My fiancée has Raynaud's disease, which ends up turning her fingertips white when they get cold. In other words, cold weather is tough because her fingers start to hurt.

I have thought about making some mittens with a heating element built in and powered by something like a 9V battery, but I'm not sure how to go about doing that. Any thoughts? In its ultimate form, it would have some sort of thermostat in there too, but I'd be fairly content with an on/off switch for the time being.

A quick google search gave me a couple of patents and a product or two, but nothing that useful in DIY terms. We seem to have a pretty sizeable knowledge base here when it comes to electrical concepts, so I thought you guys might have some ideas on where to start.