58 BELL SYSTEM TECHNICAL JOURNAL 



and by head phones at frequencies within the audible range. For 

 measurements at higher frequencies, a heterodyne detector is needed. 

 Detection with a galvanometer is feasible when used in connection 

 with a rectifier and filter, the filter being required so as to eliminate 

 errors due to currents of extraneous frequencies. 



The galvanometer used for d-c. bridge balancing should be sensitive 

 enough to secure resistance readings of precision equal to that of the 

 a-c. balance. The d-c. supply to the bridge should be limited to a 

 current of the same order of magnitude as the a-c. supply, to guard 

 against permanent magnetization of the magnetic material under test. 



The usual test procedure is to set the oscillator at the lowest desired 

 frequency, and measure the inductance and resistance of the test coil 

 at several currents beginning at the lowest, increasing to the highest, 

 and returning again to the lowest, so as to detect any tendency for 

 permanent magnetization or magnetic aging. Direct current balances 

 are taken as often as required to keep up with gradual changes of 

 circuit resistance due to room temperature changes, the direction of 

 current through the bridge being reversed each time to detect and 

 eliminate stray currents and thermal e.m.f.'s. The differences between 

 the observed a-c. and d-c. resistances gives the a-c. increment resistance 

 of the test coil, except for corrections on account of the calibration of 

 the bridge and coils. This process is repeated at successively higher 

 frequencies until a suitable range of data has been covered. The 

 resulting data can then be analyzed to show the characteristic of the 

 magnetic core by the appropriate method as described above. 



The Test Core 



The design of the test core depends upon the physical and magnetic 

 characteristics of the material to be tested. In general the radial 

 thickness should be small in comparison with the diameter. Strain 

 sensitive materials must be protected from mechanical stresses of 

 handling and winding. Types of insulation and winding depend upon 

 the loss characteristics of the core. 



In any practical core, the diameter ranges between an inside value 

 di and an outside value do. Since the diameter enters into the de- 

 nominator of the expression for // and certain other magnetic quan- 

 tities, the efifective diameter must be calculated and used in such 

 expression rather than the simple mean diameter. The effective 

 magnetic diameter of a core having a rectangular cross-section is 



