210 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



per nautical mile at 75°F. In practice, the spread of resistance values was 

 well within these limits. 



The 20-cycle core capacitance was also measured under constant- 

 temperature conditions. For this measurement, the two ends of the 

 central conductor were connected together and the measurements made 

 l)etween the central conductor and ground, which was provided by the 

 water. A capacitance-conductance bridge was used for this purpose. The 

 capacitance limits set initially were from 0.1726 to 0.1740 microfarads 

 per mile at 75°F. Analysis of the core measurements indicates that at 

 each factory the range of capacitance was held more closely than indi- 

 cated, which illustrates the benefits of servo control to the insulating 

 process. 



The dc insulation resistance of the core was also measured by applying 

 500 volts for one minute. A minimum insulation resistance requirement 

 of 100,000 megohm-miles at 75°F was established, but any lengths that 

 had less than 500,000 megohm-miles were scrutinized for possible sources 

 of trouble and were subject to rejection. As a general rule, insulation 

 resistances considerably in excess of 500,000 megohm-miles were ob- 

 tained. 



The core was tested also at a voltage of 90,000 volts dc for a period 

 of one minute. This test was designed to catch any gross faults in the 



SPRING-LOADED 

 ELECTRODES 



RECORDER 



Fig. 9 — SimpUfied block diagram of concentricity gauge for continuous meas- 

 urement of centering of central conductor. 



