Barometric light
Encyclopedia
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.
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.
: "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. After learning of the phenomenon from Bernoulli, the Englishman Francis Hauksbee
investigated the subject extensively. Hauksbee showed that the full vacuum
was not essential to the phenomenon, for the same glow was apparent when mercury was shaken with air only partially rarefied, and that even without using the barometric tube, bulbs containing low-pressure gases could be made to glow via externally applied static electricity. The phenomenon was also studied by a contemporary of Hauksbee's, the Frenchman Pierre Polinière
.
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
that is emitted by a mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
-filled barometer
Barometer
A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather...
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
Meniscus
The meniscus is the curve in the upper surface of a liquid close to the surface of the container or another object, caused by surface tension. It can be either convex or concave. A convex meniscus occurs when the molecules have a stronger attraction to each other than to the material of the...
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 PicardJean Picard
Jean-Felix Picard was a French astronomer and priest born in La Flèche, where he studied at the Jesuit Collège Royal Henry-Le-Grand. He was the first person to measure the size of the Earth to a reasonable degree of accuracy in a survey conducted in 1669–70, for which he is honored with a...
: "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
Johann Bernoulli
Johann Bernoulli was a Swiss mathematician and was one of the many prominent mathematicians in the Bernoulli family...
studied the phenomenon while teaching at Groningen, the Netherlands, and in 1700 he demonstrated the phenomenon to the French Academy. After learning of the phenomenon from Bernoulli, the Englishman Francis Hauksbee
Francis Hauksbee
Francis Hauksbee the elder , also known as Francis Hawksbee, was an 18th-century English scientist, and a Fellow of the Royal Society...
investigated the subject extensively. Hauksbee showed that the full vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
was not essential to the phenomenon, for the same glow was apparent when mercury was shaken with air only partially rarefied, and that even without using the barometric tube, bulbs containing low-pressure gases could be made to glow via externally applied static electricity. The phenomenon was also studied by a contemporary of Hauksbee's, the Frenchman Pierre Polinière
Pierre Polinière
Pierre Polinière was an early investigator of electricity and electrical phenomena, notably "barometric light", a form of gas-discharge light, which suggested the possibility of electric lighting...
.