To those not in the know, these are high precision (for a toy) gyroscopes, whose main function is ostensibly "wrist and arm exerciser", for either building up strength or rehabilitating after an injury. Mine was an enlightened Christmas gift. I love gyroscopes.
They consist of a plastic sphere about the size of a tennis ball, with an opening at the bottom about 35mm across, exposing the surface of a spherical rotor inside. This spins about a thin steel axis with a circular cross-section, which extends a few mm either end. These ends fit into what is best described as an equatorial groove, only rather than being analagous to a canyon here on our humble earth, it's the opposite: more like a square section groove cut into the sky. Crap analogy I know, but I can't think of a better way of visualising it. The groove, essentially, is cut into the inside surface of the sphere. Here's what it looks like exploded.
The plastic rotor is weighted, and perfectly balanced with the aid of precision-placed metal ballasts, but there is a not inconsiderable additional mass at one pole. So it spins perfectly about its axis but there is a tendency for it to want to change orientation as different forces are applied, because of the extra mass at one of the poles.
The idea is that you start the rotor manually (with a cord, or without, once you're good enough) and then twist the wrist in a circular motion in such a way that the rotor spins faster and faster. This takes a bit of practice. As it spins faster, the torsional forces become amazingly strong. In fact, the upper limits of the RPMs are due to the fact that to make it spin ever faster you have to move your wrist in smaller and smaller circles, like the classic analogy of the skater spinning faster and faster by bringing in his or her arms. Keeping your wrist still then becomes the weak point: you need tremendous strength to do it, and this is where the exercising and muscle toning come into play.
The whole process is very addictive, and made more so by the fact that buried in the top (of the 'Pro' models) is a digital RPM meter. So the need to beat your previous high score is quite compelling, and in fact there exists a table of the world's fastest Powerballers, which anyone can top by simply spinning the fastest, and videoing the attempt in order to prove it. The record has been held for some time now by a chap called "Akis" who seems invincible and has reached an incredible 16,218 RPM (video here). All the videos of him are actually quite funny (and look potentially indecent; see animated gif above), because the action of spinning the thing up is an unnatural one and it just looks, well, bizarre as you approach the very high revs. It's clearly a lot of effort, and I can vouch for that despite my meagre personal best of only 12,357 RPM. It is interesting to see his technique, which illustrates well the point about having to keep your wrist rotating in as small a circle as possible to achieve the highest speeds.
There's more. There are versions of this fabulous instrument which have no rev counter, so avoid these, but there are also versions with LEDs -- on the rotor -- which are powered by a small generator within. The top speeds of these ones are about 5% lower, because of the power needed by the LEDs, so they are not used by the out-and-out speed freaks, but they look quite spectacular in the dark, and the very principle is fantastic. Speaking of which, there's also the Techno version, which displays the RPMs on the surface of the rotor itself, I think using some sort of persistence of vision technique. Again these are slower, but look like good fun.
Because of their speed, the Powerballs have been given the tag "250Hz", which equates to 15,000 RPM (250Hz x 60 seconds => 15,000), their nominal top speed.
Then, as if that wasn't enough, there's the chrome one. This is, as the name implies, metal, having a heavy metal rotor as well as a chromed metal case. Because the rotor is twice the mass of the plastic ones, the top RPMs are less (Akis manages just over 12,000), but there's a lightweight rotor in production which should be available soon, which is predicted to provide RPMs in excess of 20,000 (hence its tag "350Hz" => 350x60 => 21,000 RPM). The whine of the normal ones at 12,000 is quite phenomenal, and at 20,000 the chrome one must sound like it's about to take off. I want one.
It occurred to me that there might be those among us on Novacaster who would be very interested in these from their sciencey point of view... after all, it is essentially a "precession machine". I think we know to whom I am alluding :-) Anyway, I highly recommend that everyone buys one, immediately.
Approximate price for the plastic ones is 20 quid, the metal ones are hard to find and are 70 quid.
It's worth mentioning the after sales service of these folk, which looks like the icing on the cake. They offer a lifetime guarantee on all powerballs, such that if anything ever breaks or wears out they will replace it for free, including reimbursing postage, if you send them the old parts. For ever. How enlightened is that?!
If you want to buy one I'd suggest kelkoo-ing around rather than buying from the powerballs.com site, because you'll find them a lot cheaper. And if you want to have a go with mine I'm sure that could be arranged.
Despite that this reads a lot like a press release, I have no affiliation with the Powerballs company.
This has in fact been a public service announcement.