1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook