Yesterday I wrote about a rare earth material called gadolinium that loses its attraction to magnets when it reaches room temperature. And I wondered if you could use this strange phenomenon as the basis for a generator. I thought it was obvious that a device depending on outside heat was not a perpetual motion machine any more than a wind mill is, but but many of you left comments to clarify that point. Apparently my writing was sloppy because the earlier paragraphs were about perpetual motion and I didn't call out the transition to non-perpetual motion and the gadolinium generator. In the interest of clarity, the rest of this post is NOT about perpetual motion in the strictest sense.
Someone pointed out that gadolinium doesn't lose its attraction to magnets; it just becomes less of a magnet itself, but would still be attracted to magnets like metal. There appears to be some conflicting information on the Internet on that point. The stuff I read indicates that a warmed piece of gadolinium wouldn't be attracted to a magnet in any fashion. Your unreliable strangers might be more reliable than my unreliable strangers.
But here's the interesting part. When you expose gadolinium to a magnet, it heats up. That property has been used to create refrigeration, although I don't see any recent articles about it. http://www.eurekalert.org/features/doe/2001-11/dl-mrs062802.php
So if what I read on the Internet is correct (which seems hugely unlikely) you could build a (nearly) perpetual motion device using a natural magnet and gadolinium as long as you could control the room temperature without any extra energy. Here's how it would work: Imagine a natural magnet suspended over a piece of gadolinium in a room that is just below room temperature. The gadolinium is attracted to the magnet and jumps from its resting point to attach to the magnet. Now the gadolinium is experiencing a stonger magnetic force, and according to its natural and unique properties, it heats up. That extra heat puts it above room temperature and it becomes suddenly unattracted to the magnet, falls off, and begins to cool. And repeat, forever, or until someone opens a window.
I suppose you'd use more energy keeping the room at the right temperature than you'd create by the process. But still, if it worked outdoors for half an hour every day, as the world went from cool to warm, in certain climates on certain days, it would still be nifty.
Clearly this won't work, or you'd have a toy like this on your desk as a novelty item that only operates when the room temperature is in a particular range. So I assume I am misinterpreting the qualities of gadolinium. But that doesn't make it any less fun to think about.