  Crookes' Radiometer exploits thermal forces arising from temperature
gradients in rarefied gases. This instrument has remained confined to
educational physics because of the very small size of the radiometric effect
and because of the necessity of an evacuated environment for operation. In this
article the physics of this instrument is reviewed, different theories are
compared and some general principles are derived. A method for enhancing the
radiometric effect and for generating radiometric forces at higher pressures is
proposed.
