1. Unit 9 Measuring Instruments
Objectives:
Discuss the operation of a d’Arsonval meter movement.
Connect a voltmeter to a circuit.
Read an analog multimeter.
Connect an ammeter.
Measure resistance using an ohmmeter.

2. Unit 9 Measuring Instruments
Analog meters are characterized by the fact that they use a pointer and scale to indicate their value.
One of the common types of meters uses the d’Arsonval type of meter movement.
Analog meters use a moving coil placed between the poles of a magnet.

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Basic d’Arsonval meter movement.

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Basic d’Arsonval meter movement with rectifier to change AC voltage to DC voltage.

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The voltmeter is designed to be connected directly across the source of power.

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Reading an analog meter is similar to reading a speedometer or fuel gauge.

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The ammeter is used to measure current and must be connected in series with the load to permit the load to limit the current flow.

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A shunt is used to set the value of the ammeter.

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Many electricians use the clamp-on type of AC ammeter.
Please note the clamp-on ammeter reads only one conductor at a time.

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Ohmmeters
The ohmmeter is used to measure resistance.
There are two basic types: analog or digital.
The common VOM (volt-ohm-milliammeter) contains an ohmmeter.
An ohmmeter should always be readjusted to zero when the scale is changed.

11. Unit 9 Measuring Instruments
Digital Ohmmeters
Digital ohmmeters display the resistance in figures instead of using a meter movement.
They operate by measuring the voltage drop across a resistor.
The ohmmeter, whether digital or analog, must never be connected to a circuit when the power is turned on!

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Digital Ohmmeter

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Digital Voltmeter

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Low-impedance Voltage Tester
The low-impedance voltage tester has a very large current draw compared to other types of voltmeters and should never be used to test low-power circuits.

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Low-impedance Voltage Tester
This tester has an impedance of 5000 Ω and can generally be used to measure voltages as high as 600 V.

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Low-impedance Voltage Tester
This is also known as a solenoid type tester. This tester is not susceptible to giving the misleading voltage readings caused by high-impedance ground paths or feedback voltages.

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Low-impedance Voltage Tester
High-impedance ground paths can produce misleading voltage readings.

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Low-impedance Voltage Tester

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Reading a Digital Meter
Many digital meters are auto-ranging, which means that they select the range scale automatically. This type will display a notation beside the numerical digits to indicate the meter scale.

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Review:
The d’Arsonval movement is one of the common types of meters.
The d’Arsonval movement operates only on DC currents or rectified AC current.
Voltmeters have a high resistance and are designed to be connected directly across the power line.

21. Unit 9 Measuring Instruments
Review:
Ammeters have a low resistance and must be connected in series with a load.
Shunts are used to change the value of DC ammeters.
Clamp-on ammeters read only one conductor at a time.

22. Unit 9 Measuring Instruments
Review:
7. Ohmmeters are used to measure the resistance in a circuit.
8. There are two basic types of Ohmmeters: analog and digital.
9. Ohmmeters must never be connected to a circuit that has power applied to it.

23. Unit 9 Measuring Instruments
Review:
10. Digital multimeters display their value in digits instead of using a meter movement.
11. Digital ohmmeters measure resistance by measuring the voltage drop across an unknown resistor when a known amount of current flows through it.

24. Unit 9 Measuring Instruments
Review:
12. Low-impedance voltage testers (solenoid type) are not susceptible to indicating a negligible voltage caused by a high-impedance ground or feedback.