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.

The great thing about dealing with reality is that each session of “playing with the phenomena” leads to new questions and ultimately to greater understanding. For example: » Try different designs for your turbine. Any light-weight pinwheel or propeller shape is worth a try, and some will work spectacularly well! » Will the wheel spin if the air temperature is essentially the same as the surface temperature of your hand and fingers? Nope, if at all only erratically and randomly due to small regions of air turbulence, which is the clearest demonstration of the fact that we have a heat engine. » Will the turbine spin if it is placed inside an inverted glass or plastic box? Usually, depending on where your hands are. If you close your hands against the sides of the glass or box, you are heating the sides by conduction, and then convection occurs inside the enclosure to drive the turbine... not too different from heating the air near your fingers directly by conduction, and then having it convect away upward. What if you don't touch the container with your hands, but keep them very close to it? Well, remember from Middle school that there are three commonly-encountered methods of heat transfer: convection, conduction... and radiation. Under normal conditions the human body loses about 2/3 of its heat by radiation. You can still produce convection inside the enclosure by radiation from your fingers and palms. It can be a bit less efficient, but heat transfer is heat transfer. Any approach that will transfer heat from your hands to the air just below the turbine will get it into motion.

Now here is a tragic part to this adventure. As mentioned elsewhere, a significant fraction of the world population is still mentally deep within those utterly Dark Ages of 6 centuries ago. For the U.S., the time-trapped fraction seems to be about 1/2! Down in the darkness there's no science, only competing superstitions, and so to this literally benighted segment of the crowd, our little heat engine has been sold and re-sold and sold again as a miraculous, supernatural entity... a little device driven by the mysterious powers of whatever, a mind-powered motor, a psychic propeller, a telekinetic turbine, a... you get the idea! Concentrate, concentrate, and the little thing spins magically. Don't worry about why your hands have to be there, it's your mind that's doing it, we guarantee! Don't hope to learn anything about how the world you live in actually works from this nifty example... instead marvel at the wonder of being trapped at the bottom of the mindless, meaningless black pit of “incomprehensible, mysterious POWERS!” Here are some nice videos of simple experiments that show clearly how the engine works by convection, rather than by imaginary, unspecified, untestable psychic something or the other. This site also explains how to make a similar engine using a plastic drinking straw hung from a thread, or balanced on a bottle, but actually two different physical phenomena are often mixed in this straw example, which we will treat in detail later. Stay tuned. By the way, for something truly magical yet based in simple physics, consider the Stirling Heat Engine.

Magicians have worked out a variety of methods to control “psychic spinners” invisibly, so if someone shows you any sort of spinner that doesn't appear to use heat transfer, suspect one of several available magical methods which I have no intention of revealing here! One method that can be very deceptive but takes great practice is to learn to project a narrow stream of air silently from nearly-closed lips. I have seen a shady character who was actually able to turn the pages of a book by this means!

This video summarizes almost all relevant “psychic wheel” physics, without explanation.

Next science page?