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Bill Nye Demonstration:  Radiometer

Bill Nye Demonstration: Radiometer

Author: Bill Nye

There's nothing than a perpetual motion machine sitting on your desk to strike the curiosity of everyone around. Let's take a look at one of my favorites, the radiometer. It's not really perpetual motion, but it does do a nice job of faking it.

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Simple Radiometer

Find yourself barefoot on a hot day? Where would you rather stand - an asphalt road or a concrete sidewalk? Good question.

How Does it Work?

The Radiometer provides an introduction to the study of thermodynamics that sits on your desk or table top. Set it in sunlight or incandescent light and watch the mill inside spin!

The radiometer consists of four vanes, each has one blackened side and one silvered side. It is encased in a clear glass bulb that has a near perfect vacuum. So what happens to make the radiometer "work"? 

Crookes originally believed that sunlight falling on the light-mill turned the vanes so that the black surfaces were being pushed away by the light. He thought that this demonstrated that light radiation pressure on the black vanes was turning it round just like water in a water mill.

The problem with Crookes' explanation is that light falling on the black side should be absorbed, while light falling on the silver side of the vanes should be reflected. This means that there would be twice as much radiation pressure on the metal side as on the black and the mill would actually turn in the opposite way.

What really happens is that the faster molecules from the warmer side of each vane strike the edges obliquely and impart a higher force than the colder molecules. The movement of the vane as a result of tangential forces around the edges, is away from the warmer gas and towards the cooler gas with the gas passing round the edge in the opposite direction. Therefore, the movement is actually due to what happens near the edges of the vanes rather than at the faces. (This correct answer, thermal transpiration, was first provided by Osborne Reynolds in 1879.)


Try This!

Here is one inquiry activity that may get your class spinning!


Source: GK-12 Down to Earth Science (ASU)

Learn More!!

Check out more explanations and extensions!

  • Energy Pathway - lessons developed for NGSS MS-PS3-3
  • The Sun and Its Energy - a series of 14 activities developed by the US Department of Energy for grades K-4 (NGSS PS3 - Energy)
  • Science of Energy - hands-on activities for grades 5-8 (NGSS PS3 - Energy)
  • Getting Energized! - teacher's activity guide for grades 3-6 developed by the National Renewable Energy Laboratory (NGSS PS3 - Energy)

What Else is There?

If you're interested in watching more of my demonstrations, checkout my playlist,

Your Turn!

Have cool science demonstrations you show your classes regularly? Save time and materials by making a quick video tutorial of your own for students. By using demonstrations and lab investigations regularly, students not only will know science, they can apply it!

Check out some resources on doing it yourself: