. Bell telephone magazine . able, and so cores of magnetic material areemployed, but it must be a material with extremely low hys-teresis and eddy current loss. The requirement is met by acore of compressed powder of molybdenum permalloy. Thismaterial is itself one of remarkably high intrinsic permeabilityand low hysteresis, and by insulating the separate grains of thepowder from which the core is pressed, eddy currents are keptvery small. These coils have another property which is quite important 37 BELL TELEPHONE QUARTERLY in this application. They maintain the same inductance overa very wid
. Bell telephone magazine . able, and so cores of magnetic material areemployed, but it must be a material with extremely low hys-teresis and eddy current loss. The requirement is met by acore of compressed powder of molybdenum permalloy. Thismaterial is itself one of remarkably high intrinsic permeabilityand low hysteresis, and by insulating the separate grains of thepowder from which the core is pressed, eddy currents are keptvery small. These coils have another property which is quite important 37 BELL TELEPHONE QUARTERLY in this application. They maintain the same inductance overa very wide range of temperatures. This is unusual in coilswith ferromagnetic cores, since permeability in general in-creases rather rapidly with rise of temperature. Molybdenum-permalloy is no exception in this regard, but its increase ofpermeability with temperature has in this case been compen-sated by incorporating with it a small proportion of an alloywhose permeability decreases rapidly with temperature. The ELECTRICAL AXIS = X. MECHANICAL AXIS= Y OPTICAL AXIS = ZFig. 8. The Quartz Plates Are Cut with Their Face Perpendicular to theElectrical Axis of the Crystal, and with Ends and Sides Making a Small Anglewith the Mechanical and Optical Axes. resulting combination displays negligible change of perme-ability over the range of temperatures to which it is subjectedin use. The evolution of this core presents still another interestingillustration of physical research extending over a long periodof time. In this case we must go back thirty years to the firstapplication of inductive loading to telephone lines. The prob-lem then, as now, was to obtain inductance with as little added 38 THE EVOLUTION OF THE CRYSTAL WAVE FILTER resistance as possible. The best that could be done in the firstloading coils was to use fine iron wire for the core. This wirehad to be drawn to .004-inch diameter to hold the eddy-currentloss to the required low value, and this feature made the coilsvery expensive.
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