Electrical Power Systems Technology

Power Measurement Equipment 63

standard resistance (Rs) is marked on the meter, so that the value of the un-

known resistance (Rx) can be determined. Resistors R 1 and R 2 form the “ra-

tio arm” of the indicator. The value of an unknown resistance (Rx) may be

mathematically expressed as:

R 1

Rx = — × Rs

R 2

Sample Problem:

Given: in the bridge circuit of Figure 3-3, R 1 = 500 ohms, R 2 = 2000

ohms, and Rs = 1000 ohms.

Find: the value of unknown resistance (Rx).

Solution:

R 1

Rx = — × Rs

R 2

5002

——— × 1000 Ω

20000

Rx = 250 Ohms

The Wheatstone bridge will measure most values of resistance with

considerable accuracy. Several other types of comparative instruments

also use the Wheatstone bridge principle.

Cathode Ray Tube (CRT) Instruments

cathode ray tube display instrument, A usually called an oscilloscope, is

an important type of instrument. Using an oscilloscope makes it possible

to visually monitor the voltages of a system. The basic operational part of

the oscilloscope is the cathode ray tube. Figure 3-4 shows the construction

and electron gun arrangement of a CRT.

Several different types of CRT display instruments are available.

General-purpose oscilloscopes are used for electronic servicing and for dis-

playing simple types of waveforms. Triggered-sweep oscilloscopes are used

when it is preferable to apply an external voltage to the oscilloscope for

comparative purposes. Other oscilloscopes, classified as laboratory types,

have very high sensitivity and good frequency response over wide ranges.

A CRT display instrument may be used to measure AC and DC voltages,

frequency, and phase relationships, and for various timing and numeri-