The radiometer: a 130-year-old mystery

Radiometers are today just considered to be a novelty, but once these devices constituted serious science. Most commercial radiometers are constructed of a machine-blown, lightbulb-shaped glass envelope, which encloses four vanes spinning on a central pivot within a partial vacuum. When set near a sunny window it spins rapidly around in circles. There are a few commercial variations in the design but the principle of operation is the same; the problem is that no one knows for sure what that principle is. Many theories exist and a good number of scientists have grappled with the problem over the years. Fortunately for us, most of the current theories can be easily disproved using a little logic and some basic 21st century science. However, the question remains as to what makes it spin. This article looks at past and present theories, and suggests some experimental approaches.