FAQ of demagnetize a magnet

How do you demagnetize a magnet?

Magnetism is a result of the alignment of magnetic dipoles in material into a general direction. (refer to the domain theory). In general, a piece of iron, for example, would not be magnetic unless on average we find that the magnetic dipoles or unit magnets are aligned in some direction. Hence, to demagnetize a magnet what you need to do is to cause disorder to the unit magnets, randomizing them. All of the below methods act to randomize the unit magnets or magnetic dipoles in the magnet.

Answer 1:Heating, Hammering and/or Jarring

Heating a magnet past its Curie point will destroy the long range ordering. In the case of heating, energy is provide to the magnetic dipoles causing them to free themselves from the initial order, thus destroying or reducing the ordering of the magnetic dipoles. This causes the magnetic poles to point in different direction in space. So on average along any direction you have very little magnetic poles alignment. Since magnetism arise from such alignment, in this case we have very little or no magnetization. Similarly when we hammer or drop the magnet many times on the floor, the vibration induced on the magnet causes the magnetic dipoles to randomize. Hammering and/or Jarring, such activity will destroy the long range ordering within the magnet.

Answer 2 :Using an AC(alternating current)field

The method uses a magnetic field generated by a current flowing through a solenoid to change the ordering of the magnetic dipole. An AC current is used to produce magnetic field of changing direction near the magnet. The(electro-)magnetic field caused the magnetic dipoles to switch direction many times. Each time the field point to one direction some magnetic dipoles will try to align along that direction. If the field is big enough, many will align along that direction. When the field is reduced and reversed not all these magnetic poles will reverse. Hence, by repeating this process many time (that's why an AC current is needed) and also by reducing the magnitude of the current as we go, to reduce the strength of the (electro-)magnetic field, the magnetism of the magnet can be reduced to almost zero or perhaps for most practical purpose zero. In an electromagnet, ceasing the flow of current will eliminate the magnetic field. However, a slight field may remain in the core material as a result of hysteresis.

Have you noticed that sometimes when you have your computer monitor placed in some location of your house you find that the display is discolorated? Ever pressed some buttons on the monitor and select the degaussing function to remove the color distortion on your computer monitor? What do you think is happening when you do that?

Conversely, as you have learned, you can use the field generated by the current to magnetized a magnet. The magnetism may or may not stay after you remove the external magnetic field due to the current depending on the property of the material. For example the magnetic material used in our hard-disk for storing information will keep its magnetism (data) after the current is removed. On the other hand the little "horse-shoe" electromagnet in the recording head that is used to produce the magnetic field for recording becomes a strong magnet in the present of the current but weaken drastically when the current is removed.

How to use the magnetizer demagnetizer?

Magnetizer Demagnetizer tool VTMD belongs on every work bench. It will magnetize and demagnetize the tips of screwdrivers and many other kinds of steel tools. It takes only seconds to magnetize or demagnetize your tool.

To magnetize, just insert the tool tip in the magnetizer hole and gently but rapidly shake the tool from side to side in the hole for a few seconds. Repeat if necessary to achieve magnetism. To demagnetize the same tool, repeat the procedure in the demagnetizer hole.

There are two concerns when using a demagnetizer:

First, is the field from the demagnetizer strong enough to demagnetize the heads and guides? And second, when is it far enough away so that you can shut it off without remagnetizing the heads and guides?

Demagnetizing:

If the demagnetizer is to demagnetize the core laminations, then the field that it produces must be large enough to cause the induction (flux density) in the core to approach saturation. When saturation approaches, the head output voltage waveform becomes distorted. But by this time the output voltage level is about 50 dB greater than the maximum output from tape, and the playback head preamplifier will surely be completely overloaded. Thus to perform this test you must disconnect the head and connect it directly to an oscilloscope input, and look for distortion in the waveform.

Now another measurement complication arises:

The head output voltage is the derivative of the core flux. The effect is that when the core flux is sinusoidal, the output voltage is also sinusoidal. But when the flux becomes a square wave, the head output voltage becomes a series of "spikes" and there's no way to tell just how near you are to core saturation. The "fix" for this is to build an integrating amplifier. When the head output voltage is fed through an integrator, the integrator output voltage has the same waveform as that of the core flux. Therefore the 'scope waveform will show a flat-topped wave when the core saturates.

For rough estimation purposes, you can look at the head output voltage directly on an oscilloscope. Turn the head demagnetizer on, and bring it to a point about 20 mm away from the head. You should see a sinusoidal wave on the 'scope. Then bring it closer, eventually touching the core with the demagnetizer pole tips. As you bring it closer, you should come to a point where the waveform on the 'scope begins to look distorted. The sinusoid will turn into more or-less spikes. When you begin to see spikes, the core is saturating, and the head will be magnetized if you switch the demagnetizer off, or demagnetized if you move the demagnetizer away from the head before switching it off.

How to clean and demagnetizing tape recorders and duplicators

The demagnetizer must be turned on at least 3 feet from any tape or tape machine. The active end of the demagnetizer should be covered with plastic so as not to scratch. If it has a bare metal end, cover it with a layer of tape. The magnetic field of the demagnetizer drops off rapidly with distance. In order to demagnetize a part you must apply a field that is strong enough to totally magnetize the part. Since the demagnetizer is running on 60 Hz AC, the polarity of the magnetic field is reversing 120 times a second. If the power were to go off while the demagnetizer was close to something, there would be a very good chance the part would wind up strongly magnetized. In order to DEmagnetize something, the field must be very slowly be reduced in strength to close to zero. We do this by slowly moving the demagnetizer away from what we are trying to demagnetize. Fast or jerky motions can result in magnetizing not DEmagnetizing. Move like you are in molasses in January.

So the steps are:

  1. Turn the power off on the tape recorder or duplicator. Demagnetizing with the power on can damage the circuitry.
  2. Turn the demagnetizer on well away from tape or tape machines (3' or greater).
  3. Move the demagnetizer in so the tip contacts the parts of the tape recorder or duplicator you are trying to demagnetize. You want to do the head(s), guides, and (if you have a strong enough demagnetizer) the capstan.
  4. Move across the surface of each part and from one part to the next very slowly and smoothly. If you slip and move fast, go back over that part.
  5. After you have gone over the surface of every metal part in the tape path, very slowly and smoothly move the demagnetizer away until you are at least 3' away.
  6. Turn the demagnetizer off.

Some additional notes:

  1. If you use a low power demagnetizer like RS sold, you probably do not have to power to be able to demagnetize a capstan.
  2. It is not important to move slowly when moving TOWARDS the tape heads, only when near them and when moving AWAY from them.
  3. You need to get the strongest possible magnetic field applied to the metal tape path parts you are trying to demagnetize. This requires that you get the end of the demagnetizer into contact with the parts, since the magnetic field drops off rapidly with distance. This is why we want plastic or tape over the metal end of the demagnetizer, so we do not scratch anything with it.
  4. You do not need to hold the demagnetizer in position for any amount of time. All you need to do is:
    • get a strong enough alternating magnetic field
    • reduce that field strength slowly.
  5. Pulling away slowly and smoothly is very important. You want to demagnetize not only the tape head(s), but all metal parts that touch the tape.
  6. The only way you can damage things while demagnetizing would be to do it with the equipment power turned on, to kill the power while the demagnetizer is close to the equipment, or to move fast or jerky.
  7. I would be very cautious cleaning the rubber pinch roller with anything BUT alcohol. Most other solvents can damage the rubber by dissolving it and making it sticky. While it is true alcohol can dry rubber, it does so very slowly, and treatment once or twice a year with platen cleaner will restore the rubber surface.