54 BELL SYSTEM TECHNICAL JOURNAL 



If the data at lower frequencies are sufficiently reliable, this parabola 

 can be extrapolated to give the zero intercept {aBm + c) which 

 contains the sought for hysteresis constants of the core. Subtracting 

 the intercept so found, and dividing again by / gives 



This is the equation of a straight line, when plotted against/. The 

 intercept e is the desired eddy current coefficient for the core material. 

 The slope of this line, S, yields the dielectric quality 



„ Sir^CL ,_.- 



Q = ^ • (54) 



Here C is the distributed capacitance, which can be obtained from 

 eq. (46). This relation is useful in comparing the qualities of various 

 insulating and spacing materials, and in calculating the total losses to 

 be expected in any proposed coil. 



Accurate Separation by Limiting Inductance 



It appears from the above discussion that magnetic loss separations 

 can be made in spite of interference by distributed capacitance, 

 leakance and eddy current resistance of the coil windings, provided 

 that the interference is not too large, and provided that eddy current 

 shielding in the test core is negligible. When the latter condition is 

 not fulfilled, it becomes necessary to suppress the interference due to 

 capacitance, etc., to negligibly small quantities. This is facilitated by 

 proper technique in applying the windings, but any degree of sup- 

 pression can be secured by sufficient limitation of the coil inductance, 

 as will appear by reference to eq. (47), Although reduction of the 

 coil inductance by using a winding with few turns is desirable in thus 

 suppressing errors, it is undesirable in that it reduces the core loss 

 resistance (cf. eq. 50) to a value which may be difficult to measure 

 accurately on any available bridge. It is thus necessary to wind the 

 test core to an inductance which will yield the largest possible loss 

 resistance, without exceeding the allowable error from capacitance, 

 leakance, and copper eddy current loss. The value of this maximum 

 allowable inductance is obtained by calculating the inductance re- 

 quired to make the errors due to capacitance, leakance, and copper 

 eddy currents at the highest measuring frequency equal to some 

 tolerable small fraction of the core loss resistance. 



