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barometric light
Barometric light is a name for the light that is emitted by a mercury-filled barometer tube when the tube is shaken. The discovery of this phenomenon in 1675 revealed the possibility of electric lighting.
== The phenomenon and its explanation ==
The earliest barometers were simply glass tubes that were closed at one end and filled with mercury. The tube was then inverted and its open end was submerged in a cup of mercury. The mercury then drained out of the tube until the pressure of the mercury in the tube — as measured at the surface of the mercury in the cup — equaled the atmosphere's pressure on the same surface.
In order to produce barometric light, the glass tube must be very clean and the mercury must be pure.〔Even if the glass tube is clean and the mercury is pure, eventually the light will cease to appear because a thin film of mercury will form on the glass. In order to produce barometric light again, the glass must be cleaned again.〕 If the barometer is then shaken, a band of light will appear on the glass at the meniscus of the mercury whenever the mercury moves downward.
When mercury contacts glass, the mercury transfers electrons to the glass. Whenever the mercury pulls free of the glass, these electrons are released from the glass into the surroundings, where they collide with gas molecules, causing the gas to glow — just as the collision of electrons and neon atoms causes a neon lamp to glow.〔Gay L. Dybwad and C. E. Mandeville (1967) "Generation of light by the relative motion of contiguous surfaces of mercury and glass," ''Physical Review'', vol. 161, pages 527-532.〕〔R. Budakian, K. Weninger, R. A. Hiller, S. J. Putterman (1998) Letters to ''Nature'' : "Picosecond discharges and slip-stick friction at a moving meniscus of mercury on glass," ''Nature'', vol. 391, pages 266-268. Available on-line at: (University of California, Los Angeles ). Article includes photograph of barometric light occurring in a glass cylinder partially filled with mercury and low-pressure neon gas.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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