Ammeters
and voltmeters, except electrostatic voltmeters, operate on the same principle.
The ammeters carry the current to be measured of a definite fraction of it and
this current or its definite fraction produces the deflecting torque whereas
voltmeter carries the current proportional to the voltage to be measured, which
produces the deflecting torque.
Ammeter,
which is connected in series with the circuit carrying the current under
measurement, must be of very low resistance so that the voltage drop across the
ammeter and power absorbed from the circuit are as low as possible.
Voltmeter,
which is connected in parallel with the circuit across which the voltage is to
be measured, must be of very high resistance so that the current flowing
through the voltmeter, and the power absorbed from the circuit are minimum
possible.
Apparently
it is, therefore, impracticable in most of the cases, and dangerous in some,
for an instrument manufactured as a voltmeter to be employed as an ammeter, or
alternatively to use an ammeter as a voltmeter. The low resistance winding of
an ammeter, will suffer serious damage if connected directly across a high
voltage source (i.e. more than a fraction of volt), and the high resistance
winding of the voltmeter will prevent all but the smallest current flowing and
therefore would normally be quite unsuitable to be used as an ammeter, except
of course in the special case where it is required to read only very small
currents.
However,
an ammeter of low range may be used as a voltmeter by connecting a high
resistance in series with it provided the current through the series combination
is within the range of the ammeter when connected across the voltage to be
measured.
