Light Mill

This captivating physics toy transforms light into action. Invented in 1873 by the chemist Sir William Crookes as the by-product of some chemical research, the radiometer is made from a glass bulb with a set of four two-tone vanes inside. All the air has been removed from bulb which creates a partial vacuum and allows vane to rotate freely on a central spindle.

When exposed to sunlight, artificial light, or even heat, the vanes begin to spin—the dark sides retreating from the radiation source and the light sides advancing.

Experiment with different energy sources to watch the interior of the Light Mill spin faster or flower. For those who don't know the science of how a radiometer works, the movement can seem inexplicable. A simple explanation may seem to dispel the mystery but the more deeply you think about the thermodynamics at play, the more profound the wonder becomes.