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Ammeter and voltmeter in circuit

  1. Nov 1, 2005 #1


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    Why is that when I introduce an ammeter or a voltmeter into a circuit, it alters it. In other words how does a *meter affect the circuit? I read somewhere that when you measure the resistance [using an ammeter], it is slightly different from the actual value. Why does this happen?

    And one more question. I did an experiment and they said that I should neglect the resistace of contacting leads used to hook up the circuit. That must mean that it could affect my results then. How so?

    --thank you.:cool:
  2. jcsd
  3. Nov 1, 2005 #2


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    The ampmeter has a shunt of known resistance and can determine the current simply by the voltage drop across that known resistance (or hall effect).

    The voltmeter has a high internal resistance of something like 1 MOhm but in some cases this will affect the circuit as well. And some voltmeters will have a much lower internal resistance.

    Look up Heisenberg's Uncertainty Principle about how the more you know about a subatomic particle's speed the less you know about its position or vice-versa. Its about how just observing the particle changes it.

    Hopefully the effect is minimal and can be discarded.

    Resistors have a tolerance (meaning they will vary a certain percent from the number on them), and meter's have only a certain level of precision as well with most being only good to a certain percentage of the value displayed.

    So if you measured 7.69V one time and 7.61V the next, that is easily within the range of values that should be considered the same because it is smaller than the margin of error that is likely +/- 0.1V or more.
  4. Nov 2, 2005 #3


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    Thanks for the input Cliff_J.
  5. Nov 4, 2005 #4
    See in science you must know that nothing is just accurate, you must accept to have some tolerance somewhere and the amount of tolerance you have mainly depends on what your working on, in very simple circuits you can have about 5% to 10% tolerance of the values written and measured but in more complicated circuits like VLSI & ULSI ,tolerance tends to be very small so the type's of instrument you use to measure differ from that your using right now.
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