Original Crookes radiometer, first series, 1900. The Crookes radiometer (also known as a light mill) consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. The vanes, built with light metal, rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. It was invented in 1873 by the chemist Sir William Crookes as the by-product of some chemical research. The radiometer on auction, unlike some imitations with a slight bump on top, has a perfectly rounded glass bulb. Dimensions: 140x80x80 mm PRO insured shipping with proper material (sturdy hard box, 1 cm polystyrene layer on all sides of the box, protective film and 200 g bubble wrap to protect your parcel from shocks and falls). The item of this auction will be shipped via DHL/NEXIVE.