These curious and mesmerising instruments measure the power of electromagnetic radiation. Made from a glass bulb with much of the air removed to form a partial vacuum, inside is a set of fine metal vanes, each with one dark and one light side, mounted on a spindle. The vanes rotate, as if by magic, when exposed to sunlight, artificial light, or infrared radiation (even the heat of a hand nearby can be enough) – the more intense the light, the faster the rotation.