012-07124B
Basic Electrometer
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Measuring Voltage
The electrometer can be thought of as an
infinite impedance voltmeter. To show that the
electrometer is like a voltmeter, connect the
leads to the battery, set the range (3, 10, 20, or
100 volts full scale), and read the voltage on the
meter.
Measuring Charge
Induction Charges
Under most conditions, the best way to measure
charge is by induction. Use a proof plane and a
Faraday ice pail, such as those included with
PASCO’s Demonstration Electrostatics System, and
shown in Figure 2. The proof plane is simply a
small conductive disk mounted on an insulating
handle. To sample the charge distribution on a
charged object, simply touch it with the proof
plane, then place the proof plane inside the inner
cylinder of the ice pail, without touching the
cylinder. A charge of equal magnitude and sign is
induced on the surface of the ice pail and can be
read by the electrometer. By always using the
proof plane and the ice pail, the capacitance will be
the same for all your measurements, and the charge
on the proof plane will always be proportional to the
voltage reading of the electrometer.
Figure 2. Using a Faraday Ice Pail
Contact Charges
Charges can also be measured by contact. If you
touch the inner cylinder of the ice pail with a
charged object, for example a charged proof plane,
the electrometer reading will generally remain
relatively unchanged. This is because the total
capacitance is only negligibly affected by the proof
plane. This may not always be the case, however.
Any charge measurement with the electrometer is
indirect, based on the knowledge that the amount
of charge on an object is proportional to the
potential difference. The readings of the
electrometer will always be of volts, not of
coulombs. The polarity of the voltage, however,
directly shows the type of charge being sampled. If
you need quantitative rather than qualitative charge
measurement, values for the charge can be
computed according to the relationship
Q
=
CV
,
where
V
is the voltage across a known capacitor of
capacitance
C
.
The electrometer can be thought of as an infinite
impedance voltmeter in parallel with a capacitor,
C
E
,
as shown in Figure 3.
C
E
represents the internal
capacitance of the electrometer, plus the capacitance
of the leads and the capacitance of the Ice Pail, if
being used.
Figure 3. Ideal Schematic of the Electrometer
When a charged object is placed across the
electrometer leads (or in the ice pail), a voltage
V
displays on the meter. If the value
C
E
is known, the
value of the charge can be calculated as
Q
=
C
E
V
.
The capacitance of the electrometer alone is around
27 pF. However, if the sampled object adds
significant capacitance, the situation becomes as
shown in Figure 4.
Electrometer
shield
ice pail
proof
plane
electrometer
insulator
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clip
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