© Science Museum/Science and Society Picture Library
William Crookes (1832-1919) devised the radiometer in 1875 while investigating the behaviour of hot bodies in a vacuum. This example was made in Crookes' laboratory in 1877. The pivoted vanes, each blackened on one face, rotate inside the nearly evacuated globe when it is placed in sunlight. Crookes made many important contributions to science. His hope that the radiometer would be used to measure heat radiation was not fulfilled, but it remains a fascinating scientific toy. How it works was not explained fully for many years.