The Lightmills of Leadenhall

I was in Leadenhall Market grabbing my lunch this week and I wandered over to  the market stalls under the cupola.

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I had seen one particular stall before selling what I thought was glass ornaments.

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When I looked at them I realised they were Lightmills, otherwise known as a Crookes radiometer, which consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity.

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It was invented in 1873 by the chemist Sir William Crookes. He was weighing samples in a partially evacuated chamber to reduce the effect of air currents, and noticed the weighings were disturbed when sunlight shone on the balance. Investigating this effect, he created the device named after him. It is still manufactured and sold as a novelty item.

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Taken with my X-pro1 and 35mm lens.

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