This wooden BBC crest (1934), carved by George Kruger Gray, is found in the Council Chamber of Broadcasting House, London.

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The Crookes radiometer (also known as a light mill) consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. Mechanical idents or screen clocks were shown at major junctions between programmes and always before the news. This design does not experience the Einstein effect because the faces are parallel to the temperature gradient.The first experiment to test this theory was done by Arthur Schuster in 1876, who observed that there was a force on the glass bulb of the Crookes radiometer that was in the opposite direction to the rotation of the vanes. In this case, the black side of the vane becomes hotter than the other side, as radiant energy from a light source warms the black side by absorption faster than the silver or white side.

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Directly heated glass gives off enough infrared radiation to turn the vanes, but glass blocks much of the far-infrared radiation from a source of warmth not in contact with it. It is found at the centre of the original mosaic floor in the reception of Broadcasting House, London. When a radiant energy source is directed at a Crookes radiometer, the radiometer becomes a heat engine.However, the idents matured over the years, becoming individual characters on a yellow background, and latterly as a cut out 'window on the world'. Abram Games was commissioned to design an on-air image for the BBC Television Service, probably hastened by the imminent arrival of commercial competition. The prefix " radio-" in the title originates from the combining form of Latin radius, a ray: here it refers to electromagnetic radiation.

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From a molecular point of view, the vane moves due to the tangential force of the rarefied gas colliding differently with the edges of the vane between the hot and cold sides. The Reynolds paper went unpublished for a while because it was refereed by Maxwell, who then published a paper of his own, which contained a critique of the mathematics in Reynolds's unpublished paper.The white or silver side of the vanes are slightly warmer than the internal air temperature but cooler than the black side, as some heat conducts through the vane from the black side. The glass stays much closer to ambient temperature than the temperature reached by the black side of the vanes.

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Note that the explanation given in the caption to the clip doesn't agree with the modern explanation. It is very unusual in being the only BBC crest to carry the alternative motto - 'Quaecunque' (whatsoever), and not the usual BBC motto, 'Nation Shall Speak Peace Unto Nation'. Mechanical clocks were also seen regularly, often before important broadcasts such as the national news. 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.In the event that you consider anything on this page to be in breach of the site’s House Rules please Flag This Object. Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. 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. The wheel turns backwards because the net exchange of heat between the black sides and the environment initially cools the black sides faster than the white sides.