DIRECT CAPACITY MEASUREMENT 



25 



Maxwell Discharge Method', Fig. 5 



Connect the terminals between which the direct capacity C12 is 

 required, to A, B and the remaining accessible terminals of this 

 electrical system to D. The adjustable standard capacity is C and 

 any associated direct capacities in this standard are shown as C", 





y 4>— ♦ 



B 



VTmS^/!' 



Fig. 5 — Maxwell Discharge Method for Direct Capacity 



C". If C\2 is a direct capacity to ground, interchange C and C12. 

 Balancing involves the following repeated cycle of operations: 



1. Make connections 1, 2, 3 and 7 for an instant (thus charging 

 C12, Ci3, C", C and discharging the electrometer). 



2. Make connections 4, 5 and then 6 (to discharge condensers C13, 

 C" , mix charges of C12, C with polarities opposed and connect 

 electrometer). 



3. Adjust C to reduce the electrometer deflection when the cycle 

 is again repeated. 



When a null deflection is obtained Cn = C; the required direct 

 capacity is equal to the standard direct capacity irrespective of the 

 magnitudes of the four associated direct capacities. If all capacities 

 are free of leakage and absorption, this remarkable method accurately 

 compares two direct capacities by means of a single null setting, and 

 it requires the irreducible minimum amount of apparatus. 



Balanced-Terminal Capacity Measurement, Fig. 6 



This is defined as the direct capacity between two given terminals 

 with all other terminals left floating and ignored, after a hypothetical 

 redistribution of the total direct capacity from the given pair of 

 terminals to every third terminal which balances the two sides of the 

 pair. The balanced-terminal capacity, as thus defined, is equal to 

 the direct capacity between the pair augmented by one-quarter of 



• Electricity and Magnetism, v. 1, p. 350 (ed. 1892). 



