LRAD-COVRRRD PAPER-TNSULATKD TRLFPHONE CABLK 463 



insulation resistance, D.C. capacitance, A. C. capacitance meter, and 

 conductor resistance test desk which appears in the foreground of 

 Fig. 30. Insulation resistance measurements are made between 

 conductors and between all conductors and the sheath by observing 

 the deflection obtained with a high sensitivity reflecting type D'Arson- 

 val galvanometer through which a potential of 500 volts D.C, is 

 impressed on the insulation of the conductors under test. Due to the 



Fig. 31 — D-C. insulation resistance test desk — front view. 



high insulation resistances involved and the extreme sensitivity of the 

 measuring circuit, considerable difficulty is likely to be encountered 

 with leakage in the test apparatus itself, especially during times of high 

 relative humidity. To overcome this source of error special test 

 circuits have been designed which employ a shield (Fig. 33) to eliminate 

 from the measurement all extraneous leakage other than that of the 

 cable. The direct reading capacitance meter is used extensively for 



