134 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



and effective resistance, to transmission characteristics in the frequency 

 band 20-174 kc. What was unusual were the extremely narrow limits 

 imposed and the number and variety of tests involved as compared to 

 those usually specified for commercial counterparts. 



The following two examples will serve to illustrate the extreme meas- 

 ures taken to prove the integrity of the product: 



(A) One type of Resistor was wound with No. 46 mandrelated nichrome 

 wire to a value of 100,000 ohms plus or minus 0.3 per cent. This resistor 

 received six checks for dc resistance, five for instantaneous stability of 

 resistance and two for distributed capacitance, at various steps in the 

 process which included six days' temperature cycling for mechanical 

 stabilization. This resistor was considered satisfactory, after final anal- 

 ysis of the test results, if: (a) The difference in any two of the six resist- 

 ance readings did not exceed 0.25 per cent, (b) The change in resistance 

 during cycling was not greater than 0.02 per cent, (c) The "instantaneous 

 stability" (maximum change during 30 seconds) did not vary more than 

 0.01 per cent. In addition, it was required that the distributed capaci- 

 tance, minimum 7, maximum 10 mmf, should not differ from any other 

 resistor by more than 2 mmf. 



(B) For high voltage paper capacitors, the 0.004-inch thick Kraft 

 paper, which constitutes the dielectric, was selected from the most 

 promising mill lots which the manufacturers had to offer. This selection 

 was based on the results obtained from tests that involve examination 

 for porosity, conducting material and conductivity of water extractions. 

 These tests were followed by the winding and impregnation in Halowax 

 of test capacitors. The test capacitors were then subjected to a direct 

 voltage endurance test at 266 degrees F for 24 hours. 



Samples of prospective lots of paper, which have passed the above 

 test, were then used to wind another group of test capacitors that were 

 subsequently impregnated with Aroclor and sealed. 1,500-volt dc was 

 then applied to the capacitors at 203° F for 500 hours. In case of failure, 

 a second sampling was permitted. 



After the foregoing tests had been passed, the supplier providing the 

 particular mill lot was authorized to slit the paper. Upon receipt, six 

 special capacitors were wound, using a group of six rolls of the paper 

 being qualified. These capacitors were then impregnated, checked for 

 dielectric strength at 3,000-volt dc, and measured for capacitance and 

 insulation resistance. The capacitors were then given an accelerated life 

 test at 2,000-volt dc, temperature 150° F, for 25 days. Each lot of six 

 satisfactory test capacitors qualified six rolls of paper for use. 



Product capacitors were then wound from approved paper, and the 

 dry units checked for dielectric strength at 300-volt dc. Capacitance 



