The Measurement of Small Capacities 85 



Connect up as in the figure, leaving out C. If R3, R3' are the values 

 of R, R', when balance is obtained, we have, 



C7C" = R37R3. (3) 



Now, in place of C, put in a known capacity Ci. If R4, R/ and Rb* 

 R'5 are the values of R, R' when balance is obtained with Ci connected 

 to B and A respectively, we have, 



(Ci + C0/C' = R4/R/, ^ (4) 



and, 



(Ci+C')/C" = RVR5. (5) 



Equations (3), (4), and (5) give two determinations of C and C", 

 the mean of which may be taken. 



Taking as standard a Muirhead concentric cylinder air condenser, 

 whose capacity, as found by measurement, was 0.00520 mfd., the follow- 

 ing capacities were found for one, two, and three ordinary ten inch sus- 

 pension-type insulators. 



Table I. 



Lead 

 Capacity, Capacity, Net Capacity, 



Specimen Micro-mfd. Micro-mfd. Micro-mfd. Cm. 



A 29.6 7.5 ^ 22.1 19.9 



B 33.4 7.5 25.9 23.3 



Mean for one 



Insulator. 24.0 21.6 



AandB Series 18.4 7.5 10.9 9.8 



C and D Series 20.3 7.5 12.8 11.5 

 Mean for two 



Insulators 11.8 10.7 



B, C and D Series 16.5 7.5 9.0 8.1 



II. — Comparison with a Guard-Ring Condenser 



The Muirhead condenser had over one hundred times the capacity 

 of a single insulator; to obtain more accurate results, it was found 

 necessary to compare the insulator capacities with a capacity of approxi- 

 mately the same value, and a special guard-ring condenser had to be 

 built. In addition, the capacity between the leads and the sheath was 

 not strictly constant, so that this capacity had to be removed to a part 

 of the bridge where a small variation in it would have no appreciable 

 effect. 



