56 BELL SYSTEM TECHNICAL JOURNAL 



maximum measuring frequency the eddy current resistance will be 

 1.325 ohms and the hysteresis resistance 0.265 ohm. At the lower 

 end of the frequency range, say at 1000 ~ these figures become Re 

 = 0.0132 ohm and Rh = 0.0265 ohm. This indicates that a bridge 

 will be required for such measurements capable of measuring in- 

 crements of resistance to an accuracy of about 0.0002 ohm at 1000~, 

 and about 0.002 at 10,000 ~. 



Computations of this sort show the high quality of a-c. bridge 

 generally demanded for core loss measurements. Such measurements 

 require equipment and a bridge with a maximum of sensitivity of 

 both a-c. and d-c. balances, and a minimum of losses in standards, 

 pick-up, unbalanced impedance to ground, and variable contact. 



Bridges, and Test Procedure 



The essential features of bridges suitable for core loss measurements 

 will now be mentioned in general terms. It will be appreciated from 

 the above discussion that the specific range of frequencies, and the 

 required resistance sensitivity must be adapted to the loss charac- 

 teristics of the magnetic core to be measured. 



Although certain types of resonance bridges ^^ have advantages for 

 measurements at high frequencies, the most suitable bridge for the 

 usual measurement of magnetic core coils is an equal arm inductance 

 comparison bridgCj^" on which inductances can be measured directly, 

 and on which a-c. and d-c. resistance measurements can be made in 

 prompt succession, to eliminate the effect of gradual temperature 

 changes on the resistances of the bridge and test coil. A suitable 

 circuit is shown in Fig. 1. 



Inductance coils for bridge standards should be as stable as possible 

 against frequency and current. Although low effective resistance per 

 unit of inductance is desirable, it is more important for core loss 

 measurements to design such standards for a minimum increase of 

 resistance with frequency, so as to keep calibration corrections small 

 in comparison to the resistance increments to be measured. A satis- 

 factory type of standard coil consists of an air core toroidal form, with 

 a bank winding of finely stranded wire. The bank winding minimizes 

 capacitance effects on the observed inductance and resistance of the 

 coil, and the fine stranding minimizes eddy current losses in the copper; 

 cf. eq. (49). The small residual corrections must finally be included 

 as calibrations when making measurements with the aid of standard 

 coils. 



" \V. J. Shackelton and J. G. Ferguson, B. S. T. J. 7, 82 (1928). 

 =» W. J. Shackelton, B. S. T. J. 6, 142 (1927). 



