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The tracking ability of a read/write head on a flexible disk is largely determined by the dynamic and dimensional stabilities of the substrate. Dimensional changes are caused by thermal and hygroscopic expansion/contraction as well as time-dependent shrinkage due to stress relaxation. High-performance tracking system design requires knowledge of the substrate expansion characteristics including mean coefficients, anisotropy and anisotropy frequency spectrum. Efforts to study and control these changes require techniques which provide for their rapid and precise measurement. Methods previously employed have proven either coarse or difficult to apply. This paper describes a novel technique which facilitates the rapid collection of relatively precise data on dimensional changes. The technique, which depends upon a magnetic recording surface being present on the substrate, involves a pair of corresponding skewed core gaps in a single head. Magnetic transitions are prerecorded in closely located tracks; subsequent dimensional changes in the substrate are manifested by timing differences in the signals read by the head. Examples of data from several PET films are presented and clearly illustrate their anisotropic thermal and hygroscopic expansion behavior.