32 BELL SYSTEM TECHNICAL JOURNAL 



one-half of the difference between the capacities on the two sides, as 

 explained above in connection with conductance. The capacity 

 sections employed to extend the range of the revolving condenser 

 include both air condensers C and mica condensers C" , the latter 

 being calibrated by means of the air condensers and the conductance 

 standard. 



A novel feature of our standard air condensers is a third terminal 

 called the leakage terminal, and indicated at L in Figs. 4, 9. Attached 

 to it are plates so arranged that all leakages either over, or through, 

 the dielectric supports from either of the two main terminals, must 

 pass to the leakage terminal. There can be no leakage directly 

 from one of the main terminals to the other. There is thus no phase 

 angle defect in the standard direct capacity due to leakage, and that 

 due to dielectric hysteresis in the insulating material is reduced to a 

 negligible amount by extending the leakage plates beyond the dielec- 

 tric, so as to intercept practically all lines of induction passing through 

 any support. This leakage terminal is connected to corner C of the 

 bridge; in the revolving condensers, it is one of the fixed plates. 



Direct Impedance Measurements 



The reciprocal of a direct admittance is naturally termed a direct 

 impedance; substituting impedance for capacity, the definition of 

 direct capacity, given above, becomes the definition of direct im- 

 pedance. The complete set of ciirect impedances constitutes an exact, 

 symmetrical, physical substitute for any given electrical system. 

 Direct impedances are often, in whole or in part, the most convenient 

 constants since many electrical networks are made up of, or ap- 

 proximate to, directly connected resistances and inductances. To 

 make direct impedance measurements which will not involve the 

 calculation of reciprocals, we naturally employ inductance and re- 

 sistance standards in series, the associated direct impedances being 

 eliminated as with direct capacities. 



Conclusion 



It has been necessary to preface the description of methods of 

 measuring direct capacities by definitions and a brief discussion, since 

 direct capacities receive but scant attention in text-books and hand- 

 books. By presenting direct capacities, direct admittances, and 

 direct impedances as alternative methods of stating the constants of 

 the same direct network, employed as an equivalent substitute for 

 any given electrical system, it is believed the discussion and measure- 



