PHYSICS 328 LAB -- METERS
Objectives: To determine the resistance and full scale current sensitivity of a
D'Arsonval meter movement and to use these parameters to construct ammeters
and voltmeters of different ranges.
Introduction:
One of the least expensive and most versatile devices for
electrical measurements is the electromechanical meter movement, or D'Arsonval
meter. In simple terms, it consists of a coil of wire which carries the current and which
is suspended between the poles of a permanent magnet. The resulting torque on
the coil is balanced at equilibrium by the torque produced by a restoring spring. The
equilibrium position, indicated by a pointer connected to the coil, represents the
magnitude of the current through the coil. The design results in two important meter
characteristics -- meter resistance and current required for full-scale deflection, i.e., full
scale current sensitivity. If we know the magnitudes of these two parameters we can
alter the meter to read voltage or current by using the appropriate shunt and series
resistors.
Procedure:
I.
Meter Characteristics
A. Resistance
Connect the circuit as shown, using the Analog Trainer, with the 100 µA meter
and the 0 - 15 V variable supply and adjust V until the meter reads full scale.
Shunt the meter with a decade resistance box and adjust its value until the
meter reads half of its full scale value. What is the meter's resistance ?
Discuss the significance of any assumptions or approximations made with this
technique.
10 kΩ
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A
R
V
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Physics 328 - Revised 1/28/99
B. Current Sensitivity
Use the digital voltmeter to adjust V to 10 volts. Connect the circuit as shown
and determine the full scale current sensitivity.
II.
III.
Ammeter
How would you alter the meter to make a device which has a full scale current
sensitivity of 50 mA ? Draw a schematic diagram of the 50 mA meter circuit.
What would be the effective resistance of this device ? (Because of the
difficulty with obtaining precision resistors of arbitrary value, we do not do this
experimentally.)
Voltmeter
Alter your meter movement to make a voltmeter which measures 5 volts full
scale. Use the digital voltmeter to set the voltage source to 4 volts. Measure
this voltage with your 5 volt voltmeter. What is the effective resistance of
your voltmeter ? One figure of merit for a voltmeter is its ohms/volt value,
that is, its resistance divided by its full scale voltage sensitivity. What is the
ohms/volt figure for your voltmeter ?
Rs
A
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Physics 328 - Revised 1/28/99
IV.
Meter Effects
A. Ammeter
Connect the circuit as shown, using the digital voltmeter to set the voltage to
0.5 volts. Theoretically determine the current flowing in the circuit. Insert the
meter (as a 5 mA current meter) in series with one of the150Ω resistors to
measure the current. What value do you get? What is the effect of the meter
on the circuit? Calculate the percentage error caused by your meter.
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0.5 V
150 Ω
150 Ω
B. Voltmeter
Connect the circuit as shown. Theoretically determine the voltage across the
1000Ω and 5000Ω resistors. Using the meter movement as a 5 volt
voltmeter, measure the voltage across the 1000Ω and 5000 Ω resistors.
Compare these values with the theoretical values and calculate a percentage
error. Qualitatively explain the discrepancy. Use the digital voltmeter to
measure the same voltages. What is the percentage error now and why are
these readings much closer to the theoretical predictions ?
V.
Questions
A. What is the ideal internal resistance for an ammeter ? For a voltmeter ?
B. For part IV A, theoretically determine what your non-ideal ammeter should
read and compare with your actual reading.
C. For part IV B, theoretically determine what the readings of your non-ideal
voltmeter should be and compare with your actual readings. Repeat this
calculation and comparison for the digital voltmeter.
Physics 328 - Revised 1/28/99