DIRECT CAPACITY MEASUREMENT 27 



Any failure to afljust condenser / to perfectly balance the given 

 pair of terminals will decrease the measured capacity Ce,. This fact 

 may be utilized to measure the capacity with the second bridge ar- 

 rangement alone (contacts a, a' open and b closed) by adjusting con- 

 denser / so as to make the reading C" of condenser II a maximum. 

 This procedure presents no difficulty, since the correct setting for 

 condenser / lies midway between its two possible settings for a balance 

 with any given setting of condenser //; furthermore, C" is not sensi- 

 tive to small deviations from a true balance in C. 



Balanced-terminal capacity is of practical importance as a measure 

 of the transmission efficiency to be expected from a metallic circuit, 

 if it is subsequently transposed so as to balance it to every other 

 conductor. In practice, when the unbalance of the section of open 

 wire or cable pair, which is being measured, is relatively small, it is 

 sufficient to set condenser /, once for all, to balance the bridge itself 

 and ignore the unbalance of the pair. This favors an unbalanced 

 pair, however, by the amount [Gi — GiY/^ {On -f Geo) where 

 Gi2 + GcD is the grounded capacity of the pair augmented by that 

 of the bridge.^ For rapid working, condenser II is graduated to read 

 2C" and by auxiliary adjustment Cg is made zero, so that the re- 

 quired capacity is read directly from the balance. 



Additional Methods of Measuring 

 Direct Capacity 



Measurement of the capacity between the terminals, taken in 

 pairs with all the remaining terminals left insulated or floating, gives 

 n(n — l)/2 independent results, from which all the direct capacities 

 may be derived by calculation of certain determinants^. Practically, 

 however, we are in general interested in determining individual direct 

 capacities from the smallest possible number of measurements, and 

 the first step is naturally to connect all of the remaining conductors 

 together, so as to reduce the system to two direct capacities in addi- 

 tion to the one the value of which is required. Three measurements 

 are then the maximum number required, and we know that two, or 

 even one, is sufficient if particular devices are employed. 



The three measurement method of determining direct capacities 

 from the grounded capacities of the two terminals taken separately 

 Gi, Gi, and together G12, is given by Maxwell.'" If d = €', Gn = 

 a + C\ and G2 = C" + C"\ 



* See appendix, section 6. 

 ' See appendix, section 7. 

 "/6t(/., p. 110. 



