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).