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Potentiometer vs Voltmeter.Which should be preferred for greater accuracy?

  1. Oct 21, 2011 #1
    I want to find out the potential difference between two given points with maximum accuracy. Which one should i choose and why.
    I think both will work fine in most of the cases.
     
  2. jcsd
  3. Oct 21, 2011 #2

    xts

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    Could you be a bit more specific?

    Are you going to measure voltage (potential difference) between two points of some electric circuit, or measure electrostatic potentials between points in some configuration of static charges, where no current actually flows?

    The answers may be diametrically different for various experimental setups. Please provide detailed description of the system you want to measure.
     
  4. Oct 21, 2011 #3
    Here i am going to measure potential difference between the points in a electric circuit.
     
  5. Oct 21, 2011 #4

    xts

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    So just use the best voltmeter you can afford. Depending on the properties of your circuit (its characteristic resistance, its vulnerability to parasitic currents), pay attention either to high resistance of the voltmeter, or to its high accuracy.

    If your circuit operates on high frequency rather than DC - pay also attention to parasitic capacitance introduced by your voltmeter.
     
  6. Oct 21, 2011 #5
    thanks!
    I am not going to buy a voltmeter as you are saying.
    Still now i am doing questions based on voltmeter from my book. In a question i remove voltmeter check current then add voltmeter and then calculate current. Due to resistance of voltmeter there came a change in circuit current which makes little changes. however very high resistance of voltmeter can minimize it.
    Now i think i have got the point.
     
  7. Oct 21, 2011 #6

    xts

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    Actually modern electronic voltmeters utilise idea of potentiometr - they generate some voltage (and display it), compare it with the voltage to be measured, and adjust generated voltage to match the measured one.

    In idealistic platonic world of school physics, voltmeter (which measures a current flowing through some high resistance resistor) always disturb the measured circuit, while potentiometer+microampermeter doesn't disturb it at all when is balanced.
    But that is idealistic view - in reality our tools always disturb measured circuits, and those devices are designed such to disturb it as little as possible - but we have different tools optimised for different purposes. And, of course, we do always balance between price we are ready to pay for the device and acceptable disturbance.
     
  8. Oct 21, 2011 #7
    !!Nothing is ideal in real world; !!
     
  9. Oct 21, 2011 #8

    sophiecentaur

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    But when you're talking about Electricity and Electronics you are in the Engineers' World.
    In that world, near enough is real enough. It gets the job done, which is what most of life is about.

    Even a Potentiometer will take some current - because the point of balance is still not certain. The zero that you detect will still have a degree of imprecision about it, which can give a measurement error of the PD.
     
  10. Oct 21, 2011 #9
    Of course potentiometer.
    Voltmeter deflects to show reading. That deflection requires electrical energy.
    But in potentiometer there is no deflection. No deflection means no utilization of electrical energy. This suggests that voltmeter is useless for accurate readings.
    Potentiometer and wheat stone bridge which use balanced circuit for measuring potential and resistance respectively were designed just to eliminate unnecessary errors of galvanometers and all.
    But none of these are 100 pc accurate.
     
  11. Oct 21, 2011 #10

    sophiecentaur

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    Yes but you missed my point that you cannot detect an exact zero. There will be a current that is small enough not to be detected by your 'galvanometer'. You could be a small amount either side of the true zero. This is fact of measurement that you can't avoid - in the end, it's a signal to noise limit.

    A voltmeter can have as high an input resistance as you want, using feedback. Feedback is just what you are using, in fact, when you wiggle the potentiometer position - with the feedback signal being what you see on the galvanometer and the feedback amp being You. So "Useless" is rather a harsh word. Along with every other form of measurement, a voltmeter has its limitations.
    I would frankly be happier with the answer that a good voltmeter gave me than what I obtained with a potentiometer - with all its mechanical problems - like 'where is the actual position of the contact on the variable resistance?' and 'how uniform is the resistance track?'.
     
  12. Oct 21, 2011 #11

    xts

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    @Himal
    I see you live in a school world. Of course - in this world voltmeters always draw some current, while potentiometric bridges take no current at all.
    The differences between school world and real world are:
    - (as Sophiecentaur pointed) you never balance a bridge to zero current, so simple bridges also draw some current;
    - electronic voltmeters utilise potentiometric bridge idea, so they also draw only residual current;
    - modern measuring tools (even those 5€ boxes from Castorama) are pretty well optimised for their purpose, so I bet that for any budget (up to 10,000 € - above that we are entering an area of very specialised devices dedicated to single purpose) standard voltmeter in that price range will measure voltage better than microampermeter+potentiometer you may achieve for the same price.
     
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