492 DOC. 48 MEASUREMENT OF SMALL
CHARGES
Published in
Physikalische Zeitschrift
9
(1908):
216-217.
Dated
Bern, 13
February
1908,
re-
ceived
15
February
1908,
published
1
April
1908.
[1]
An
explanation
of
the
quadrant
electrome-
ter
is
found
in
Einstein's
ETH
notes,
H. F. We-
ber's Lectures
on Physics, ca.
December 1897-
ca.
June
1898
(Vol.
1,
Doc.
37),
pp.
156-158.
The
quadrant
electrometer
was
first
proposed
by
William Thomson
(see,
e.g.,
Thomson,
W.
1867);
later versions
of
this instrument
are
dis-
cussed
in Cermak
1918,
pp.
110-123. At
the
time
of
this
paper, a sensitivity
of
10-6
volt had
only
been achieved with the instrument
pro-
posed by
Kleiner
(see
Kleiner 1906 and
Cermak
1918, pp.
141-142). For the order
of
magnitude
of
the effects Einstein
hoped
to
measure,
see
Einstein 1907b
(Doc. 39), pp.
571-572.
[2]
Einstein's conclusion follows from the fol-
lowing
expression
for the deflection
p
of
the
needle in
Thomson's
quadrant
electrometer: p
=
K
(A
-
B)
(C
-
1/2
(A
+
B)),
if
C,
the
auxiliary potential
of
the
needle, is
assumed to
be
high;
A, B
are
the
potentials
of
the
two quad-
rants,
and K
is
a
constant.
For
a
discussion
of
this
formula, see, e.g.,
Maxwell
1891,
part
I,
chap.
13,
§
219;
it is also mentioned in Ein-
stein's
ETH
notes,
H.
F.
Weber's
Lectures
on
Physics,
ca.
December 1897-ca. June 1898
(Vol.
1,
Doc.
37),
p.
157.
[3]
Inductive machines
were
further
develop-
ments
of
potential multipliers,
which in fact
were
intended
to
measure
small
quantities
of
electricity,
in
a way
similar
to
the device
pro-
posed
by
Einstein
(see,
e.g.,
Schmidt
1918,
pp.
33-40).
However,
at
the time
of
this
paper,
in-
ductive
machines
were
used
mainly
for the
gen-
eration
of
high potentials,
and
not
as measuring
instruments.
For
a description
that includes
a
historical
account
of
their
development, see,
e.g.,
Schmidt
1918,
pp.
40-67.
[4]
The
method
proposed by
Einstein
was
later
severely
criticized for
not
taking
into
account the
phenomenon
of
self-induction,
caused
by
the
different
metal
parts
of
the instrument that have
to
be
brought
into contact with each other.
See,
e.g., Schmidt
1918,
p.
35;
see
also Vol.
5,
the
editorial
note,
"Einstein's 'Maschinchen'
for
the Measurement
of
Small
Quantities
of
Elec-
tricity."
[5]
For
a
discussion
of
the
use
of
electrometers
in
contemporary investigations
of
radioactivity,
see,
e.g., Rutherford
1906,
pp.
23-36
(a
Ger-
man
edition
appeared
in
1907,
Rutherford
1907).
The
significance
of
the instrument
pro-
posed by
Einstein for research
on radioactivity
is
also
emphasized
in
Habicht
and
Habicht
1910,
p.
535.
[6]
Einstein's
paper
aroused immediate inter-
est. On 30 March
1908,
Joseph
de
Kowalski,
Professor
of
Physics
at the
University
of
Fri-
bourg,
wrote
to
Einstein:
"Today,
in the
Physi-
kalische
Zeitschrift, I
read
of
your proposal
for
the
measurement
of
electrical
quantities.
This
interested
me greatly
and
I
would like
to
have
such
an
instrument
built"
("Heute
lese ich in
der
Physik.
Zeitschft. Ihren
Vorschlag zur
Mes-
sung
der
Elektricitätsmengen.
Derselbe hat mich
sehr interessirt und ich möchte
gern
ein
entspre-
chendes Instrument ausführen lassen."). Before
the
publication
of Einstein's
paper,
his
friends,
Conrad
and Paul
Habicht, had
started
working
on
the
measuring
device
proposed by
Einstein;
see
the editorial note,
"Einstein
on
Brownian
Motion,"
pp.
221-222, and Vol.
5,
the editorial
note,
"Einstein's 'Maschinchen'
for the Mea-
surement
of
Small
Quantities
of
Electricity."
[7]
Einstein 1907b
(Doc. 39).