Before you speak into the tape recorder, is the tape non-magnetic because half of the magnets face one way and half the other way?
Exactly. When you switch your tape recorder to the record mode, it has a special erase head that becomes active. This erase head deliberately scrambles the magnetic orientations of the tape’s magnetic particles. The erase head does this by flipping the magnetizations back and forth very rapidly as the particles pass by the head, so that they are left in unpredictable orientations. There are, however, some inexpensive recorders that use permanent magnets to erase the tapes. This process magnetizes all the magnetic particles in one direction, effectively erasing a tape. Because it leaves the tape highly magnetized, this second technique isn’t as good as the first one. It tends to leave some noise on the recorded tape.