Battery arm heater

It gets bloody cold here in WI, so I made an arm heater to prevent more frostbite (I'm looking at skin grafts if I do it again). All it is is a high resistance wire wrapped around the socket attached to a 1.5v battery. Put a battery in the holder and the resistance in the wire causes the wire to heat up as the current passes through it. My prosthetic guys were kind enough to inset the battery pack for me so I dont have to keep screwing it to the side, which in turn causes it to get caught on everything. I have a neoprene cover that protects it when not in use. The heater uses a D battery and will stay warm for about 9 hours or more.