  The existence of two motive forces on a Crookes radiometer has complicated
the investigation of either force independently. The thermal creep shear force
in particular has been subject to differing interpretations of the direction in
which it acts and its order of magnitude. In this article we provide a
horizontal vane radiometer design which isolates the thermal creep shear force.
The horizontal vane radiometer is explored through experiment, kinetic theory,
and the Direct Simulation Monte Carlo (DSMC) method. The qualitative agreement
between the three methods of investigation is good expect for a dependence of
the force on the width of the vane even when the temperature gradient is
narrower than the vane which is present in the DSMC method results but not in
the theory. The experimental results qualitatively resemble the theory in this
regard. The quantitative agreement between the three methods of investigation
is better than an order of magnitude in the cases examined. The theory is
closer to the experimental values for narrow vanes and the simulations are
closer to the experimental values for the wide vanes. We find that the thermal
creep force acts from the hot side to the cold side of the vane. We also find
the peak in the radiometer's angular speed as a function of pressure is
explained as much by the behavior of the drag force as by the behavior of the
thermal creep force.
