BRIDGE METHODS OF MEASURING IMPEDANCES 453 



Due to the wide field of usefulness and great flexibility of the 

 impedance bridge, a very large amount of development work has been 

 done and a considerable amount of literature has been published 

 covering various types and modifications. In fact, the subject has 

 become so broad and the information so voluminous that the engineer 

 who has not specialized in the subject has every excuse for a feeling of 

 considerable confusion when he finds it necessary to make a choice 

 among the numerous circuits available. Perhaps the greatest single 

 obstacle to a still more extensive use of the impedance bridge in 

 industry is this very multiplicity of types combined with a rather 

 complete lack of any practical guide for the engineer who is interested 

 principally in the measurement itself and looks on the bridge simply 

 as a means to this end. 



V^ery little information is available as to the relative merits of the 

 various types of bridges, the great majority of published articles being 

 confined to a description of a particular circuit used by the author for a 

 particular purpose. 



The present article furnishes a comparison of the relative merits 

 of the large number of circuits which are available for making the 

 same measurement and should serve as a guide to the engineer who is 

 more interested in results than in acquiring a broad education in 

 bridge measurements. An outline is given of the fundamental require- 

 ments which must be met by bridges used for impedance measure- 

 ments, and a classification is made which serves as a help in the choice 

 of a bridge for any particular type of measurement. The relative 

 merits of the simpler types of bridge are discussed from the standpoint 

 of the measurement of both components of an impedance, particularly 

 with reference to measurements in the communication range of 

 frequencies from about 100 to 1,000,000 cycles. Where only the 

 major component of an impedance is desired, for instance where only 

 the inductance of a coil or the capacitance of a condenser is desired, 

 the requirements are not so severe and many forms of bridges may 

 be used which are not suitable for the purpose here outlined. Bridges 

 are also used to a large extent for other purposes than impedance 

 measurements, such as for frequency measurements. These applica- 

 tions will not be considered here. 



The General Bridge Network 



Any bridge may be considered as a network consisting of a number 

 of impedances which may be so adjusted that when a potential differ- 

 ence is applied at two junction points, the potential across two other 

 junction points will be zero. For this condition, there are relations 



