If you're looking to erase magnetic tape, it can be done with DC or AC, or even a permanent magnet. The idea is that the information on the tape is stored as an alternating magnetic field, so as long as you rearrange the magnetic medium to have a stationary field when played, there will be silence. To demagnetize for any other purpose requires AC running through a coil or hoop. When demagnetizing, you must energize the coil as far away from the object you wish to demagnetize as possible, then bring it slowly and steadily towards the object. After about ten seconds, you gradually move the coil away from the object before powering down the coil. Failure to do so will result in inducing a new magnetic charge on the object, as any interruption in the fluctuation of the magnetic field will permanently freeze the magnetic field of the object in whatever state the coil was when interrupted. The size of the coil and the power supplied to it will depend on the strength of the magnetic charge you wish to remove. To demagnetize tape transports, a small coil with low power requirement will suffice; CRT's require a larger hoop with a substantial charge.
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Fire makes it better.
Well, I would repeatedly bash it to demagnetise it, or heat it up a tad....in a big fire.....
batteries produce DC... which will MAGNETIZE, not DE-MAGNETIZE. You need to use a voltage that ALTERNATES from plus to minus...to de-magnetize. That is why housevoltage is used.
A demagnetiser / de-Gausser pumps something with alternating current, to produce an alternating magnetic field. The usual job is to decrease the power over a second or so. These aren't easily built, so I'll echo steve' in asking what you want to demagnetise? (Bank-cards can be done with a permanent-magnet) L
What do you want to demagnetise ?