Glass Solar Radiometer on a Tapered Stem
Crooke's Solar radiometers are also known as light mills because light moves their blades as opposed to wind for windmills.
The Crookes radiometer was invented by the chemist Sir William Crookes as the by-product of some chemical research. In the course of very accurate quantitative chemical work, he was weighing samples in a partially evacuated chamber to reduce the effect of air currents, and noticed the weighings were disturbed when sunlight shone on the balance. Investigating this effect, he devised the device named after him, still manufactured and sold to this day as a curiosity item.
A Crooke's Solar Radiometer comprises an airtight bulb containing a partial vacuum, which normally either sits upon a glass stem or is suspended in the case of a hanging radiometer.
Inside the glass bulb is a fine vertical spindle with a set of vanes mounted at the top. The vane panels are darker coloured on one side, and lighter on the other. When the glass bulb is exposed to light, the 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 gift and talking point, 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.
Height: 19cm (7.48 in)
Diameter: 6 cm (2.36 in)