Observe the power of sun and thermodynamics with the Crookes Radiometer. When placed in direct sunlight, the metal vanes will rotate inside the glass vacuum. The vanes are silver on one side and black on the other—sunlight heats the black sides faster than the silver, causing a small current of air. The more intense the sunlight, the faster the radiometer will spin. No power necessary! The perfect high end gift for a sci-curious friend or coworker.