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Ammeter and voltmeter

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  • Ammeter and voltmeter

    If the resistance adds up in a series and is smaller in parallel, how come a voltmeter, wired in parallel, has a "very high resistance",
    while an ammeter, wired in series, has a very low one?

  • #2
    Both instruments are essentially galvanometers, which measure current flow employing the electromagnetic principle. To operate as an ammeter a small resistor is placed in parallel with the galvanometer, most of the current passes through this shunt resistor protecting the sensing coil and allowing it to read loads higher than it would otherwise.

    The voltmeter has a large resistor in series with it and is measuring the PD across the power source ideally not drawing any current, but it must do to provide the magnetic field.The size of the resistor is determined by the maximum value to be measured.

    The ammeter has relatively low resistance compared to the voltmeter, ideally zero resistance would be preferable so as not to influence the circuit current.

    So you see the reference in the manual to high and low resistances is referring to the construction of the meters rather than the method of summing resistances in series or parallel.

    Rgds

    Fraser

    BGS TKI

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    • #3
      Thank you Fraser.
      You know your stuff!

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