312
MEASUREMENT
OF
SMALL CHARGES
Doc. 48
A NEW
ELECTROSTATIC
METHOD FOR THE MEASUREMENT
OF
SMALL
QUANTITIES OF
ELECTRICITY
by
A.
Einstein
[Physikalische
Zeitschrift
9
(1908):
216-217]
As
we
know,
electric potentials
as
low
as
about
10-6
volts
can
be
measured
by
sensitive electrostatic
quadrant
electrometers if
a
sufficiently
[1]
great auxiliary
potential
is supplied
to
the
needle.
However,
an
increase
in
this
auxiliary potential
results
in
a
decrease, rather than increase, in the
sensitivity
of
the instrument if
one measures
quantities
of
electricity.
The
higher
the
potential of the needle, the
smaller
the
deflection
caused
by a
given
quantity of
electricity. If the absolute value
of
the
potential of
the
needle is
large
compared
with the
potential difference
between
the
quadrants,
then the deflection
depends
only
on
the
product of
the potential
and
the
quantity of
electricity supplied,
and
thus
on
the electric
energy
supplied,
[2]
and
the
energy
required
for
the
deflection
must
be
taken
from
the
energy
of
the
system
being
measured.
This circumstance determines the limit
actually
achievable
of
the sensitivity
of
the
quadrant
electrometer
and of
analogous
instruments
when
measuring
quantities
of
electricity
or
energy,
respectively.
However,
it is possible
to construct
measuring
instruments
in which
the
energy
required
for deflection is
not
taken
from
the
system
being measured,
but
from
an
auxiliary
source, so
that it will
be
possible
to exceed
the actual
sensitivity limit
mentioned above. In
the
following,
I describe the
scheme
of
[3]
an
induction
machine
with
which,
in
my
opinion,
this
goal
can
be
achieved.
Let
A1
and
A'1
(Fig.
1)
be two
fixed
conductors,
along
which
shall
slide
two
rigidly
connected metal
leaflets
B,
fastened, for
example, to
a
small
wheel.
The
leaflets
have
two contact pins
b,
which
are
within the
range
of
fixed
contact springs
K1
and
K'1,
respectively.
Let
K1
be
grounded
and
K'1
conductively
connected with
A'1.
A1
shall
be
kept permanently
at
a
positive potential
P1.
When
the
leaflet that
is just
passing
touches
K1,
the electric
charge
on
A1
induces
an
opposite
electric
charge
-e on
b.
Once
this
leaflet is
opposite
A'1,
so
that it touches
K'1,
it delivers
negative
electricity
to
A'1.
In
such
a
way