QUOTE(AmandaL @ Jan 14 2007, 07:12 PM)

If you live in the UK it's unlikely you'll need a humidifier. As an island surrounded by water and with the amount of rain we usually get, the atmosphere is damp enough already.
Generally speaking, smaller instruments fair better than large ones. This is simply due to the amount of surface area. Therefore a violin is fair more likely to survive without problems in a spell of dry weather, than maybe a cello or double bass.
Last winter, when we had weeks (months) of cold dry weather I did put Dampit humidifiers into my instrument cases, just to be on the safe side. I normally wouldn't use such things, other than if I was going abroad to very dry climates - hot or cold.
If you have central heating (and I don't) then this is also a very real threat, especially if instruments are inadvertently stood near radiators. Humidifying the room/air is more practical and more effective than trying to humidify each instrument in its case. The use of a potato sounds like a cheap one, but possibly a bit smelly....
Digital hygrometers are a cheap and easy way of keeping an eye on how humid the air is in any particular room.
Remember that too much humidity can also play havoc with musical instruments.
*Remembers* My clarinet once got drenched in a rainstorm (What a way to discover your case isn't waterproof!), I stupidly left it near the radiator overnight, and wondered why it simply refused to play the wierdest combination of notes (Just one, over the entire range, just one note. It would be the one I need to play most as well.)