This document discusses various types of measuring instruments used in electrical work. It describes the operation of analog meters like the d'Arsonval movement and how they differ from digital meters. It explains how to properly connect and read voltmeters, ammeters, ohmmeters, and digital multimeters. The key points are that d'Arsonval meters use a moving coil, voltmeters have high resistance and connect across circuits, ammeters have low resistance and connect in series, and ohmmeters must not be connected to live circuits.

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.

3. Basic d’Arsonval meter movement.
Unit 9
Measuring Instruments

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

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

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

7. 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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Measuring Instruments

8. Unit 9
Measuring Instruments
A shunt is used to set the value of the ammeter.

9. Unit 9
Measuring Instruments
Many electricians use the clamp-on type of AC
ammeter.
Please note the clamp-on ammeter reads only one
conductor at a time.

10. Unit 9
Measuring Instruments
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!

12. Unit 9
Measuring Instruments
Digital Ohmmeter

13. Unit 9
Measuring Instruments
Digital Voltmeter

14. Unit 9
Measuring Instruments
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.

15. Unit 9
Measuring Instruments
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.

16. Unit 9
Measuring Instruments
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.

17. Unit 9
Measuring Instruments
Low-impedance Voltage Tester
High-impedance ground paths can produce
misleading voltage readings.

18. Unit 9
Measuring Instruments
Low-impedance Voltage Tester

19. Unit 9
Measuring Instruments
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.

20. Unit 9
Measuring Instruments
Review:
1. The d’Arsonval movement is one of the
common types of meters.
2. The d’Arsonval movement operates only on
DC currents or rectified AC current.
3. Voltmeters have a high resistance and are
designed to be connected directly across
the power line.

21. Unit 9
Measuring Instruments
Review:
4. Ammeters have a low resistance and must
be connected in series with a load.
5. Shunts are used to change the value of DC
ammeters.
6. 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.