The Bell System Technical Journal 



July, 1927 



Measurement of Inductance by the Shielded Owen Bridge 



By J. G. FERGUSON 



Synopsis: The study described in this paper shows that the Owen 

 bridge is well adapted to the accurate measurement of inductance and 

 effective resistance to above 3,000 cycles. The construction of a shielded 

 bridge for audio frequencies is described and a theoretical discussion is also 

 given. It was found possible to measure inductances ranging from 0.1 

 to 3 henrys with an error of measurement less than 0.1 per cent, and for 

 10 henrys the accuracy is better than 0.25 per cent. As a means of meas- 

 uring effective resistance the bridge shows an accuracy of about 2 per cent. 

 The sources of error and method of eliminating or correcting them are 

 discussed. 



Introductiox 



THE accurate measurement of inductance and capacitance is 

 essential to the correct design of practically all precision electrical 

 apparatus. Particularly is this so in the field of electrical communica- 

 tion where the successful introduction of new circuits and equipment, 

 such as the carrier telephone and the telephone repeater, depends 

 largely on the accuracy with which the elements can be adjusted to 

 the nominal values, this accuracy in turn depending on the accuracy 

 with which the electrical measurements can be made. 



Owing principally to the ease with which a telephone receiver may 

 be used to indicate a balance at audio frequencies, bridge measurements 

 are very generally used for the measurement of capacitance and 

 inductance in telephone work. The simplest type of bridge and the 

 one used most for the comparison of like impedances is the equal 

 ratio arm bridge described by Shackelton.^ This bridge requires 

 standards of the same kind and magnitude as the impedances which 

 are to be measured. The calibration of these standards is a separate 

 problem, for which a distinct type of bridge is required. 



Either capacitance or inductance may be measured by a bridge 

 method in terms of time and resistance, both of which are fundamental 

 quantities. However, since condensers may be obtained with very 

 low losses and small changes with frequency, this type of measurement 

 is usually made with capacitance,- inductance measurements being 



^ W. J. Shackelton, "A Shielded Bridge for Inductive Impedance Measurements," 

 Bell System Technical Journal, January, 1927. 



* J. Clerk Maxwell, Electricity and Magnetism, Vol. 2, pp. 776-7. 

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