Measuring Instruments--part 6: Summary/Quiz

Summary

• The d'Arsonval type of meter movement is based on the principle that like magnetic fields repel.
• The d'Arsonval movement operates only on DC.
• Voltmeters have a high resistance and are designed to be connected directly across the power line.
• The steps to reading a meter are as follows:
• a. Determine what quantity the meter is set to measure.
• b. Determine the full-range value of the meter.
• c. Read the meter.
• Ammeters have a low resistance and must be connected in series with a load to limit the flow of current.
• Shunts are used to change the value of DC ammeters.
• AC ammeters use a current transformer to change the range setting.
• Clamp-on ammeters measure the flow of current by measuring the strength of the magnetic field around a conductor.
• Ohmmeters are used to measure the resistance in a circuit.
• Ohmmeters contain an internal power source, generally batteries.
• Ohmmeters must never be connected to a circuit that has power applied to it.
• Digital multimeters display their value in digits instead of using a meter movement.
• Digital multimeters generally have an input impedance of 10 megohms on all ranges.
• The oscilloscope measures the amplitude of voltage with respect to time.
• The frequency of a waveform can be determined by dividing 1 by the time of one cycle ( f = 1/t).
• Wattmeters contain a stationary coil and a movable coil.
• The stationary coil of a wattmeter is connected in series with the load, and the moving coil is connected to the line voltage.
• The turning force of the dynamic wattmeter is proportional to the strength of the magnetic field of the stationary coil and the strength of the magnetic field of the moving coil.
• Digital ohmmeters measure resistance by measuring the voltage drop across an unknown resistor when a known amount of current flows through it.
• Low-impedance voltage testers are not susceptible to indicating a voltage caused by a high-impedance ground or a feedback.
• A bridge circuit can be used to accurately measure values of resistance, inductance, and capacitance.

QUIZ:

1. To what is the turning force of a d'Arsonval meter movement proportional?

2. What type of voltage must be connected to a d'Arsonval meter movement?

3. A DC voltmeter has a resistance of 20,000 V per volt. What is the resistance of the meter if the range selection switch is set on the 250-V range?

4. What is the purpose of an ammeter shunt?

5. Name two methods used to make a DC multirange ammeter.

6. How is an ammeter connected into a circuit?

7. How is a voltmeter connected into a circuit?

8. An ammeter shunt has a voltage drop of 50 mV when 50 A of current flow through it. What is the resistance of the shunt?

9. What type of meter contains its own separate power source?

10. What electrical quantity does the oscilloscope measure?

11. What is measured on the Y axis of an oscilloscope?

12. What is measured on the X axis of an oscilloscope?

13. A waveform shown on the display of an oscilloscope completes one cycle in 50 ms. What is the frequency of the waveform?

14. What is the major difference between a wattmeter and a d'Arsonval meter?

15. What two factors determine the turning force of a wattmeter?

+++++ The multirange voltmeter operates by connecting different values of resistance in series with the meter movement.

+++++ Ayrton shunt. DC ammeter.

On-the-Job Assignments and Practical Applications:

Measuring Instruments:

1. A d'Arsonval meter movement has a full-scale current value of 100 mA (0.000100 A) and a resistance of = kV (5000 V). What size resistor must be placed in series with this meter to permit it to indicate 10 V full scale?

2. The meter movement described in (1) is to be used to construct a multirange voltmeter. The meter is to have voltage ranges of 15 V, 60 V, 150 V, and 300 V. Find the values of R1, R2, R3, and R4.

On-the-Job Assignments and Practical Applications:

You are an electrician on the job. You have been give a multimeter that has the following AC voltage ranges: 30, 60, and 150. The meter states that it has a resistance of 5000 V/ V. You need to be able to measure a voltage of 277 volts. How much resistance should be inserted in series with the meter to make the 30-volt range indicate a full scale value of 300 volts?

3. A meter movement has a full-scale value of 500 mA (0.000500 A) and 50 mV (0.050 V). A shunt is to be connected to the meter that permits it to have a full-scale current value of 2 A. What is the resistance of the shunt?

4. The meter movement in Question 3 is to be used as a multirange ammeter.

An Ayrton shunt is to be used to provide full-scale current ranges of = A, 1 A, and 0.5 A (). Find the values of R1, R2, and R3.

5. A digital voltmeter indicates a voltage of 2.5V when 10 mA of current flow through a resistor. What is the resistance of the resistor?

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