Therefore a voltmeter consists of an ammeter (always a milliammeter) in series with a resistance. The resistance is used as a fixed load to send known currents through the armature as different voltages are applies. The fixed load is known as a multiplier and it is a permanent part of a voltmeter.
A voltmeter is connected across a source of e.m.f. (in parallel) to measure its voltage. Likewise when measuring any voltage, the voltmeter is connected in parallel to the points where the difference in potential is measured.
In measuring voltage it is desirable to us a current meter which will draw very little current. Consequently, the multiplier must be of high resistance.
A galvanometer which is used in experimental and laboratory work is nothing more than a very sensitive ammeter, designed
(a)
(b)
Fig. 25
to read in milliamperes or microamperes. The principles involved in its construction are the same as those of the voltmeter and ammeter.
By using resistances or shunts of known values, a galvanometer can be used as a very accurate voltmeter or ammeter.
A.C. METERS
Up to this point we have been speaking of meters used in measuring D.C. currents. Different types of meters must be used with A.C. current--if an ordinary galvanometer were put in an A.C. circuit, the needle would tend to swing back and forth with each reversal of current flow--and if the frequency of the current was anything but very low, the reversals would be too
