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How does a voltmeter work?

  1. Jan 21, 2016 #1

    Suraj M

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    In a recent Viva one of my friends was asked this- " Why don't we use a normal Voltmeter instead of converting a galvanometer into a voltmeter?"
    I didn't quite understand what the examiner meant
    Isn't a galvanometer with a high resistance is what present inside a voltmeter?
    Same goes for an ammeter( except shunt low and in parallel)
    So what should I answer for this question?
     
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  3. Jan 21, 2016 #2

    sophiecentaur

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    This is an 'Engineering' question, based on practicability. Look at the way a galvanometer works and you will see that it's different from a 'normal' Ammeter meter movement. It's basically the same but it is far more sensitive. The return spring is extremely weak (often just a thin ribbon suspension). Just not robust enough for everyday use. If you try to carry a sensitive galvo', its needle will wave all over the place. It has no place in an electrician's carry case.
     
  4. Jan 21, 2016 #3

    Suraj M

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    Well isn't that a bit contradicting
    The question is why we don't use a voltmeter? It was asked in an experiment where we had to convert a galvanometer into voltmeter,
     
  5. Jan 21, 2016 #4

    sophiecentaur

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    Yes, I take your point but the question is not really complete enough for a proper answer. What was the application? How did the question arise in the conversation? What is a "normal Voltmeter"? Nowadays, it would be normal to use an electronic system for measuring volts, if the circumstances are at all unusual. A bit more background about the OP could be useful, I think. The examiner had something in mind and it's not clear from the wording of the OP.
     
  6. Jan 21, 2016 #5

    Svein

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    So, what is a "normal voltmeter" then? Historically, you have something called an Electroscope (https://en.wikipedia.org/wiki/Electroscope), but that only showed the existence of a potential, not the magnitude. Therefore, given the precision of the galvanometer, it was easy to insert a resistance in series and recalibrate the scale in volts (since V = R⋅I). Today, we have better methods, but we still have to interfere with what we are measuring.
     
  7. Jan 21, 2016 #6

    Suraj M

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    Honestly Sophie that's all the examiner said
    Exact words
    I'm in 12th grade and the experiment was just to convert the galvanometer Into a voltmeter using a high resistance and verifying the readings by varying the rheostat, that's it.
    The question didn't make much sense to me though, I used to think what goes on inside a voltmeter is the same as our setup, isn't it?
     
  8. Jan 21, 2016 #7

    sophiecentaur

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    OK - no context to the question. Well then, I think my answer about ruggedness is the only one that can apply as a galvo is really just a very sensitive meter with a light suspension and virtually no return spring.
    Perhaps the examiner was after the answer "they are both virtually the same and it's just the details that distinguish one from the other"
    Interviewers can be human, you know and they can often use an examination as an educational tool, rather than a test.
     
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