Classification of Bridge Methods of Measuring 

 Impedances * 



By JOHN G. FERGUSON 



An analysis is made of the requirements for satisfactory operation of 

 the simple four-arm bridge when used for impedance measurements. The 

 various forms of bridge are classified into two major types called the ratio- 

 arm type and the product-arm type, based on the location of the fixed 

 impedance arms in the bridge. These two types are subdivided further, 

 based on the phase relation which exists between the fixed arm impedances. 

 Eight practical forms of bridges are given, three of them being duplicate 

 forms from the standpoint of the method of measuring impedance. These 

 bridges together allow the measurement of any type of impedance in terms 

 of practically any type of adjustable standard. The use of partial substi- 

 tution methods and ot resonance methods with these bridges is discussed and 

 several methods of operation are described which show their flexibility in 

 the measurement of impedance. 



Introduction 



BRIDGE methods have been used for the measurement of im- 

 pedance from the very beginning of alternating current use. In 

 fact, the history of the impedance bridge dates back to the earUer 

 bridges developed for the measurement of direct current resistance. 

 While some objection may be raised to this method of measurement on 

 the count that it is not direct indicating, in the sense that an ammeter 

 or voltmeter is, this has been more than offset by the high accuracy of 

 which it is capable. Bridge methods of measuring impedance have 

 accordingly continued to hold a high place in the field of electrical 

 measurements and except perhaps at the higher radio frequencies are 

 considered supreme for this purpose over the whole frequency range, 

 where high accuracy is the principal requirement. 



The peculiar advantages of the bridge method are most evident 

 where emphasis is laid on the circuit characteristics rather than on 

 power requirements. In power engineering it may be more logical 

 to make measurements in terms of current, voltage, and power, since 

 these are the quantities of immediate interest. In communication 

 engineering,, however, where design is based for the most part on 

 circuit characteristics, and power considerations are only of secondary 

 interest, it is natural that bridge methods, which furnish a direct 

 comparison of these circuit characteristics should be generally pre- 

 ferred. 



* Presented at Summer Convention of A.I.E.E., Chicago, Illinois, June, 1933. 

 Abstracted in June 1933 Elec. Engg. 



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