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Measuring Potential Differences in a circuit

  1. May 29, 2008 #1
    Is there anyway of measuring the potential difference between 2 points on a electrical circuit other than using a voltmeter? The reason I'm raising this question is because Voltmeters are developed purely using Ohm's Law and so certainly Ohm might have had his own way of measuring the p.d. for formulating the law. So.. Essentially the question is.. How did Ohm measure p.d. without a voltmeter..? Thats interesting isn't it?
  2. jcsd
  3. May 29, 2008 #2
    Perhaps this URL may provide some insight. "http://www.sparkmuseum.com/ELECTROSCOPE.HTM"

    A typical (older) gold leaf electrometer had gold leaves attached to a metal plate (lid) of a capacitor (in the form of a container with glass separating a top plate from a bottom plate).

    After the electrometer (capacitor) was charged, both gold leaves acquired the same charge as the lid. Since like charges repel, electrostatic repulsion between the leafs caused them to separate.

    Note: The capacitor was necessary to retain leaf separation after removal of the charging body. In other words, without retention of voltage's energy, no voltage's energy would retain separation of the leaves.

    If a person acquires enough static electricity to make their hair stick out, perhaps the hair is similar to gold leaves. Perhaps if the person's head has a positive charge, their feet would be negatively charged, thus an attraction would exist between head and feet.
    Last edited: May 29, 2008
  4. May 29, 2008 #3
    Well.. But it does not answer my question..which was to know if there is a way of measuring p.d. across 2 points in a circuit other than using a volmeter and to measure current without an ammeter.. .. There certainly is a way cos..Ohm obviously wouldn't have used a votmeter or an ammeter(cos we make use of his law for devicing it!!:wink:) to get the relation between p.d. and the current through it..:confused:
  5. May 29, 2008 #4
    A voltmeter is any device that measures voltage, even if the device is something that measures kinetic energy of a point charge after voltage's acceleration.

    Some aspects of voltage's energy that can be used to determine existence and/or amount of voltage are:
    1. Volts can convert to magnetism, thermal energy, or particle acceleration. For example, magnetism in a mechanical meter's coil can cause its attached needle to deflect.
    2. Volts can switch on a transistor, for analog to digital or analog to analog conversions in an electronic voltmeter.
    3. Objects with voltage attract or repel each other. For example: a gold foil electrometer.
    4. Pushing objects with voltage toward and away from each other generates additional voltage (as happens in a Wimshurst generator).
    5. Volts can cause chemical reactions.
  6. May 29, 2008 #5


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    Excellent question and reasoning! It is hard or impossible to find the answer in textbooks.

    The experimental 'proof' palmed off on students, having them 'measure' the current through and voltage across a resistor, only proves at most that the ratio of the current through two resistors remains constant, since the 'voltmeter' is nothing but an ammeter with high resistance; then you have to ask how the two ammeters have been calibrated.

    I have never seen it said but I assume that Ohm must have used something like the torsion electrometer seen in pzlded's link. That is at some point you have to be able to measure electrical force by balancing it with a mechanical one. Anything else must be traceable back to that, various other instruments have been calibrated by that. E.g. the magnetic ones etc. mentioned in pzlded's second post.
  7. May 30, 2008 #6
    Most students harbor some misconceptions about point charges and magnetism. For example, many do not know that magnetism cannot accelerate point charges.

    I was thinking exposing the misconceptions as magnetism myths. Perhaps twenty would be enough to present within the education section of a physics conference, possibly as a talk or a poster session. Do you have any ideas or suggestions of things you would like included?
  8. May 30, 2008 #7


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    Perhaps these will help.

  9. May 30, 2008 #8


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    Nowadays the ammmeter is actually the least "fundamental" instrument from a metrological point of view since it is the only instrument that can't be calibrated directly. The calibration of a good voltmeter is traceble to national standards that consist of arrays of Josephson junctions that will output e.g. 1V when irradiated with microwaves of a known frequency (which in turn is traceble to the standard for the second).
    The same thing is true for resistance where the standard is based on the quantum hall effect.
    There are also a few other effects where you can have a known voltage even without knowing the current or resistance, but they are less precise.

    Anyway, the point is that it is NOT correct to say that you need to use Ohms law in order to measure voltage or resistance. Of course that doesn't answer the question about how Ohm did it; but an electrometer is probably a good guess.
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