Basic Electrical Theory
Subelement ZLB
Section ZLB13
Meters and Measuring
An ohmmeter measures the
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Correct Answervalue of any resistance placed between its terminals
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impedance of any component placed between its terminals
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power factor of any inductor or capacitor placed between its terminals
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voltage across any resistance placed between its terminals
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A VSWR meter switched to the "reverse" position provides an indication of
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power output in watts
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Correct Answerrelative reflected voltage
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relative forward voltage
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reflected power in dB
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The correct instrument for measuring the supply current to an amplifier is a
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wattmeter
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voltmeter
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Correct Answerammeter
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ohmmeter
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The following meter could be used to measure the power supply current drawn by a small hand-held transistorised receiver
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a power meter
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an RF ammeter
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Correct Answera DC ammeter
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an electrostatic voltmeter
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When measuring the current drawn by a light bulb from a DC supply, the meter will act in circuit as
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an insulator
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Correct Answera low value resistance
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a perfect conductor
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an extra current drain
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When measuring the current drawn by a receiver from a power supply, the current meter should be placed
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in parallel with both receiver power supply leads
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in parallel with one of the receiver power leads
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in series with both receiver power leads
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Correct Answerin series with one of the receiver power leads
Correct answer: in series with one of the receiver power leads
To measure current, the meter must be placed in series with the load so that all of the supply current flows through the meter.
In a receiver power circuit, this is done by opening one of the power leads (positive or negative) and inserting the ammeter in series at that point. The same current flows in both leads, so measuring either lead gives the correct value.
- in parallel with both receiver power supply leads would short the supply and could damage the meter or power supply.
- in parallel with one of the receiver power leads would not measure load current and may create a short circuit.
- in series with both receiver power leads is unnecessary and impractical, since the same current already flows through each lead.
Therefore, the current meter should be placed in series with one of the receiver power leads.
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An ammeter should not be connected directly across the terminals of a 12 volt car battery because
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Correct Answerthe resulting high current will probably destroy the ammeter
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no current will flow because no other components are in the circuit
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the battery voltage will be too low for a measurable current to flow
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the battery voltage will be too high for a measurable current to flow
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A good ammeter should have
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a very high internal resistance
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a resistance equal to that of all other components in the circuit
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Correct Answera very low internal resistance
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an infinite resistance
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A good voltmeter should have
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Correct Answera very high internal resistance
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a resistance equal to that of all other components in the circuit
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a very low internal resistance
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an inductive reactance
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An rms-reading voltmeter is used to measure a 50 Hz sinewave of known peak voltage 14 volt. The meter reading will be about
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14 volt
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28 volt
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Correct Answer10 volt
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50 volt
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