Simple potentiometric voltmeter question

Discussion in 'General Electronics Chat' started by thori, Jul 1, 2008.

  1. thori

    Thread Starter New Member

    Jul 1, 2008
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    0
    Hello everyone,

    This is probably a very elementary problem I've encountered. In Chapter 8, part 3 (Voltmeter impact on measured circuit, http://www.allaboutcircuits.com/vol_1/chpt_8/3.html ), I'm confused about how the potentiometric voltmeter employs Kirchhoff's Voltage Law. The text states,

    "Likewise, the null detector simply indicates when the voltage between points 1 and 2 are equal, which (according to Kirchhoff's Voltage Law) will be when the adjustable voltage source (the battery symbol with a diagonal arrow going through it) is precisely equal in voltage to the drop across R2."

    How does KVL apply here? The definition only states equality to zero of the algebraic sum of voltages. Is the text speaking in terms of the loop 1-2-1, or what's the deal?

    Thanks for your help!
     
  2. thori

    Thread Starter New Member

    Jul 1, 2008
    3
    0
    I forgot to ask,

    Once the voltages are equal, there should be no current flowing through the voltmeter circuit. Is this because the voltmeter's power source resists current flow, or simply because there is no potential drop in the voltmeter circuit? As you can tell, I'm quite new to all this.

    Thanks!
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It is just as simple as it appears. When two points have the same voltage, there is no potential difference to cause a current flow. The potentiometric meter will be invisible to the circuit being measured. Since it does not affect the voitage across R2 at null, the meter circuit does not in any way change the circuit.
     
  4. thori

    Thread Starter New Member

    Jul 1, 2008
    3
    0
    OK, cool. It is a pretty elegant solution.

    Thanks for the reply!
     
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