Barometer

Schematic drawing of a simple mercury barometer with vertical mercury column and reservoir at base

Goethe's device

A barometer is a scientific instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.

History

Although Evangelista Torricelli is universally credited with inventing the barometer in 1643^[1][2][3], two other noteworthy efforts must be cited. Historical documentation also suggests Gasparo Berti, an Italian mathematician and astronomer, unintentionally built a water barometer sometime between 1640 and 1643.^[1][4] French scientist and philosopher René Descartes described the design of an experiment to determine atmospheric pressure as early as 1631, but there is no evidence that he built a working barometer at that time.^[1]

On July 27, 1630, Giovanni Batista Baliani wrote a letter to Galileo Galilei explaining an experiment he had made in which a siphon, led over a hill about twenty-one meters high, failed to work. Galileo responded with an explanation of the phenomena: he proposed that it was the power of a vacuum that held the water up, and at a certain height (in this case, thirty-four feet) the amount of water simply became too much and the force could not hold any more, like a cord that can support only so much weight.^[5]

Galileo's ideas reached Rome in December 1638 in his Discorsi. Rafael Magiotti and Gasparo Berti were excited by these ideas, and decided to seek a better way to attempt to produce a vacuum than with a siphon. Magiotti devised such an experiment, and sometime between 1639 and 1641, Berti (with Magiotti, Athanasius Kircher and Nicolo Zucchi present) carried it out.^[5]

Four accounts of Berti's experiment exist, but a simple model of his experiment consisted of filling with water a long tube that had both ends plugged, then standing the tube in a basin already full of water. The bottom end of the tube was opened, and water that had been inside of it poured out into the basin. However, only part of the water in the tube flowed out, and the level of the water inside the tube stayed at an exact level, which happened to be thirty-four feet, the same height Baliani and Galileo had observed that was limited by the siphon. What was most important about this experiment was that the lowering water had had left a space above it in the tube which had had no intermediate contact with air to fill it up. This seemed to suggest the possibility of a vacuum existing in the space above the water.^[5]

Torricelli, a friend and student of Galileo, dared to look at the entire problem from a different angle. In a letter to Michelangelo Ricci in 1644 concerning the experiments with the water barometer, he wrote:

Many have said that a vacuum does not exist, others that it does exist in spite of the repugnance of nature and with difficulty; I know of no one who has said that it exists without difficulty and without a resistance from nature. I argued thus: If there can be found a manifest cause from which the resistance can be derived which is felt if we try to make a vacuum, it seems to me foolish to try to attribute to vacuum those operations which follow evidently from some other cause; and so by making some very easy calculations, I found that the cause assigned by me (that is, the weight of the atmosphere) ought by itself alone to offer a greater resistance than it does when we try to produce a vacuum.^[6]

It was traditionally thought (especially by the Aristotelians) that the air did not have lateral weight: that is, that the miles of air above us don't weigh down on the air at our level. Even Galileo had accepted the weightlessness of air as a simple truth. Torricelli questioned that assumption, and instead proposed that air had weight, and that it was the weight of air (not the attracting force of the vacuum) which held (or rather, pushed) up the column of water. He thought that the level the water stayed at (thirty-four feet) was reflective of the force of the air's weight pushing on it (specifically, pushing on the water in the basin and thus limiting how much water can fall from the tube into it). In other words, he viewed the barometer as a balance, an instrument for measurement (as opposed to merely being an instrument to create a vacuum), and because he was the first to view it this way, he is traditionally considered the inventor of the barometer (in the sense in which we use the term now).^[5]

Due to rumors circulating in Torricelli's gossipy Italian neighborhood, which included that he was engaged in some form of sorcery or witchcraft, Torricelli realized he had to keep his experiment secret or run the risk of being arrested. He needed to use a liquid that was heavier than water, and from his previous association and suggestions by Galileo, he deduced by using mercury, a shorter tube could be used. With mercury, then called "quicksilver", which is about 14 times heavier than water, a tube only 32 inches was now needed, not 35 feet.^[7]

In 1646, Blaise Pascal along with Pierre Petit, had repeated and perfected Torricelli's experiment after hearing about it from Marin Mersenne, who himself had been shown the experiment by Torricelli toward the end of 1644. Pascal further devised an experiment to test the Aristotelian proposition that it was vapors from the liquid that filled the space in a barometer. His experiment compared water with wine, and since the latter was considered more "spiritous", the Aristotelians expected the wine to stand lower (since more vapors would mean more pushing down on the liquid column). Pascal performed the experiment publicly, inviting the Aristotelians to predict the outcome beforehand. The Aristotelians predicted the wine would stand lower. It did not.^[5]

However, Pascal went even further to test the mechanical theory. If, as suspected by mechanical philosophers like Torricelli and Pascal, air had lateral weight, the weight of the air would be less at higher altitudes. Therefore, Pascal wrote to his brother-in-law, Florin Perier, who lived near a mountain called the Puy de Dome, asking him to perform a crucial experiment. Perier was to take a barometer up the Puy de Dome and make measurements along the way of the height of the column of mercury. He was then to compare it to measurements taken at the foot of the mountain to see if those measurements taken higher up were in fact smaller. In September 1648, Perier carefully and meticulously carried out the experiment, and found that Pascal's predictions had been correct. The mercury barometer stood lower the higher one went.^[5]

Types

Water-based barometers

The concept that 'decreasing atmospheric pressure predicts stormy weather' was postulated by Lucien Vidie -- and it's the basis for a weather prediction device called a 'storm glass' or 'Goethe barometer' (who popularized it in Germany). It consists of a glass container with a sealed body, half filled with water. A narrow spout connects to the body below the water level and rises above the water level, where it is open to the atmosphere. When the air pressure is lower than it was at the time the body was sealed, the water level in the spout will rise above the water level in the body; when the air pressure is higher, the water level in the spout will drop below the water level in the body. A variation of this type of barometer can be easily made at home.^[8]

Mercury barometers

A mercury barometer has a glass tube of at least 33 inches (about 84 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury actually creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.

Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere.^[1] He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".

The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters (torr): the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is equivalent to about 29.9 inches, or 760 millimeters, of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type normally measure atmospheric pressures between 28 and 31 inches of mercury.

Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers. Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes. Fortin barometers use a variable displacement mercury cistern, usually constructed with a thumbscrew pressing on a leather diaphragm bottom. This compensates for displacement of mercury in the column with varying pressure. To use a Fortin barometer, the level of mercury is set to the zero level before the pressure is read on the column. Some models also employ a valve for closing the cistern, enabling the mercury column to be forced to the top of the column for transport. This prevents water-hammer damage to the column in transit.

On June 5, 2007, a European Union directive was enacted to restrict the sale of mercury, thus effectively ending the production of new mercury barometers in Europe.

Aneroid barometers

Old aneroid barometer

Modern aneroid barometer

An aneroid barometer uses a small, flexible metal box called an aneroid cell. This aneroid capsule (cell) is made from an alloy of beryllium and copper.^[9] The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. In addition, the mechanism is made deliberately 'stiff' so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves. It also was invented by Blaise Pascal.

Barographs

A barograph, which records a graph of some atmospheric pressure, uses an aneroid barometer mechanism to move a needle on a smoked foil or to move a pen upon paper, both of which are attached to a drum moved by clockwork.^[10]

Applications

Digital graphing barometer.

Barograph using five stacked aneroid barometer cells.

A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. When used in combination with wind observations, reasonably accurate short-term forecasts can be made.^[11] Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, gives a contour map showing areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activities. Typically if the barometer is falling, deteriorating weather or some form of precipitation is indicated; however, if the barometer is rising, it is likely there will be fair weather or no precipitation.

Compensations

Temperature

The density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument. Temperature compensation of an aneroid barometer is accomplished by including a bi-metal element in the mechanical linkages. Aneroid barometers sold for domestic use seldom go to the trouble.

Altitude

As the air pressure will be decreased at altitudes above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is then converted to an equivalent sea-level pressure for purposes of reporting and for adjusting aircraft altimeters (as aircraft may fly between regions of varying normalized atmospheric pressure owing to the presence of weather systems). Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.

Patents

Table of Pneumaticks, 1728 Cyclopaedia

• US 2194624, G. A. Titterington, Jr, "Diaphragm pressure gauge having temperature compensating means", issued 1940-03-26, assigned to Bendix Aviat Corp 
• U.S. Patent 2,472,735 : C. J. Ulrich : "Barometric instrument"
• U.S. Patent 2,691,305 : H. J. Frank : Barometric altimeter"
• U.S. Patent 3,273,398 : D. C. W. T. Sharp : "Aneroid barometer"
• U.S. Patent 3,397,578 : H. A. Klumb : "Motion amplifying mechanism for pressure responsive instrument movement"
• U.S. Patent 3,643,510 : F. Lissau : "Fluid displacement pressure gauges"
• U.S. Patent 4,106,342 : O. S. Sormunen : "Pressure measuring instrument"
• U.S. Patent 4,238,958 : H. Dostmann : "Barometer"
• U.S. Patent 4,327,583 : T. Fijimoto : "Weather forecasting device"

See also

• Automated airport weather station
• Barograph
• Bert Bolle Barometer
• Microbarometer
• Robert FitzRoy#Meteorology
• Storm glass
• Surface weather analysis
• Weather forecasting

References

Further reading

• Burch, David F. The Barometer Handbook; a modern look at barometers and applications of barometric pressure. Seattle: Starpath Publications (2009), ISBN 978-0-914025-12-2.
• Middleton, W.E. Knowles. (1964). The history of the barometer. Baltimore: Johns Hopkins Press. New edition (2002), ISBN 0801871549.

External links

• The largest barometer in the world, Bert Bolle's Water Barometer in the Barometer Tower in the Denmark Visitor Centre, Western Australia
• Various Barometer Designs
• Early use of barometers on surveys
• Wheel and stick barometer images
• Mercury barometer