A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 



167 



TABLE III 

 Data on Coils for Inductance Standards 



Frequency Range 



500 CM — 5,000 00 



5,000 ~ — 50,000 CO 



Overall Dimensions 



Diameter of case 



Length of case 



Inductance Characteristics 



Nominal value 



Change with frequency. . 



Change with current . . . . 



Temperature coefficient. 



Residual magnetization 

 effect of one ampere 



d^ 



Resistance Characteristics 



At an alternating current 

 (effective value) of 



2.0 milliamperes. . . 



lO.C) milliamperes. . . 



Temperature coeffi- 

 cient 



6J^ in. 

 4 in. 



0.100 henry 



+0.5% (500 cs. 



5,000 cv.) 



+0.01% per milliampere 

 -0.013% per deg. Fahr. 



Less than 0.01% 



1,000 ~ — 5.2 ohms 



.S,000 cs= — 8.5 " 



5,000 cv. — 13.0 " 



1,000 CM — 5.5 " 



3,000 CM — 9.1 " 



5,000 CM — 13.9 " 



-0.017% per deg. Fahr. at 

 3,000 CM 



3}4 in. 

 3H in. 



0.010 henry 

 +2.5%, (5,000 CM 



— 50,000 cm) 

 +0.007% per milliampere 

 -0.005% per deg. Fahr. 



Less than 0.01%, 



10,000 CM — 3.6 ohms 



30,000 CM — 13.6 " 



50,000 CM —34.0 " 



10,000 CM — 3.7 " 



30,000 CM — 13.9 " 



50,000 CM —34.7 " 



- < 0.01% per deg. Fahr. 

 at 30,000 CM 



Performance 



As was stated earlier, the operation of the bridge involves an initial 

 balancing of its capacitances. It is then ready for impedance testing 

 which is done by suitably connecting the unknown and standard 

 impedances to the proper terminals and adjusting the latter until a 

 balance of the bridge is obtained. The corresponding constants of 

 the two impedance arms are then taken as being equal. Those of the 

 standards being known, by calibration, it follows that those of the 

 impedance under test can be simply derived. The degree of precision 

 obtained depends upon two major factors, the accuracy of the calibra- 

 tion of the standards and the accuracy of the bridge comparison. 

 The matter of calibration is beyond the scope of this paper and it will 

 be assumed that a suitable calibration of the standards is available. 



Due to the construction used the factors determining the accuracy 

 of the bridge comparison of impedances are reduced to the following: 



1. The resistance balance of arms AB and AD. 



2. The effective shunt capacitance balance of these arms. 



