The original Radiometer was invented by Sir William Crookes in the mid-nineteenth century to measure the intensity of radiant energy. The glass bulb contains an object that looks like a weather vane. Opposite sides of each metal vane are matt black and silvered. As light hits the vanes (sunlight, indoor light), the polished side reflects the light while the dark side absorbs it and this results in a temperature difference between the two sides of the vane. As the air molecules inside the bulb kick away from the hot dark side, they transfer momentum causing the vane to spin. Note: The rotation of the radiometer vane is not related to the momentum of the photons (the particles associated with light itself) as often stated; as Crookes found, if the bulb is (almost) completely evacuated the vanes do not turn."