462 BELL SYSTEM TECHNICAL JOURNAL 



Table II shows the imaginary type subdivided into two subtypes, 

 depending on the sign of the angle. 



As an example of the use of this table: Suppose it is desired to 

 measure the series resistance and inductance of an unknown im- 

 pedance. This may be done by using adjustable standards of series 

 resistance and inductance, series resistance and capacitance, parallel 

 resistance and capacitance, or parallel resistance and inductance, by 

 choosing the particular type of bridge for the purpose. For instance, 

 referring to Table II, if it is desired to measure the series resistance in 

 terms of conductance, and the series inductance in terms of parallel 

 capacitance, the product arm bridge with real ratio, that is either 

 Fig. 2// or 2i, would be used. 



Since there are six types of balance equations given in Table II, 

 it follows that five of the circuits of Fig. 2 are duplicates of others from 

 the standpoint of the balance equations which they give. For instance, 

 there is no difference whatever in the theoretical operation of the 

 bridges of Figs. 2a, 2b, and 2c. The choice must be determined entirely 

 from other considerations. In the same way, as indicated by the figures 

 tabulated in Table II, Figs. 2d and 2/ give identical results as do 

 Figs. 2e and 2g, and Figs. 2h and 2i. From the practical standpoint, 

 there may be, and actually there is, considerable difference in the merits 

 of these different forms. At this time, we may simply state that 

 where a choice is possible, resistance is the preferred form of fixed arm 

 and capacitance is preferred to inductance. This allows us to choose 

 our preferred forms as Fig. 2a, Fig. 2d, Fig. 2e, and Fig. 2h. 



A study of Table II shows that bridges of fixed ratio arm type 

 always measure the series components of the unknown in terms of 

 series components of the standard and, conversely, they measure the 

 parallel components in terms of parallel components of the standard. 

 Bridges of product arm type measure the series component of the 

 unknown in terms of parallel components of the standard and con- 

 versely. 



None of the balance equations of Table II includes frequency, that 

 is, all of them allow the evaluation of each component of the unknown 

 directly in terms of a corresponding component of the standard with 

 the exception that in some cases the relation is a reciprocal one. 

 Practically any form of standard may be chosen in order to measure 

 a given type of unknown impedance. 



Practical Considerations 



So far the question whether the requirements for the fixed arm 

 impedances given in Fig. 2 can be met in practice has not been con- 



