to

increase

a

potential

difference

between

two cylinders

by means

of

falling

water

drops.

The

drops fall

in two

streams,

each

through

one

of the

cylinders,

and

acquire

an

induced

charge

of

opposite

sign,

which

is

then

given

off in

a

reservoir connected with the other

cylinder

(see

the

figure).

In this

way

the

potential

difference

between the

cylinders is rapidly

increased. For

more

details,

see, e.g.,

Graetz

1905a,

pp.

51-54.

[30]The

electron

theory

and

its

interpretation

of

dielectrics

is

discussed in Abraham

1905,

§28.

[31]In the last

boundary

condition,

S

should

be

S1.

[32]Both

methods

of

determining

dielectric

constants

that

are

mentioned

here,

Perot's

method

of

using

the

refraction of

field lines

(Perot

1891)

and the

hydrostatic

method

(which was

originally

devised

by Quincke

for the determination of

magnetic susceptibilities;

see Quincke

1885), are

described in

Drude

1894,

pp.

295-301.

See

also Einstein's discussion of the

measure-

ment

of

magnetic permeabilities

in

these lecture

notes

([p.

60]).

[33]"Polarisationselektrizität" should

be "Polarisationsmagnetismus."

[34]Einstein

uses

the

term "eingeprägte

Kraft"

("impressed

force")

to denote

an

external

force

of

unknown

origin.

See

Abraham/Föppl

1907,

pp. 194-197,

for

a

similar

usage

of

the term,

and for

a

discussion of

the

need

to

introduce such

forces to explain

certain

phenomena.

[35]From

here

on

cp

is

the

potential;

the

integration angle

is denoted

by S.

[36]In

the following equation should

be

(~YI

[37]The

magnetic

double

layer is at

r.

Its normal

is

n,

it

has surface

"charge" density

rj

and thickness

Ö; £ =

rjS

is

the

magnetic moment

of

the double

layer.

[38]A

magnetized

bar of

magnetic

moment M

performs

horizontal oscillations

in

the

mag-

netic

field

of the earth

(which

has horizontal

component

H).

See

Kohlrausch

1910,

§73,

for

a

discussion

of several methods

to

determine MH and

M/H, including

a

method due

to Gauss.

Kohlrausch

1910 is

mentioned in

Einstein's "Scratch Notebook"

(Appendix

A),

[p.

8].

[39]The

tangent galvanometer

is

constructed

in

such

a way

that

the

field

produced

by

the

unknown

current

(Hi),

and the horizontal

component

of

the

magnetic

field

of the earth

(H) are

perpendicular.

The

tangent

of the

angle

between

the total

field

and the

field

of

the earth

is

thus

proportional

to

Hi,

which

is

proportional to

the

current

(see

also the

picture,

in which this

angle

is

denoted

by

a).

The

angle (and accordingly

the

current) is

determined with the

help

of

a

magnetic

needle. See

Auerbach

1905, pp.

256-268,

for

a

description

of

various kinds of

tangent

galvanometers.

[40]See

note 43.

[41]l

is

the

length

of the

moving part

of the current

loop (see

the

figure);

ö

is its

displacement

in

the direction of

the

arrow.

[42]9)zz

below should

be

§z.

[43]In the

Deprez-d'Arsonval

instrument

a

coil

of

n

windings hangs

between the

poles

of

a

magnet

(see

the

figure).

A current in

the

coil

causes

it

to

rotate. In the

expression

for the

torque

D,

l

and

2R

are presumably

the dimensions of the

coil;

0

is

the torsion

constant

of

the wire

that

holds the

coil,

and

x

is

the

angle

of

rotation. For

more details, see,

e.g.,

Auerbach

1905,

pp.

293-294.

[44]In

this

expression

E

is

the

quantity

of

electricity;

the

square

root

should be

omitted.

[45]This

should

be arctg-

= (p.

a

[46]See

note

25

for

a

description

of Thomson's balance

(or

Thomson's

electrometer).