20 BELL SYSTEM TECHNICAL JOURNAL 



circuit, with three corners 1, 2, 3 assumed to be accessible, and the 

 fourth inaccessible, or concealed. Generalizing, we have the following 

 definition: 



The direct capacities of an electrical system with n given accessible 

 terminals are defined as the n{n — i) /2 capacities which, connected 

 between each pair of terminals, will be the exact equivalent of the system, 

 in its external reaction upon any other electrical system with which it is 

 associated only by conductive connections through the accessible terminals. 



r \P--L '^^^t 



w 



c.c 



1^2 



^1 ^2 Ci+Co 



(1>HI-HKD - © ^1 © 



^14^24 



Fig. 1 — Equivalent Direct Capacities, d = C^ -\- C24 + C34 = Grounded Capacity 



of Branch-Point 4 



The total direct capacity between any group of the terminals and all 

 of the remaining accessible terminals, connected together, is called 

 the grounded capacity of the group. 



This definition of direct capacity presents the complete set of direct 

 capacities as constituting an exact, symmetrical, realizable physical 

 substitute for the given electrical system for all purposes, including 

 practical applications. Direct capacities are Maxwell's " coefificients 

 of mutual induction," but with the sign reversed, their number being 

 increased so as to include a direct capacity between each pair of 

 terminals. 



In considering direct capacities we exclude any direct coupling, 

 either magnetic or electric, from without with the interior of the 

 electrical system, since we have no concern with its internal structure; 

 we are restricted to its accessible, peripheral points or terminals; 

 some care has been taken to emphasize this in the wording of the 

 definition. 



