It is a radiometer with vanes consisting of a glass bulb (diameter 6.5 cm) ending with a stem, it too of glass, which lies on a black-lacquered, lathed wooden base (overall height 18.5 cm). Inside is a rotor with four very lightweight arms which can rotate, with no appreciable friction, around a vertical axis, since it lies with a glass head on a steel point. At the end of each arm there is a very thin, square mica vane (1.5 x 1.5 cm) having one side blackened; the blackened faces are all oriented in the same direction with respect to the axis of rotation.