Green Light Mill, two sizes
The light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity.
Important: This is not a mass produced item, but is handmade, made using natural materials including slate. Because of this, the colour and size of each mill can vary slightly. The panes of slate are painted black on one side and it is entirely normal that very small flecks of paint should (not always) fall off - so long as the light mill is spinning when exposed to light, this does not constitute a fault.