Solar radiometers are also known as light mills because light moves their blades as opposed to wind for windmills. They consist of an airtight bulb containing a partial vacuum. Inside the glass bulb is a fine vertical spindle with a set of vanes mounted at the top. When the glass bulb is exposed to light, the weather vanes are propelled around, the speed of propulsion increases with the intensity of light, providing a qualitative way of measuring electronmagnetic radiation. These days the Solar radiometer, or Crooke's radiometer (named after Sir William Crooke who invented them), is more of a beautiful science gizmo than a means of accurate measurement. Our Solar Radiometer shop stocks a range of exquisite radiometers which make unusual and interesting gifts for scientists and science enthusiasts.