Editing Crookes radiometer (section)

==General description==
[[File:Radiometer 9965 Nevit.gif|thumbnail|250px|right|A Crookes radiometer in action]]
The [[radiometer]] is made from a glass bulb from which much of the air has been removed to form a partial [[vacuum]]. Inside the bulb, on a low-[[friction]] spindle, is a rotor with several (usually four) vertical lightweight vanes spaced equally around the axis. The vanes are polished or white on one side and black on the other.

When exposed to [[sunlight]], artificial light, or [[infrared]] radiation (even the heat of a hand nearby can be enough), the vanes turn with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing.

Cooling the outside of the radiometer rapidly causes rotation in the opposite direction.<ref>{{Cite web|title=the radiometer using inquiry to teach energy conversions|url=https://www.uakron.edu/polymer/agpa-k12outreach/professional-development-modules/the-radiometer-using-inquiry-to-teach-energy-conversions|access-date=2021-10-10|publisher=The University of Akron, Ohio}}</ref>

=== 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.

===Origin of the name===
The [[prefix]] "[[Wikt:radio-|radio-]]" in the title originates from the combining form of Latin ''radius'', a ray: here it refers to [[electromagnetic radiation]]. A Crookes radiometer, consistent with the [[suffix]] "[[Wikt:-meter|-meter]]" in its title, can provide a quantitative measurement of electromagnetic radiation intensity. This can be done, for example, by visual means (e.g., a spinning slotted disk, which functions as a simple [[stroboscope]]) without interfering with the measurement itself.

Radiometers are now commonly sold worldwide as a novelty ornament; needing no batteries, but only light to get the vanes to turn. They come in various forms, such as the one pictured, and are often used in [[science museum]]s to illustrate "radiation pressure" &ndash; a scientific principle that they do not in fact demonstrate.