Magnets: How do they work? Differently on the new iPad than on the iPad 2, it has emerged — and that's bad news for anyone with an old or third-party smart cover.

A number of users who bought new iPads over the weekend (this reporter included) were dismayed to discover that the smart covers they'd bought for the iPad 2 didn't work on the new model.

Smart covers, which attach to the iPad's built-in magnets, are supposed to turn the tablet on automatically when you flip them open. But as dozens of iPad users in this Apple forum concurred, that was no longer the case with many smart covers on the new iPad.

That seemed odd, as the new iPad is physically no different from the iPad 2 — on the surface, at least. But it turns out Apple has been messing with the polarity of its magnets under the hood.

Photographer Mark Booth uncovered this when he did some experiments with magnets, iPads and a couple of smart covers. You can see the results in a video on Booth's blog.

"The iPad 2's sleep/wake sensor wasn't polarity specific," Booth explains. And that apparently led to an issue for iPad 2 users who flipped their smart covers around so that they sat flush with the back of the tablet — an everyday act that could cause the iPad 2 to switch off unintentionally.

So it seems the new iPad's sleep/wake sensor does require a specific polarity. But in fixing one issue, Apple appears to have caused another for users who want to use their old smart covers with their new tablets. (We've asked Apple to comment, and will update you if we hear back.)

Booth theorizes that Apple quietly made the polarity change in their smart covers at some point in 2011, since newer Apple-made smart covers do seem to work with the new iPad. If you got yours for the holidays, there's a good chance you won't see a problem.

If your smart cover hails from early 2011, try taking it back to the Apple store; Booth says he's hearing from users that the store will exchange old smart covers for new ones. Owners of third-party smart covers, however — such as the beautiful wood covers from Miniot — appear to be out of luck.

And it may not simply be a case of having to turn the new tablet on manually. In tests using my Miniot as a stand — the other purpose of the smart cover — the new iPad's screen was plagued by fuzzy electronic lines and flashing artifacts.

Do you own a smart cover and a new iPad? Have you noticed this issue? Let us know in the comments.

BONUS: The New iPad