Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

From : Kimball, Arthur Lalane, A College Text-Book of Physics, 3rd ed., H. Holt and Co., New York (1923) pp.474-5.
© Copyright 1998 R. Paselk

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.
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.
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.
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
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.
Such a voltmeter cannot be used for alternating currents.

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