PERMALLOY, A NEW MAGNETIC MATERIAL 103 



Permeability is the magnetic characteristic of permalloy in which 

 we first became interested and we have used its numerical value as an 

 index in establishing the effects of mechanical and thermal treatments. 

 Most of the measurements have been made in a ring permeameter of 

 special design. The ring sample is prepared by winding twenty or more 

 turns of tape around a disk about three inches in diameter. The disk is 

 then removed leaving the material in the form of a spirally laminated 

 ring with a rectangular cross-section approximately 3.2 mm. by 6 mm. 

 A single massive copper conductor is linked with this ring, and con- 

 stitutes also the secondary of a transformer whose primary winding 

 forms one arm of an inductance bridge. From the bridge measure- 

 ments, and the dimensions of the ring the permeability of the latter 

 may readily be computed. For most of the measurements 112-cycle 

 alternating current has been employed, permitting the use of tele- 

 phone receivers in adjusting the balance of the bridge. The ring is 

 sufficiently well laminated so that no serious troubles are introduced 

 at this frequency by eddy currents. This fact was verified by making 

 a number of permeability determinations at alternating current fre- 

 quencies both above and below that chosen for routine use, and also 

 by comparing the results of ring permeameter tests with those of 

 ballistic tests on specially wound ring samples. The bridge method 

 is particularly well adapted to the measurement of permeability in 

 very weak magnetic fields since amplifiers may readily be used to 

 increase the delicacy of the bridge adjustment to almost any degree 

 desired. As a matter of convenience we have usually included in our 

 test program measurements with fields of 0.002, 0.003, and 0.010 

 gauss, and on the graph of permeability against magnetizing field 

 strength the straight line through these points has been extended to 

 field strength zero. We have called the permeability read from the 

 graph at this point the "initial permeability" of the sample. 



The form of permeameter used is especially adapted to making 

 measurements quickly and with minimum handling of the sample, 

 since it makes use of but a single magnetizing turn. The ring is laid 

 on suitable insulating supports in an annular copper trough, and 

 placing the copper cover on this trough completes the electrical circuit. 

 In a modified instrument, the "hot permeameter", provided with a 

 heating device, permeabilities may be measured from liquid air tem- 

 peratures up to about 1000°C. without altering the position of the 

 sample. 



The heat-treatment of permalloy is of the utmost importance. To 

 develop its maximum initial permeability it must be cooled not only 

 through the proper temperature ranges, but also at the proper rates. 



