How Do 'Singing Magnets' Work?

dpbsmith asks: "Singing magnets are available at all of the usual geek-toy emporia, and, for all I know, ordinary toy emporia as well. They consist of a pair of magnets made of a polished substance with the general appearance of hematite. What is surprising, pleasing, and unexpected is that when the magnets strike each other under their own power, they produce a sharp, loud buzz that rises in pitch. The sound lasts a good fraction of a second and climbs somewhere into what sounds like the 200-500 Hz range. The exact sound and its duration are somewhat unpredictable and depend on how the magnets happen to strike each other. It is a little like the sound that you get when you mash a pingpong ball against a pingpong table with a paddle. What physics are involved in the production of these sounds?" "Google searches turn up some forum postings that indicate that it is a synthetic magnetic substance similar to hematite that's available cheaply in China as an industrial byproduct. The singing magnets are a little larger than size of olives; the shape is similar to a (U. S.) football but slightly more elongated. Their major axis is about 5 cm long, their minor about 1 cm. They are fairly powerful and will jump together when placed on a desk about three inches apart. They can distort the colors on a CRT display from a distance of over 20 cm.

Contrary to expectation, the poles of the magnets are oriented along one of the minor axes of the ellipsoid, not the major axis.

Neodymium magnets in 'ordinary' shapes produce boring 'plinks' when they snap together. Something about the shape of these magnets makes the sound much longer-lasting and entertaining. It is not simply the bounding rebound of two objects made of stiff-but-elastic material. Transfers of linear to angular momentum are clearly involved.

If course, I'd love to know whether these things were 'invented' or 'discovered', and by whom, trying to do what.

Comments

Um.

[Solder Fumes]

Despite what you say, they are just bouncing off each other. The modulus of elasticity is high, and because of the shape of the objects they can only contact at one point. The magnet pulls them together and they bounce apart, the point of contact possibly traveling along the curve of the objects depending on how much energy remains. They might maintain a semi-constant tone because this contact point travels to where the mass of the objects is less, at the ends, allowing them the bounce apart at the same frequency despite energy being absorbed in the material.

I don't have any so I can't verify this theory.

Re:Singing Sword

[s0l0m0n]

You are unaware of the basics of metallurgy in steel, as well as of 'singing' in swords.

A properly made sword rings when tapped. This is because the fittings (hilt, pommel and handle) are tight, and do not dampen vibration. If the blade has any cracks, these will also dull the ring of a sword.

Additionally, a good western sword flexs a fair amount during cutting or thrusting. This in itself is not a positive or negative feature. However, a blade that returns too much vibration to the hand may be uncomfortable to use (think about how a bat stings your hands when you hit something with it).

The addition of chromium to a steel in quantities of over 13% makes 'stainless' steel. Not only does the addition of high levels of Cr make the steel resistant to stains, it makes in more 'deep hardening'. This is in refference to the cooling of steel from the the point at which all of the carbon in the steel is disolved in the solution(AC1, also called the Currie point, which is generally above 1350 degress F) down to below the point(MS, below 900 degrees F) at which martensite (hardened steel) is formed.

In this process, a variety of different crystaline forms can be produced. If you cool slowly, you will probably end up with pearlite. This is soft, and relitively flexible, and not at all good for blades. If you cool faster, you will end up with grains (crystals) of martensite, which is harder, and more springy (once tempered) much better for knives and swords.

Now, back to why stainless is bad for swords. Stainless is deep hardening because the chromium pins the edges of the grains (crystalline stuructures of carbon and iron), preventing them from growing when heat is applied. Smaller grain sizes lead to increased hardness. Unfortunately, the introduction of the chromium into the edges of the crystals causes them to be less strong. This leads to lower flexiblity. Lower flexility leads to swords that fail castastrophically durning use.

I'm not a metallurgist by any means, but I have made a half dozen swords, a hundred plus knives, and been studying heat treating of swords for about a dozen years. Please, spend 300+$ on good old fashioned carbon steel if you must have a sword. Heck, even get a good stainless steel sword from Rob Criswell or one of the Dawsons, but quite buying that cheap stainless crap on ebay and in the Mall cutlery stores. Support a sword smith with real talent, here in the US. There are lots of us, It's a better deal in the long run.

Josh Powell, owner and operator of Josh Powell Custom Knives.