(also, radiometric action), the repulsive force between two surfaces maintained at different temperatures (T1 > T2) and located in a vessel containing a rarefied gas. The effect arises because molecules that strike the surface with T1 rebound with a mean kinetic energy greater than that of the molecules rebounding from the surface with T2.
Suppose a “cold” plate and a “hot” plate face each other in such a vessel. The side of the cold plate facing the hot plate is bombarded by molecules having a higher mean energy than the molecules striking the cold plate’s other side—that is, the side facing the wall of the vessel, for which T = T2. Owing to the difference in the momenta imparted by the molecules to the opposite sides of the plate, a repulsive force arises. At a sufficiently low gas pressure p, the mean free path of the molecules is greater than the distance between the surfaces, and the repulsive force per unit area is
As ρ increases, F decreases even though a greater number of molecules is participating in the transfer of energy. The reason for this is that the fast molecules lose some of their energy when they collide with slower molecules. Thus, at low pressures the repulsive force F is directly proportional to p, but at high pressures F is inversely proportional to p. At some intermediate p, the value for F passes through a maximum. The operation of the radiometric manometer is based on radiometric effect.