62 Electrical Power Systems Technology
multifunction meters.
The basic meter movement may be modified so that it will measure
almost any electrical or physical quantity.
Comparative Instruments
Another group of measuring instruments can be classified as compar-
ative instruments. Usually, a comparative instrument is designed to com-
pare a component of known value to one with an unknown value. The ac-
curacy of comparative instruments is ordinarily much better than that of
analog instruments, described above.
A Wheatstone bridge is a typical type of comparative instrument. The
technique used for measurement with a Wheatstone bridge is illustrat-
ed in Figure 3-3. A voltage source is used in conjunction with a resistive
bridge circuit and a sensitive zero-centered moving-coil meter movement.
The bridge circuit is completed by adding the external unknown resistance
(Rx) that is to be measured. When Rs is adjusted, so that the resistive path
formed by Rx and Rs is equal to the path formed by R 1 and R 2 no current
will flow through the meter. In this condition, the meter will indicate zero
(referred to as a null), and the bridge is said to be balanced. The value of
Figure 3-2. Operating principle of a meter movement