356 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1954 



order of 0.001 fifj-F and 0.003 mmF, respectively. Meeting requirements of 

 this order, even under controlled laboratory conditions, presents some 

 difficulties — and yet these requirements had to be met on a production 

 line, on moving cable in the climatic and operational conditions prevail- 

 ing in a large cable plant. 



It was evident, therefore, that conventional grounded-capacitance 

 measurements would not be practical. For instance, a shielded cable 

 connecting the probes with the bridge circuit alone could produce wider 

 random capacitance variations than the capacitance increments under 

 measurement. Thus a new system which would meet all the necessary 

 requirements had to be developed. 



2 CIRCUIT DESCRIPTION 



The measuring system which Avas developed consists of an impedance 

 bridge, a phase sensitive detector, an unbalance indicator (recorder), 

 capacitance probes and associated auxiliary equipment (See Figure 3). 



2.1 The Impedance Bridge for Grounded Direct Ca'pacitance Measure- 

 ments. 



The circuit shown on Fig. 4 employs a bridge having ratio arms 

 magnetically coupled. An application of this type of circuit for capaci- 

 tance measurements has been known for some time.' Such a circuit is 

 capable of performing in one balancing operation direct capacitance 

 measurements while the center point (B) of the transformer ratio-arms 

 winding is grounded. In our case, the ''D" corner of the bridge consists 

 of the metal covering of the cable core, which, as was mentioned above, 

 is necessarih' at the ground potential. Therefore, the "B" corner cannot 

 be grounded. However, by connecting to this ''B"-corner a shielding,^ 

 surrounding the "A-D" and "C-D" measuring arms, including cables 

 and probes, the following results can be achieved: 



(a) Admittances from the measuring electrodes to the "B"-shielding 

 are not critical. These admittances appear across the transformer-arms 

 and, as a result of a close magnetic coupling reaUzable between these 

 arms, any loading effects across any one of them are sjTnmetricall}^ re- 

 flected at the "A" and "C" corners of the bridge, thus essentially not 

 affecting its balance. 



(b) Stra}'' admittances from the "B" shielding to ground appear across 

 the opposite corners of the bridge (detector diagonal). Therefore, they 

 also have no essential effects on the circuit balance. 



(c) As a result of the "B "-shielding, stray admittances-to-ground 



