1. Measuring Voltage and Current
d’Arsenval analog meter movement
ECE 201 Circuit Theory 1

2. 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.
ECE 201 Circuit Theory 1

3. Analog Voltmeter
Use a d’Arsenval meter movement in series with a resistor (multiplier).
The resistor limits the voltage drop across the meter movement.
ECE 201 Circuit Theory 1

4. 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.
ECE 201 Circuit Theory 1

5. 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.
ECE 201 Circuit Theory 1

6. 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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7. 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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8. Solution
1 mA
999 mA
ECE 201 Circuit Theory 1

9. Ammeter Example continued
How much resistance is added to the circuit when the meter is inserted to measure current?
ECE 201 Circuit Theory 1

10. 1A +
50 mV -
50/999 Ω
50 Ω
ECE 201 Circuit Theory 1