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Subelement L
Installation, Maintenance & Repair
Section 74
Indicating Meters
What is a 1/2 digit on a DMM?
  • Smaller physical readout on the left side of the display.
  • Correct Answer
    Partial extended accuracy on lower part of the range.
  • Smaller physical readout on the right side.
  • Does not apply to DMMs.
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A 50 microampere meter movement has an internal resistance of 2,000 ohms. What applied voltage is required to indicate half-scale deflection?
  • 0.01 volts
  • 0.10 volts
  • 0.005 volts.
  • Correct Answer
    0.05 volts.

50 microamps is full scale. Half scale would be 25 microamps.
Use Ohm's Law to calculate voltage:

   V = (25/1000000 amps) x 2000 ohms = 0.05 volts

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What is the purpose of a series multiplier resistor used with a voltmeter?
  • Correct Answer
    It is used to increase the voltage-indicating range of the voltmeter.
  • A multiplier resistor is not used with a voltmeter.
  • It is used to decrease the voltage-indicating range of the voltmeter.
  • It is used to increase the current-indicating range of an ammeter, not a voltmeter.
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What is the purpose of a shunt resistor used with an ammeter?
  • A shunt resistor is not used with an ammeter.
  • It is used to decrease the ampere indicating range of the ammeter.
  • Correct Answer
    It is used to increase the ampere indicating range of the ammeter.
  • It is used to increase the voltage indicating range of the voltmeter, not the ammeter.
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What instrument is used to indicate high and low digital voltage states?
  • Ohmmeter.
  • Correct Answer
    Logic probe.
  • Megger.
  • Signal strength meter.
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What instrument may be used to verify proper radio antenna functioning?
  • Digital ohm meter.
  • Hewlett-Packard frequency meter.
  • Correct Answer
    An SWR meter.
  • Different radio.

SWR = Standing Wave Ratio.

The Standing Wave Ratio is defined by wikipedia as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line.

Put in simpler terms, one of the most common ways to measure SWR is to use a directional wattmeter and measure the amount of power from the transmitter to the antenna (\(P_f\) = Forward power) as well as the amount of power coming back from the antenna to the transmitter (\(P_r\) = Reflected power).

The SWR can then be calculated using the formula:

\[\text{SWR} = \frac{1+\sqrt{P_r\over P_f}}{1-\sqrt{P_r\over P_f}}\]

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