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If you just want to know how to choose the right magnet, skip this section.
While magnets have been known for at least 2500 years, they were very weak. Metallic iron, nickel and cobalt, or an alloy from them, could become a magnet naturally or by stroking with a lodestone. Lodestones are natural magnets composed of a particular kind of Magnetite crystal.
It does seem likely that some people gained healing through using such magnets -
We have to come to the practical use of electricity to begin to make progress towards more powerful — and more effective — magnets.
Powerful electromagnets are essentially a nineteenth century invention. They are at the heart of dynamos, generators and electric motors and they powered the Electric Age. The magnetic fields they generated were huge compared to anything previously known and they captured the public imagination, being a popular science and comedy feature of Vaudeville and the Music Halls.
Electromagnets are still the best way to generate a very large magnetic field. In medical use magnetic therapy in hospitals uses powerful electromagnetic machines, and Magnetic Resonance Imaging equipment (MRI) uses the strongest magnetic fields. Even more powerful are the huge electromagnets which pick up tonnes of scrap steel at a time. The ongoing search for atomic fusion power is based on enclosing the nuclear fusion reaction inside enormously powerful electromagnetic fields.
Iron Magnets. When a coil of wire has an electric current passing through it, the coil becomes a magnet, with the highest flux (magnetic strength) inside the coil. A piece of iron or steel placed in the coil will become a magnet, and if it is heated then cooled while the electromagnet stays on, the iron will remain a powerful magnet even when it is taken out.
What actually happens is this. The iron (or other magnetic material) is composed of groups of millions of molecules called domains. These are normally separate crystals in the iron, each of which can become a tiny magnet. Usually, they are weakly magnetized in random directions, so there is no overall magnetic effect. The whole lump becomes an effective magnet when a lot of the domains -
When the iron is placed in a strong magnetic field and heated above a certain temperature, the domains remagnetize in the direction of the field they are in. If the iron is then cooled while the field is still active, the piece of iron becomes a permanent magnet. Magnetic strength drops over time for a variety of reasons, as domains randomise again. Common reasons are banging the iron or dropping it, the influence of other magnets and heating the iron to near the temperature needed to make it in the beginning.
This is the origin of the modern ‘permanent’ magnet (doesn’t lose the magnetism when the current is switched off), also called a ‘static’ magnet (not needing a moving current). The earliest strong magnets were of various iron alloys (called steels), and by the 1920s they were a common product. The mental image of a strong magnet even today is the comic cartoon of a little boy holding out a horseshoe-
Not until the 1940s did the research into magnetisable materials get a real boost. Basic research into atomic and molecular structure and magnetic domains led to a better understanding of the nature of materials. This led to more accurate prediction of how alloys might behave, so trials on hundreds of alloys began to find useful magnetic materials.
Advances in mineralogy, refining and metallurgy made the rarer elements for alloys more affordable, and now we have many good permanent magnet types commonly available. Low-
This is a quick summary of the properties and usefulness of each type of magnet commonly in use today. Five new magnetic materials have relegated older iron and steel magnets into history. For the uses I’m concerned with — therapy and magnetohydrodynamics — I’ve omitted several kinds of static magnet because either they lose their power quickly or they have other disadvantages for ordinary use, and so they’re rarely seen today. The order below is by strength and it’s a practical rather than a scientific one.
Plastic >> Ferrite >> Strontium Ceramic >> Samarium Cobalt >> Neodymium
WEAK >>>> STRONG >>>> EXTRA-
Plastic magnets are simply magnet powder used as the filler in a flexible plastic which is set in a strong magnetic field. They’re very versatile and are ideal for magnetic ‘sticky notes’ such as fridge magnets. But they lose their magnetism quite quickly. Now you know why older fridge magnets tend to fall off when you slam the door!
Cheap ferrite magnets are a ceramic including some ferrite . They are brittle, so they chip and break easily. They’re more powerful than plastic magnets, though — there’s more ferrite in them — so they make better fridge magnets and they have a lot of uses in appliances and gadgets which aren’t expected to last for long. They’re the cheapest kind of magnet and are common in toys and short-
Extra: A new type of magnet is the haematite bead or ring. These are a polished, dull metallic grey stone -
are by far the best of the ceramic magnets -
They’re fairly cheap, they’re weatherproof and they last almost forever. They are the static magnet of choice where high power is needed and weight isn’t important. They are maybe three times as strong as ferrite for the weight, and cost much more.
I’ve found one maker quoting these magnets as losing 2% of their power every 50 years, so they’ll outlast you! They would take about 600 years to fall to half power.
magnets have a lot going for them.
They’re powerful for their size and keep their most of their magnetic strength at quite high temperatures (eg, in boiling water). They’re also long-
So, when a powerful magnet is needed for extreme conditions, they are the first choice. For magnetic therapy, they are better than anything except neodymium and have one advantage over that material: they don’t need protecting from rust. The disadvantages are that they’re nearly twice the bulk and weight for the power, and they’re very expensive. And — unlike 20 years ago — neodymium’s a lot cheaper.
are the tops overall for therapy.
‘Neodymium’ or ‘neo’ is the usual term for an iron-
For the size, typical neodymium magnets are over 100 times as powerful as a lodestone, 20 times the power of cheap ferrite and half as powerful again or more than samarium cobalt. Not bad! Neodymium magnets keep their magnetic flux nearly as well as strontium ceramics — according to one maker’s website, they’re quoted as losing about 5% of their magnetic strength every 20 years, so they will still have over half power 250 years after manufacture.
There’s a downside, though. They rust like crazy — especially when kept damp and salty (like in the sea, or on a sweaty wrist!). That’s because both the iron and the neodymium in the alloy are very reactive with oxygen, especially when there’s salt around. So all neodymium magnets need some protection from air and water. The best ones are triple plated with copper and nickel, then maybe covered with gold or a synthetic coating like tough epoxy paint or PVC. And they are usually encapsulated as well, to avoid damage to the coatings.
If you can get round the rust problem — and that’s no problem to a careful magnet maker — neodymium is the one to go for where weight is an issue, like in a wristband. Just beware of those therapy bangles with the magnets embedded in the inside and only a light nickel plating for protection. Skin acids soon strip it off, then the magnets rust, swell and drop out in chunks!
As magnets are often hidden, and cheap ferrite looks very similar to strontium ceramic, the best way to identify which magnetic material you have is to ask, or look at a specification. You’d be lucky to get an answer, though — people who sell the poor stuff don’t advertise it as rubbish! Don’t despair; there are a few visible tests and pointers.
1) In the UK, copper coins made since 1991 have some iron in them; in the USA, pennies are iron-
2) Neodymium magnets are easy to tell from most others: they have such fantastic power for the size. Samarium cobalt magnets look quite like plated neodymium. They’re nearly as strong and just as acceptable, if a little bulkier for the power and more expensive. And they don’t rust.
3) The plating quality on a neodymium magnet is all-
4) The same applies to bangle and bracelet materials. Many are made from plated copper, brass or — worst of all — zinc-
The Bioflow torc design used to be made of brass with a heavy chrome plating, then gold. Once the gold had been rubbed off, some people’s skin acid could get through the chrome in under two years; other people had no corrosion in six years. We now use nickel-
We will concentrate on magnets for therapy and for fuel conditioning, because that’s what we sell. There’s quite a lot to say, so... time to start a new page. Go to Evidence and Trials or Fuel Saving.
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