EINSTEIN'S "MASCHINCHEN" 51
EINSTEIN'S "MASCHINCHEN" FOR THE
MEASUREMENT OF SMALL
QUANTITIES
OF ELECTRICITY
I
In 1907
Einstein became interested in
measuring very
small
quantities
of
electricity.
These
measurements
could
provide experimental support
for the
occurrence
of volt-
age
fluctuations in
condensers,
an
electrical
phenomenon
related
to
Brownian motion
and discussed
by
Einstein
in
a
paper
written
in
December
1906.[1]
On
15 July 1907,
in
a
letter
to
his friends Conrad and Paul
Habicht,
he
announced his
discovery
of
a
"new method of
measuring very
small
quantities
of
energy."[2]
Einstein's "new meth-
od" consisted
essentially
of
amplifying
a
very
low initial
voltage
by means
of
a
spe-
cial electrostatic induction machine
(or multiplier),
whose
output voltage
could then
be measured with
a
simple
electrometer.
In
fact,
Einstein's idea
was
not
new,
but
goes
back
to
the last decades of the
eigh-
teenth
century,
a
time when the
study
of
electricity
was
based
purely
on
electrostatic
experiments
and
observations,
and
high voltages
were
generated
with friction
ma-
chines.
Globes,
cylinders,
or
disks made of
glass (or
other
insulating substances)
were
rotated and rubbed
by
one or more
leather
cushions;
the friction
charged
the
glass
and the
charges
were
accumulated
on
big
brass
conductors.[3]
Electricity
was
measured
by simple divergence electroscopes.
In
1775
Alessandro
Volta
invented the
electrophorus,
a
simple
instrument
capable
of
generating high potential
differences
by
electrostatic
induction,[4]
and
all
subsequent
electrostatic induction
generators
were
based
on
Volta's
electrophorus.
The first small induction machines invented
during
the last decades of the
eigh-
teenth
century
were
meant,
like
Einstein's, to
multiply
an
electrical
charge
mechan-
ically
so
that it could be detected and measured with
an electroscope.[5]
Unlike the
friction
machines, however,
they
were
not
powerful generators.
The doubler
pro-
posed
by
Abraham Bennet
in
1778,
for
instance,
was a
kind of double
electrophorus.
In
the
same
year
William Nicholson invented
a
doubler with
two fixed
disks and
a
rotating one,
and
in
1795
Tiberius Cavallo devised another instrument with three
fixed
plates
and
a
fourth
one
oscillating
between them.
Many
modifications and im-
provements
of these
instruments
were
proposed
in
later
years.
In the
first
decades of
the nineteenth
century Giuseppe Belli,
Professor of
Physics
at
the
University
of
Pavia,
invented
two
types
of
multipliers:
the "macchina
ad attuazione"
(actuating
machine),
an
induction machine with
a rotating glass
disk
carrying
three metallic
sec-
tors,
and
a "doplicatore" (doubler).[6]
These
machines
were
bigger
than earlier mul-
tipliers
and could
be used instead of friction machines
to
produce high voltage
elec-
tricity
for
charging Leyden jars,
for
generating sparks,
and for
many
other classical
electrostatic
experiments.
The
glass
disk of
the
"macchina
ad
attuazione"
was
rotated
between
two
massive insulated metallic boxes. At the
beginning
of the
cycle
one
of
the
boxes,
say A,
carried
a
small electrical
charge.
When
one
of the
metallic
sectors
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