Resumen de An Unexpected Heat Engine
It's common knowledge that light can produce chemical and electronic changes (photography is based on those effects); however, many people consider light to have no mechanical effect. Some are familiar with the novelty of a radiometer that spins when placed in the light. Fewer are aware that a cymbal will vibrate audibly if hit by the light from a camera flash.1 It came to our attention that the bubble in an ultra-sensitive level would move toward a flashlight beamed at the level from one end. Our investigations of this phenomenon show that it is thermal in nature and the effect large enough to be demonstrated in a classroom. With the addition of a small heating wire, the bubble level can be converted into a small heat engine.
The basic concept of a bubble level is that a bubble of gas in a transparent container of liquid rises to the highest point. They are also called �spirit levels� because the liquid usually is an alcohol. The typical carpenter's level that can be purchased in a hardware center has one or more curved tubes (oriented for vertical or horizontal alignment) with lines indicating when the bubble is centered.
The ultra-high-sensitivity bubble levels use a barrel-shaped bore rather than a curved tube. This allows for a much gentler and more precise curve, thus producing significant bubble movement with tiny angles of tilt. Because of this extreme sensitivity, tiny forces other than gravity can make the bubble move. In a poorly lighted area, if someone were to use a flashlight to check her reading on one of these levels, she would be amazed to see the bubble move as she watched (see Fig. 1).
