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V=Ed voltmeter

  1. Nov 1, 2006 #1
    Do voltmeters work by sampling current from two points in a circuit and output the voltage or potential difference between the two points? Must there be resistor in the middle of the two points? Since V=Ed. there dosen't have to be does there?

    Sometimes the voltmeter gives a negative reading. Why is that?
     
  2. jcsd
  3. Nov 1, 2006 #2

    Integral

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    Since voltmeters measure a potential difference, if there is no resistor (or some other voltage dropping device) there would be no potential difference between the leads resulting in a reading of zero. So there only needs to be a resistor between the leads if you want a non zero voltage reading.

    Voltmeters have red and black leads, generally the black lead is connected to the common terminal of the meter and the point at a lower potential in the circuit. The red is connected to the V+ terminal of the meter and to the higher potential in the circuit, this results in a positive reading. If the black lead is at a higher potential then the red, you will get a negative reading. It really does not matter in a digital meter, however, if you ever get your hands on an old fashioned meter with a D'Arsonal meter movement, you can damage the meter if it is connected backward.
     
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