Back in the Pleistocene era, when I was in radio, we used carts, reel to reel decks and spools of magnetic tape. Re-using tape required erasing that tape. Sometimes the contents was just incriminating and you wanted to erase it. We used a degausser or bulk eraser.
Degaussing is the process of decreasing or eliminating an unwanted magnetic field. It is named after Carl Friedrich Gauss a German mathematician and scientist. He was an early researcher in the field of magnetism. The terms "scientist" and "researcher" don't really describe the savant this fellow was. He was engaged in research as a teen, and published a substantial work on number theory Disquisitiones Arithmeticae in 1798 at the age of 21. He didn't even start looking at magnetic theory until the 1830s.
Here is how it works:
1. The degausser applies a a unidirectional magnetic field. Thsi would be a DC unit or a very powerful permanent magnet. (In my experience it works poorly)
2. The degausser applies an alternating field that is reduced in amplitude over time from an initial high value. These units direct you to pull away the degausser slowly.
Due to magnetic hysteresis it is generally not possible to reduce a magnetic field completely. Hysteresis is the property of metallic materials to maintain a magnetic field. Degaussing applies an external magnetic field and aligns the metallic particles to this known field.
The remaining field is called remanence. In the old reels it's actually hard to destroy all the data. You can render it unusable, even unintelligible. But as anyone who's attempted it knows, degaussing a hard drive is far more effective. When you degauss a hard drive you're not just wiping out the data on the drive. You'd damaging the control track and thus preventing the drive from ever being read again no matter what the condition of the rest of the data. Without the control track, data recovery is impossible. Hooray for digital.