Magnets

[Cienna]

What are the reasons a permanent magnet loses its magnetism? And if lost, are there other ways to restore its magnetism other than using a magnetizer?

 

[Gene-WI]

Age, shock and heat. You could draw it against another magnet but it would not get as strong as using a machine made for magnetizing. Have heard of guys wrapping welding cables around a piece they want magnetized.

 

[john d]

Wrap several turns of #12 copper wire around the object, and give it a quick "flash" through that coil with a DC welder set on 60 amps or so.

 

[John T]

I studied this in Physics and Electromagnetic Field Theory (and Maxwells Equations YUKKK) but that was over 40 years ago and my old Hard Drive is failing lol

Loss of magnetism has something to do with the the North/South poles are loosing their physical orientation BUT I JUST FORGET GRRRRRRRR

If a keeper (soft iron bar/connection from North to South pole) is used the field will be retained wayyyyyyyyyyy longer

About the only way I know to re magnetize is to pass a high DC current around the iron which puts those poles back into physical alignment

Surrounding a wire carrying current there exists an electromagnetic field which has a certain North/South Magnetic polarity field in relation to the + or - electrical current flow. If you change the current direction the North/South magnetic field polarity also changes

If you pass a magnetic field by a coil of wire, it induces an electrical potential difference (emf/voltage) in the wire ends (typical generator action) and conversely as above, if you pass current flow by soft iron it induces a magnetic field into the iron

Wheres Professor Jim (janicholson) and Bob M when we need them??????? Its been toooooooooo long ago for me

John T

 

[Cienna]

Okay,JohnD, but what if the coils of wire are wrapped the wrong direction?----Wouldn't that tend to cancel out the proper polarity of the original magnet? Or at least weaken the proper/original polarity?

I appreciate your guys' responses!

 

[Cienna]

I do have an AC welder-----Does that also produce an AC voltage at the "business" end of the leads?

So an AC welder won't work, right?

 

[Anonymous-0]

No an AC welder won't work. At high amps DC welders can have arc blow which is caused by magnetic fields. Using an AC welder is one way to eliminate arc blow.

 

[Bob M]

(quoted from post at 12:03:02 02/18/12) I studied this in Physics and Electromagnetic Field Theory (and Maxwells Equations YUKKK) but that was over 40 years ago.....

John T: So did I. Makes my head hurt now recalling the mathematics involved(!)

----

A Gene-WI suggests, time, shock and heat can result in loss of magnetism. However certain classes of magnetic materials (Alnico, neodymium, etc ) are not affected by the passage of time. Also a strong magnetic field - as from arc welding or a nearby lightning strike - can neutralize permanent magnets as well.

I had a portable generator ruined when lightning struck a tree a few feet from the shed where the generator was housed. The flywheel magnet (part of the magneto) was ruined, and the generator head lost its residual magnetism and had to be "reflashed".

And more than once I've ruined (erased the magnetic strips) on credit cards by welding too close to my wallet in a back pocket.

 

[Tony Griffey]

Do magnets ever lose their magnetism?


Answer

Yes, it is possible for a permanent magnet to lose its magnetism. There are three common ways for this to occur:

1) Via heat: ferromagnet materials will lose their magnetism if heated above a point known as the Curie temperature. At this point, the energy being put into the magnet from the heat will permanently disrupt the magnetic domain structure of the material, turning it into a paramagnetic material [a similar effect occurs in materials called hard ferrites, which exhibit a form of magnetism called ferrimagnetism; the analogous temperature for these materials is known as the Neel point]. You would have to re-magnetize the magnet again, either in a solenoid or with another permanent magnet, in order to restore the magnetism. If you heat a magnet up a little bit, it will lose some of its magnetism, but on returning to room temperature [depending on how high it was heated, and on the shape of the magnet itself], full magnetism can be restored.

2) Via a demagnetizing magnetic field: permanent magnets exhibit a characteristic called coercivity, which is the ability of a material to withstand being demagnetized by an applied magnetic field. Modern permanent magnet materials such as Sm-Co and Nd-Fe-B have high coercivities; older materials such as Alnico or ceramic [hard ferrite] materials have lower coercivities. With a strong enough magnetic field of opposite polarity, it is therefore possible to demagnetize the magnet [whether this comes from another permanent magnet, or a solenoid]. Interestingly, an opposing magnetic field is sometimes applied to a magnet in order to 'knock it down', or to lower its overall magnetic output, so that it can be used appropriately in an application.

3) Via shock: this really only applies to older materials such as magnetic steels and Alnico materials; the mechanism that creates coercivity means that they are susceptible to being demagnetized if enough energy is transmitted through the material via a shock, such as being dropped or hit with a hammer. Modern materials do not suffer this type of problem, if the magnet is made of soft magnetic material like normal iron bar then it will certainly lose its magnetism if placed against like pole of another, And also They can be restored by stroking the magnet with another magnet going in the fight direction to reallign the poles.

 

[LJD]

One non-technical comment. The magnets used in tractor magnetos supposedly lose their magnetism fairly quick if taken out of the magneto - unless a "keeper" is attached. Not so bad with newer Alnico magnets, but a big problem with the cobalt, chromium, and tungsten magnets. A "keeper" and can be just about any chunk of metal left stuck to the magnet. When replacement magnets were sold new by Fairbanks-Morse or Wico - they came with a keeper attached to them. You don't remove until ready to install into the mag. Same with freshly recharged magnets. Once IN the magneto, the "keeper" is the array of metal pole-shoe laminations already built into the mag that always surrounds the magnet.

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[bill b va]

google righthand rule

 

[Big Deal]

Have any UFO's recently flown over?

 

[john d]

The opposite direction of wrap wouldn't make any difference if the polarity of the current was also changed. It would weaken or cancel the original polarity if the current/wrap combination produced a magnetic field opposite to the original.
If the attempt is to restore magnetism to permanent magnets from a magneto or other such device, you'd probably want a better method than trial and error!
I've magnetized lots of screwdrivers with a welder. Polarity didn't matter for that purpose.