What kind of magnet is used in the the magnetic/electromagnetic bracelet things?

2 Comments

1. dogsafire says:

   April 17, 2010 at 4:29 am

   The answer to your question is that they typically use neodymium-iron-boron or samarium-cobalt magnets. These so-called "rare earth" magnets are capable of producing the strongest field for a given magnet size.

   But I feel compelled to make a couple of observations on magnetic bracelets and magnetic flux as well:

   "This high quality crafted magnetic bracelet has 3000 gauss neodymium, north magnets on every link":

   I had to laugh at this one. _North_ magnets? Someone has discovered a magnetic monopole? The physics community would be very interested in hearing about that!

   The spinalbrace link also talks about unipolar magnets. There just ain’t no such thing! They do go on to clarify that they are talking about the orientation of a magnet (which _must_ have two poles), but the use of the term "unipolar" is very misleading.

   One statement that they make that is very true is "Gauss is not a number which indicates the strength of a magnet." The short discussion following that statement is also worthwhile reading.

   Gauss is a measurement of flux density. Magnetic circuits have some close analogies to electrical circuits. For the purists out there – please bear with me; I’m making anaogies, trying to convey an idea, not stating scientific facts.

   In an electrical circuit, you have some sort of energy source and a path through which electrons from that source can flow. The available potential energy is measured in volts and the flow of electrons, called current, is measured in amperes.

   One important concept is that without any conductive path from one end of the energy source to the other, no current flows.

   The flow of current through a specific conductor can be measured in amperes/sq m or amperes/sq in or similar current/area unit. That is because a certain current is flowing through a certain cross-sectional area of wire or other conductor.

   Current per unit area is called current density. The same current flowing through a smaller cross-sectional area has a higher current density.

   For example, 20 amperes flowing through a 12 guage wire will warm the wire slightly, but the current can flow all day long with no visible effect. If, however, you pass 20 amperes through a 30 gauge wire, the wire will evaporate in a flash of light. It’s the same 20 amperes, but the current density is very much higher in the 30 gauge wire than it is in the 12 gauge wire.

   What does this have to do with magnetism? Well gauss, is a measure of flux density and is analogous to current density. A magnet is analogous to the energy source. and can be thought of somewhat like a battery or power supply that supplies the electrical potential to an electrical circuit.

   One difference between magnetic and electrical circuits is that there is no such thing as a magnetic insulator. As a result, there is always a flow of magnetic flux (analogous to current) between the poles of a magnet.

   Magnetomotive force, analogous to electromotive force, is mesured in oersted. So oersteds are analogous to volts and gauss are analogous to amperes/sq m. Note that gauss depend on 1) the fact that some sort of path exists between the poles of the magnet (the path has a certain permeability, which is analogous to electrical conductivity) and 2) the cross-sectional area of that path.

   Since the permeability of the path can change at any given point and the cross-sectional area of the path can change at any point, a gauss measurement is meaningless unless you know all the specifics of the path. I’ll say it again: A gauss value for a magnet is meaningless unless you also know all other parts of the magnetic circuit. A gauss value is valid only at the specific point at which it is measured.

   The "depth of penetration" that is discussed on the spinalbrace site is also somewhat misleading. The "depth of penetration", for lack of a better term, is a function of the shape and orientation of the magnet. Magnets that have large pole separation will produce better "depth of penetration" than those with shorter pole separation for any given magnet strength.

   By the way, the useful strength of a magnet is usually stated in terms of its maximum energy product, a concept that would take many, many more words to explain. Neo magnets have the highest available energy product and are currently available in material up to 45 mega-gauss-oersteds. Samarium-cobalt magnets are available in materials up to about 39 MGO. There may be slightly better materials now available, but I’m not too far off with those numbers.

   The difference between 39 MGO and 45 MGO has very little meaning for an application like magnetic bracelets. Both materials make for very strong magnets. Neo is more temperature sensitive and more susceptable to corrosion (rust). Both materials are very fragile and should be treated like glass.

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2. matt r says:

   April 17, 2010 at 4:29 am

   its just magnetic therepy. lots of info on wiki.

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