A completed authenticity examination tells the examiner whether or not the recording has been altered, and if so, how it was accomplished. One aspect of such determinations can be the evaluation of signals produced by strong external magnetic fields, other than from the tape recorder's magnetic heads. By making use of the changing sizes of the supply and take-up reels on most standard audio and video formats, a detailed analysis can be made of the effects of a strong external magnetic field. This periodicity technique can be used whenever an external magnetic field has affected the questioned tape sufficiently to produce artifacts that can be measured in a time-continuous waveform. This would include, but is not limited to, the nonprofessional use of bulk erasers/degaussers, permanent magnets brought into contact with the housing or tape, power transformers, etc. The use of this periodicity method is described for a Federal drug prosecution. 5 figures and appended supplementary mathematical equations