Some comments upon the reciprocity integral used in magnetic recording

The fact that the hypothetical test current in the well-known reciprocity integral used in magnetic recording studies must be at precisely the same frequency as that of the READ head output signal is emphasized. It is shown that, under the conditions which usually prevail in magnetic recording, the application of this criterion leads naturally to the concepts of the READ head efficiency temporal spectrum as well as the usual thickness, spacing and gap-loss spatial spectra