Magnetic Measurements at Low Flux Densities 

 Using the Alternating Current Bridge 



By VICTOR E. LEGG 



A resume is given of the basic relations between the magnetic character- 

 istics of the core of a coil and the inductance and resistance of the coil as 

 measured on an alternating current bridge. Modifications of the simple 

 relations to take account of the interactions of eddy currents and hysteresis 

 in the core material are developed, and are seen to reciuire a more com- 

 plicated interpretation of the data in order to obtain an accurate separation 

 of the eddy current, hysteresis, and "residual " losses. Means are described 

 of minimizing or eliminating the disturbing effects of distributed capaci- 

 tance, leakance and eddy current loss in the coil windings. Essential details 

 of the alternating current bridge and associated apparatus, and of the core 

 structure, are given. 



THE modern alternating current bridge, with its high precision 

 and sensitive balance, has almost completely superseded the 

 ballistic galvanometer for determining the magnetic properties of core 

 materials at the low flux densities employed in telephone and radio 

 apparatus. The suitability of the alternating current bridge for this 

 purpose has been recognized for some time,* but the continued im- 

 provements in magnetic materials, and the more exacting requirements 

 of modern communication apparatus, have necessitated refinements in 

 apparatus, in technique, and in interpretation of measurements. This 

 paper considers the modified technique required to take account of 

 eddy current shielding and hysteresis in the magnetic core, distributed 

 capacitance and leakance in the coil winding, and the necessary details 

 of the bridge and associated apparatus to realize the desired accuracy 

 of measurements. 



Fundamentally, the a-c. method involves measurements of the 

 inductance and effective resistance of a winding on the test specimen, 

 such measurements being made at several frequencies, and at several 

 values of current.^ From these measurements the magnetic properties 

 of the test core can be computed for the low flux density range. The 

 details of such calculations will be given below, beginning with approxi- 

 mate methods, and proceeding to successively more accurate compu- 

 tations. 



^M.Wien, Ann. d.Physik [3] 66, S59 {189S). 



^ The annular form of magnetic core, wound with a uniformly distributed test 

 winding will here be treated, but the results will be found to be readily transferable 

 to other forms of core. 



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