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- From : Kimball, Arthur Lalane, A College Text-Book
of Physics, 3rd ed., H. Holt and Co., New York (1923) pp.474-5.
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- © Copyright 1998 R. Paselk
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- 697. Current Voltmeters. - A voltmeter using current is a high-resistance
galvanometer with a scale graduated to give directly the number
of volts difference in potential between its terminals.
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- The voltmeter shown in figure 403 is a moving-coil galvanometer such as
is used in the ammeter shown in figure
401, but there is no shunt across between the terminals as
in the ammeter, and a considerable resistance is inserted in
the circuit so that only a small current passes through the instrument.
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- Voltmeters using current give correct
values only in circumstances where the current through the instrument
is so small that it does not appreciably change the potentials
to be measured.
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- For instance, the difference of potential
of two statically charged bodies could not be determined by such
an instrument, for they would be instantly discharged through
it. And if we attempt to measure the difference of potential
of the terminals of a battery cell whose internal resistance
is as great as that of the voltmeter itself, the deflection will
indicate only one-half the total electromotive force of the cell,
for the current is such that half the fall in potential takes
place in the cell itself
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- In ordinary commercial work the other resistances
in the circuit are so small compared with that of a well-constructed
voltmeter that there is no difficulty on this score.
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- Such a voltmeter cannot be used for alternating
currents.
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- © R. Paselk
- Last modified 22 July 2000
