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. 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.

History

Barometric light was first observed in 1675 by the French astronomer Jean Picard: "Experience fait à l'Observatoire sur la Barometre simple touchant un nouveau Phenomene qu'on y a découvert" [Experiment made at the [astronomical] observatory [in Paris] on a simple barometer concerning a new phenomenon that was discovered there], Le Journal des Sçavans [later: Journal des Savants ], page 112 (Paris edition) or page 126 (Amsterdam edition) (25 May 1676). Original text: "On sçait que le Barometre simple n’est autre chose qu’un tuyau de verre scellé hermetiquement par le haut & ouvert par le bas, dans lequelle il y a du vif-argent qui se tient ordinairement dans certaine hauteur, le reste au dessus estant vuide. M. Picard en a un à l’Observatoire qui dans l’obscurité, lors qu’on le remuë assez pour faire balancer le vif-argent, fait comme des éclairs & jette une certaine lumiere entrecouppée qui remplit toute la partie du Tuyau où se fait le vuide; mais cela n'arrive à chaque balancement que lors que le vuide se fait, & dans la seule descente du vif-argent. On a tâché de faire la même experience sur divers autres Barometres de même composition; mais on n’a encore reüssi que sur un seul. Comme on est resolu d’examiner la chose en toute maniere, nous en donnerons plus au long toutes les circonstances qu’on y découvrïra." Translation: "It is known that the simple barometer is nothing more than a glass tube hermetically sealed at the top and open at the bottom, in which there is some quicksilver which ordinarily stays at a certain height, the remainder above being void. Mr. Picard has one of them at the Observatory, which in the dark, when one moves it enough to make the quicksilver jiggle, flashes like sparks and throws a certain flickering light which fills the entire part of the tube where it is void; but that happens during each swing only when there is a void and only during the descent of the quicksilver. The same experiment has been attempted with various other barometers of the same construction; but it has still been successful only with the one. As it has been resolved to investigate the matter in every way, we will give in greater length everything that is discovered about it." Available on-line (in French) here: Le Journal des Sçavans. Reprinted as: "Experience fait a l'observatoire sur le baromètre, touchant un nouveau phenomene que M. Picard y a decouvert" [Experiment performed at the observatory on the barometer, concerning a new phenomenon that Monsieur Picard discovered there], Mémoires de l'Académie Royale des Sciences de Paris, vol. 10, page 566. "Towards the year 1676, Monsieur Picard was transporting his barometer from the Observatory to Port Saint Michel during the night, [when] he noticed a light in a part of the tube where the mercury was moving; this phenomenon having surprised him, he immediately reported it to the sçavans, ... " The Swiss mathematician Johann Bernoulli studied the phenomenon while teaching at Groningen, the Netherlands, and in 1700 he demonstrated the phenomenon to the French Academy. See: