Heat Resistance

You probably need special resistance wire.

The best way to control the heat is just to pulse the power on and off...don't try and adjust the heating by varying the voltage.

Easy enough to make a pulsing circuit with a 555 timer.

Hopefully someone else will know more about the specifics of the wire required.

Del

It's all in the wire selection. Different materials have different properties. Some have a positive coefficient of resistance, meaning as the temperature goes up, so does the resistance. Some have negative coefficient of resistance, meaning as the temperature goes up, the resistance goes down. I would think that for something powered by a small battery with limited life, you would want a positive coefficient; something like NiCr wire.

http://www.wiretron.com/design.html

<....connect the two battery terminals with a short low restance wire there is a good chance the battery would overheat or explode...>

A suitably-rated fuse placed in series with the resistance is all that is required to prevent this, and it will protect the wiring too.

how do I protect myself from battery fatigue?

Don't make your body part of the test. (Probably not your pet either....Oh! Was that what you're thinking about?) Use metering/measuring instead. And keep the 9V cell and your change/keys in separate pockets.

You would need to determine you wattage density required to keep warm a cold foot. You can probably use the thermal conductivity of meat to calculate how much heat you would require. I would also determine the surface area of the sock.

Once you find out the heat required to heat the meat (foot) Total wattage, then you can find out your watt/sq inch requirement. If you know your voltage you can find the resistance required.

Resistance required = Voltage² /Wattage

Once you know your resistance required you can then source a wire if that is the method you are using. I'd suggest a stranded/braided resistance wire. You can get it shielded as well.

Another thing to consider though might be resistive fabrics. There are several products out now that use a fabric as a resistive medium.

Hopefully I didn't confuse you or muddle this up.

Another point ... I hope you are not thinking of using dry cells (non rechargeable) as you will get v little battery life, although Alkaline dry cells do have better energy density than re-chargeable you will be forever replacing them...they are ok for experimentation tho' especially as they are less of a fire hazard than re-chargeables.

For info:- Alkaline dry cells start at 1.5v and can be used down to about 0.8-0.9v to get decent life. Check any battery manufacturers web site for details.

Del