Measuring Voltage and Current
• d’Arsenval analog meter movement
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Analog Ammeter
• Use a d’Arsenval meter movement with a
parallel resistor (shunt).
• The parallel resistor diverts some of the
current away from the meter movement.
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Analog Voltmeter
• Use a d’Arsenval meter movement in
series with a resistor (multiplier).
• The resistor limits the voltage drop across
the meter movement.
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Sources of Error
• Any instrument (meter) used to make
measurements extracts energy from the
circuit. (Known as “loading” the circuit).
• The amount of error due to loading
depends on the effective resistance of the
instrument compared with the resistance
in the circuit.
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Rule of the “one-tenth”
• The effective resistance of the ammeter
should be no more than 1/10th of the value
of the smallest resistance in the circuit.
– The current being measured should be the
same with or without the ammeter.
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For a Voltmeter
• Since voltage is a parallel measurement,
the “loading” effect is minimized when the
voltmeter resistance is much higher than
the circuit resistance across which the
measurement is being made.
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Ammeter Example
• A 50 mV, 1 mA d’Arsonval movement is to be
used in an ammeter with a full-scale reading of
1 A. Determine RA. (Movement resistance is
equal to (50 mV/1 mA) = 50Ω)
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Solution
999 mA
1 mA
(999mA)( RA )  50mV
50
RA 
 50.05m
999
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Ammeter Example continued
• How much resistance is added to the circuit
when the meter is inserted to measure current?
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1A
+
50
mV
50/999 Ω
-
Rm 
50 Ω
50mV
 0.050
1A
or
 50 
(50) 

999

  0.050
Rm 
 50 
50  

999


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