I’ll bet you recognize the light-bulb looking object shown here on my windowsill, but I suspect you don’t know much about it.
The first one I remember seeing was not in a science class, but in the window of an doctor’s office that I passed many times as a kid. I was fascinated by the fact that it turned without any power source. Was it a perpetual motion device? No, because it required sunlight to move. Was it my introduction to solar power? Sort of, but not accurately.
I wanted one and I finally ordered from a science supply catalog. It has been spinning near all of my desks for about 40 years.
I had heard the term “perpetual motion” and was intrigued by the notion of movement that goes on forever. The idea of something that perpetually (indefinitely) produces more energy than it consumes fascinated me.
Somewhere along the way, I was exposed to the law of conservation of energy which states that energy cannot be created or destroyed and also, disappointingly, implies that such a perpetual motion machine cannot exist.
When I was about ten years old and deeply absorbed by my basement chemistry sets and “lab,” the idea of creating a “perpetual motion machine” took hold of me for a time. The idea of making something that broke a law of nature seemed like the biggest thing you could do. It was like being a Newton or an Einstein. (I similarly remember that when my fifth grade teacher told us that every word in English had a vowel in it, I wanted very mucgh to find one that did not have a vowel.)
My quest for that machine that does not violate conservation of energy, but (like my radiometer) produces work by spontaneously extracting heat from its surroundings, thereby cooling them down, and converting the heat energy into mechanical work, never succeeded. It I had succeeded, you defininitely woould have heard about it.
It turns out that such machines are forbidden by the second law of thermodynamics.
My radiometer (AKA light-mill) was invented by the English physicist Sir William Crookes. It consists of a set of vanes reflective on one side and blackened on the other and mounted on a sensitively balanced spindle in a partially evacuated vessel. When exposed to light, the vanes rotate.
Why? As the blackened vanes become warmer and repel air molecules from the surface, the slight difference in air pressure created causes the vanes to rotate.
The speed of rotation is affected by the pressure within the vessel. Higher pressure will increase drag and will be the dominant force affecting the vanes while at low pressure the molecular recoil will dominate. If the pressure is reduced too far there will be too few recoiling molecules to drive the vanes.
The type that bears Crookes name (mine is one of those) is an early-model radiant energy-detector. A variant type is the Nichols radiometer that operates on a different principle, and is more sensitive than the Crookes type. As an eponym, radiometer usually means a Crookes radiometer.
I recommend that you go ahead and buy a radiometer
MORE
The Properties of the Force Exerted in a Radiometer
