Click here to purchase the entire book in PDF format. Retentivity Let's put a piece of magnetic tape next to a recording head. We'll apply a signal to the recording head and generate a known flux density around the head which in turn is applied to the tape. That causes the magnetic tape to become magnetized. If we turn off the signal at the record head, there will be some amount of magnetic strength left in the tape, but not as much as the amount that we applied to the tape. However, the magnetic strength of the tape won't drop to zero just because we turned off the magnetic field around the record head, either. The amount of magnetic strength left in the tape will be something between the flux density that we applied, and zero. The amount of magnetic flux density that is left on the tape after the external magnetic force (applied by the record head) has been turned off is called the tape's retentivity. Since retentivity is a measure of magnetic flux density that remains in the tape after we've turned off an external magnetic field, it's measured in Wb/m$^{2}$.