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BELL SYSTEM TECHNICAL JOURNAL 



the calculated eddy current loss gives the residual loss coefificient c. 

 Thus, the value of c is found to be a constant 20 X 10~^ at frequencies 

 above 500 cycles, but to decline toward zero as the frequency approaches 

 zero, in evident accord with the ballistic galvanometer result. 



The inductance due to the core shows a similar frequency effect. 

 The observed inductances for the 20-turn winding are given in Fig. 7. 

 The values at each frequency for the various currents fall on a straight 



1.02 



3 4 5 6 7 



COIL CURRENT IN MILLIAMPERES 



Fig. 7 — Inductance observed on 20 turn coil, at various frequencies. 



line, the slope of which gives the permeability coefficient, X = 19.6 

 X 10~^, for the lower frequencies where eddy current shielding can be 

 neglected. 



The values of L for / = 0, obtained from Fig. 7, are plotted against 

 frequency in Fig. 8. The most remarkable feature of this curve is the 

 decline of inductance (or apparent permeability) of about 1 per cent at 

 low frequencies, where very little decrease on account of eddy current 

 shielding is to be expected. The characteristic shielding curve has been 



