TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

Page 56
For a more accurate balance operate Sensitivity Key (SK) and readjust
potentiometer until voltmeter again reads zero.
The reading of the potentiometer scale multiplied by 1, 10 or 100,
according to the position of the range key, gives the value of the resistance
under test.
To confirm resistance measurements by the Wheatstone Bridge it is
desirable to operate the Reverse Battery Key
Resistance by Wheatstone Bridge Test
Fig. 19.
(RTB) after a balance has been obtained and verify that the meter remains
at zero. If the meter gives a reading other than zero this indicates that the
resistance under test is not a "pure" resistance, and probably contains a
rectifier element or is affected by some external voltage. If this condition
occurs it will generally be better to use the voltmeter method, described in
paragraph 6.
3. Line Resistance Tests by Wheatstone Bridge. (See Fig. 19).
Insert test plugs into the line to be tested and proceed as described
under Wheatstone Bridge in paragraph 2.
4. Earth Leakage Measurement, direct reading in ohms. (See Figure 19)
Proceed as in paragraph 3 but in addition operate EAK for measurement
of the resistance between line "A" and earth and key EBK for measurement
of the resistance between the "B" line and earth. The resistance of