William Crookes was working late in his lab when he made the discovery that would become an obsession and the centerpiece of his life for the next several years. He discovered that when he brought a source of heat close to a ball suspended in a vacuum, the ball would be repelled.

He thought that he had discovered a link between heat and the force of gravity. This started him on a long series of experiments that eventually led to the common Crookes radiometer that you see today.

This book tells the story of those experiments and how some of the greatest physicists of that time grappled with the enigma of how the device works. It also explains the current physical understanding of the device and presents a few basic physics problems related to how the device works.