With this circuit, one that was start to finish in 2 hours, is how I was able to ride more comfortably for 30 miles in 12F. Probably would have been able to assemble it in 30 minutes if I had an instructable.
So what is the magic in the project that I had this stuff at home, on hand. They are eight 3 watt resistors, wired in series and parallel to make approximately 80 degrees for the back of each of my hands. By the way, you are working with a circuit that if wired incorrectly could produce 140 degrees, so it's important to wire it the way I did and test it before use to make sure it is at the required temperature, not too hot. This also isn't supposed to make every part of your hand feel like it's on a Summer vacation from cold weather. It is supposed to gently warm your hand and reduce the discomfort associated with lower temperatures while riding. I also ride with my hands protected from direct wind blast, so you see where this is going, just warm.
In the wiring diagram are 4 resistors for each side, they are wired in parallel. The final product has the wires much closer as shown in the picture, but I wanted to show how it is wired in that the two sets of resistors are wired in series. The total draw on my electrical system is 200MA, or 0.2 amps. I used harvested extension cord for the wiring. Was it overkill for how much the circuit draws, absolutely, it was also decided to use the gauge of wire for engineering, so it wouldn't break.
The yellow in the circuit diagram denotes stripping about 4 inches off of the end to wrap around each wire in the resistor, clip off the excess then solder. I used flux and a propane torch. An electric soldering iron won't have the ability to heat this gauge copper.
This worked for me because I think I run hotter than other people do, and only needed a little assistance with heat. To play with the temperature, reduce one resistor from each pack totaling 3 resistors per side. Please do me one favor messing with this. As you test or rearrange your own circuit, treat it like you would respect turning on an electric stove. This circuit generates heat a lot like heated gloves do for you that are commercially available. Plug it in for at least 4 or 5 minutes before assuring how hot this will get, then put your gloves on over your hands and just sit there. I would prefer not to hear "It got really hot on my hands yo, and wrecked my bike cause it got too hot." Use your sense here, adding warmth to your hands can extend your riding to all year if there isn't snow or ice on the ground. Mod