Editing Crookes radiometer (section)

=== Effect observations ===
The effect begins to be observed at partial vacuum pressures of several hundred [[pascals]] (or several [[torr]]s), reaches a peak at around {{convert|1|Pa|Torr|abbr=off}} and has disappeared by the time the vacuum reaches {{convert|1e-4|Pa|Torr|abbr=off}} ([[Crookes radiometer#Explanations for the force on the vanes|see explanations note 1]]). At these very high vacuums the effect of photon [[radiation pressure]] on the vanes can be observed in very sensitive apparatus (see [[Nichols radiometer]]), but this is insufficient to cause rotation.