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How Does a Radiometer Bulb Work?

You may have seen one in a science or novelty shop, a glass bulb that contains a four vanes spinning on a pivot. The device, called a Crooke̵7;s radiometer, is set into motion by the sun or artificial lighting. It uses no electricity or conventional motor; instead, the vane turns from the energy of air molecules inside the bulb.

Radiometer
- A radiometer consists of a clear glass bulb about the size of a standard light bulb. Inside, a steel needle points vertically from a glass stand inside the bulb. Four thin vertical squares of metal are fixed to a central pivot, like blades on a propeller. The pivot rests on the needle point, which serves as a low-friction bearing. When you shine a light on the vanes, they spin in a circle about the bearing. The stronger the light, the faster they spin.
Vanes
- One side of each vane has a nonreflective black coating; the other side is white. All the white sides face the same direction, and the black sides face the opposite way. When light shines on the vanes, a temperature difference develops on them. The black side absorbs light and becomes significantly warmer than the white side.
Vacuum
- The glass bulb of the radiometer contains a partial vacuum; while most of the air has been removed, a tiny amount remains. The air molecules in the near-vacuum bump mostly into the vanes, they do not frequently bump into each other.A lack of air in the bulb also decreases aerodynamic drag, allowing the vanes to spin more easily.
Molecules in Motion
- Air molecules that bump into the white side bounce off with their original energy. Those that hit the vanes̵7; black sides, however, pick up energy and bounce off with more force. From Newton̵7;s Third Law of Motion, every action has an equal and opposite reaction. The air molecules that pick up energy push against the vane̵7;s black side harder than on the white side. This imbalance of forces makes the vane spin toward the white side.

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