HUMAN-POWERED MOTOR!

How about building and playing with a heat engine powered by the palms and fingers of your hand? Specifically, a paper turbine driven by convection. If you remember your Middle-school or High school physics course, you remember that heating a gas or liquid decreases its density, so that it is buoyed up by the surrounding cooler gas or liquid, the way a cork is buoyed up by water--- this process is called convection, as we said. Now, while the human body's internal temperature usually stays close to 36 to 37 Centigrade degrees, the temperature of the skin surface depends wildly on external conditions. In a room with a temperature of 22 to 25 degrees, say, the temperature of the palms and fingers might be 31 to 34 degrees Centigrade. This relatively large temperature difference drives convection currents that rise from the fingers and palm and move initially almost straight upward. An actual photo of these currents for an open palm is shown below.

Now to make a turbine for this flow of kinetic energy to drive. Take a sheet of copying paper and cut off a square that is 2 x 2 inches. Now crease the diagonals of the square sharply, both creases in the same direction. Now take the center of the square gently between your finger tips, and “close it” as if it were a little umbrella with 4 ribs. The result will be a sort of 4-cornered star, as shown in the photo at the top of the page. You now need to find a square of cardboard to act as a base, and preferably a sewing needle about 3 inches long. Drive the needle through the cardboard and gently rest the turbine (guided by the creases) so it is balanced on the needle. In a pinch you can unfold a jumbo paper clip, as seen in the sketch, and tape the curved part to the cardboard, but you will have to sharpen the opposite, straight end somehow with a file or sandpaper.

 To start your wheel, place it on a table that is not subject to drafts or disturbances from open doors or windows. Cup one or both hands around it and wait. If nothing happens, adjust your hands. Sooner or later you will find a “magic” position where the pinwheel intercepts the rising air currents with maximum efficiency. You can also place tall cups of hot water near the pinwheel, but it is more fun to use your bare hands! By controlling the relative positions of the heat sources on either side of the pinwheel, you can make it spin either clockwise or counter-clockwise as seen from above.