An Unusual Effect: Thermal Transpiration in Crookes’ Radiometer
Mentor:Timothy Heumier, Associate Professor of Physics, Azusa Pacific University
The Crookes Radiometer consists of four vanes supported on a low-friction spindle in a partial vacuum contained in a light bulb-like housing. The vanes, blackened on one side and white on the other, rotate when light is shined on them. The black side gets hotter than the white side, and this apparently drives the vanes by interaction with the tenuous gas in the radiometer. The explanation of precisely why the vanes begin to rotate has been debated since the 1800’s, and many great minds of physics, such as Einstein and Maxwell, have explored the Crookes Radiometer.
The operating vacuum is between the pressures of 10 – 80 milliTorr. Modeling of the movement of gases is difficult in this pressure regime, and it has been thought that thermal transpiration, the flow of hot gases from the black side of the vane to the non-black side, causes a torque on the vane which causes it to rotate. In studying thermal transpiration, we ran into an unusual effect: After light is removed, the vanes slow to a stop, and will sometimes reverse! This reverse effect has not been studied or discussed at length.
To study this effect, we compared the motion of the radiometer from some initial angular velocity to zero when thermally excited to the motion when solely mechanically excited. By capturing weak pulses of light passed or blocked by the vanes, we measured the angular position of the vanes as a function of time for both types of excitation. The thermally excited radiometer stopped much more quickly than the mechanically excited one. The time dependence of the deceleration of the thermally excited vane will give theoretical researchers new information in the study of thermal transpiration.