464 BELL SYSTEM TECHNICAL JOURNAL 



mutual capacitance measurements where the highest accuracy is not 

 essential and where conductance readings are not desired. D.C. 

 capacitance tests are made by the charge and discharge method, 

 employing a ballistic galvanometer. In general, D.C. capacitance 

 tests are not fully indicative of the characteristics of the cable at 

 telephonic frequencies and for this reason are not extensively employed. 



Fig. 32 — D-C. insulation resistance test desk — back view. 



Conductor resistance tests, Fig. 34, are made by means of a Wheatstone 

 bridge circuit specially arranged to read directly the conductor re- 

 sistance per mile at 68° F. 



Although the majority of mutual capacitance measurements are 

 made by means of the direct reading capacitance meter, the capacitance 

 and conductance of a percentage of all cables are measured at a 

 frequency of 900 cycles per second by means of the shielded capacitance 



