464 BELL SYSTEM TECHNICAL JOURNAL 



capacitance, but we then have a form of bridge which is, in effect, a 

 compromise between Figs. 2/ and 2g, and Figs. 2d and 2e, which 

 has no practical advantages over the latter. Accordingly, the forms 

 of Figs. 2/ and 2g must be considered impractical, particularly as 

 Figs. 2d and 2e give identical performance. 



In the case of the product arm type the requirements can be met 

 by Fig. 2h and can be met by Fig. 2i by adding a conductance in 

 shunt with the capacitance to compensate for the series resistance of 

 the inductance. However, even though this allows us to meet the 

 requirement, this form is less satisfactory than that of Fig. 2/; due to 

 the difficulty of designing an inductance standard having inductance 

 and series resistance invariable over an appreciable frequency range. 

 Again the requirements can be readily met by Fig. 2k, but in the case 

 of Fig. 2j series resistance of the inductance can be corrected only by 

 shunting the resistance arm by pure inductance, which is impractical. 

 This is unfortunate since it rules out one form of bridge for which there 

 is no duplicate and, consequently, makes the measurement of inductive 

 impedances by bridges of this type impractical. 



Summarizing the above, practical considerations rule out Figs. 2/, 

 2g, and 2j, reducing to five the number of different bridge types. 

 There are eight forms remaining, namely three of the real ratio type, 

 each capable of giving the same performance; two of the imaginary 

 ratio type which are complementary, together giving a measurement of 

 inductive and capacitive impedances; two of the real product type 

 which will measure all types of impedance; and one imaginary product 

 type which is capable of measuring only capacitive impedances. 



The only duplicate forms are in the case of the real ratio and real 

 product types. In the case of the latter. Fig. 2h is to be preferred in 

 practically all cases to Fig. 2i, as already explained, and thus we can 

 say that, practically speaking, we have duplicate forms only in the 

 case of the real ratio type. 



The three forms of this type are all used and each has certain ad- 

 vantages for certain types of measurements. This type of bridge, 

 commonly known as the direct comparison type, is probably used 

 more than any other, and is one of the most accurate types, particularly 

 in the special case of equal ratio arms. This is due to the fact that a 

 check for equality of the ratio arms may be readily made by a method 

 of simple reversal without any external measurements, and by this 

 means practically all the errors of the bridge may be eliminated. 

 Resistance ratio arms are preferable for a general purpose bridge 

 because they are more readily available and more readily adjusted to 

 meet their requirements. They also give an impedance independent 



