154 BELL SYSTEM TECHNICAL JOURNAL 



proper compromise between these conflicting factors the size and 

 impedance value of the part to be shielded must be considered. This 

 question will therefore be taken up in more detail in the following 

 section. 



Construction 



The circuit and shielding features discussed so far are of general 

 application to impedance measurements without restriction as to the 

 particular range of values to be tested or frequencies to be used. 

 The physical construction is, however, dependent upon these factors. 

 As initially stated, the bridge is intended for the measurement of 

 audio and carrier frequency inductances. By this is meant all appa- 

 ratus having reactance values nearly equal to the respective impedance 

 values. For the purpose of the present discussion, such inductances 

 will be more exactly defined as those having ratios of reactance to 

 resistance of not less than 10 (minimum phase angle of 84 deg., 20 

 min.). The difference between the reactance and the impedance of 

 any such inductance does not exceed ]4, per cent. The impedance 

 values range from about 100 to 10,000 ohms and testing frequencies 

 from 500 to 50,000 cycles. 



On the basis of these conditions, the following construction was 

 developed and is used for this bridge. 



Ratio Arms. It is desirable from the standpoint of sensitivity of 

 balance to have the ratio arm impedances of approximately the same 

 value as those of the other two arms. Considering the range of 

 impedances to be covered and giving due weight to the values which 

 are of most importance in telephone circuits, a ratio arm resistance of 

 1000 ohms was selected. The problem then was to construct two 

 1000-ohm resistances, balanced both as to effective resistance and 

 effective inductance for a frequency range from 500 to 50,000 cycles 

 when subjected to the usual temperature and humidity variations. 



Curtis and Grover have discussed the factors affecting the char- 

 acteristics of a-c. resistances and have suggested forms suitable for 

 general use at frequencies up to 3000 cycles.^ A 1000-ohm resistance, 

 constructed according to their specifications, is made by winding 

 with a 1/10-mm. diameter, double-silk-covered manganin resistance 

 wire, five 200-ohm bifilar sections on a 1-in. spool of insulating ma- 

 terial. These sections are spaced about three mm. apart on the spool 

 and are connected in series to form the 1000-ohm coil. Such a coil, 

 when shellacked, baked and coated with paraffin, was found to be 

 substantially constant in resistance and to have constant phase-angle 



^ H. L. Curtis and F. W. Grover: "Resistance Coils for Alternating Current 

 Work," Bulletin of the Bureau of Standards, Vol. 8, No. 3. 



