BarometerThe atmosphere thatsurrounds the Earth is held in place by the attraction ofgravity. Like all other matter, the air of the atmospherehas weight and exerts pressure on anything beneath it. Afew inches or feet of air weigh very little, but theEarth's atmosphere is many miles high and thereforeexerts considerable pressure. A barometer is aninstrument used to measure this pressure.
The atmosphere is constantly moving and changing.Atmospheric pressure changes too. These changes areassociated with the weather and are important inpredicting changes in the weather. For this reason, thebarometer is one of many instruments used bymeteorologists, scientists who study the weather. Homebarometers, though usually less complex than those usedby meteorologists, also provide some indication of comingweather changes. If the pressure is falling, stormy orrainy weather is probably on its way. Rising pressuregenerally indicates the return of fair weather. If thepressure remains steady, there will probably be nosignificant weather change.
Barometers are also useful to scientists other thanmeteorologists. This is because many scientificmeasurements can be made only if the atmospheric pressureis known.
Since the height of the atmosphere above any point onEarth is the major factor determining the pressure of theatmosphere at that point, atmospheric pressure isgenerally greatest at sea level (or surface level, levelof the surface of oceans; varies throughout the world;mean sea level is midway between mean high and low tides;used as standard of measurement for geographic heightsand depths) and much less at the tops of mountains orhigher. Thus, a barometer can be used by a pilot as analtimeter, an instrument that measures how high anaircraft is flying.
Mercury BarometersThe earliest barometer was the mercury barometer. It ismade of a glass tube, open at one end. The tube is filledwith mercury, the open end is covered, and the tube isthen turned upside down with the covered end placed intoa dish of mercury. The end is uncovered, and part of themercury drains into the dish. Most of the mercury remainsin the tube, however. It remains in the tube because ofthe atmospheric pressure on the surface of the mercury inthe dish. That atmospheric pressure is equal to thedownward pressure of the mercury in the tube.
Since the mercury barometer was the first one invented,atmospheric pressure is often expressed in terms of theheight of the mercury in the tube above the mercury inthe dish. At sea level, this height averages about 30inches (760 millimeters). At 10,000 feet (3,000 meters)above sea level, the height would be only about 20 1/2inches (520 millimeters). At 20,000 feet (6,000 meters),it would be about 133/4 inches (350 millimeters); and at30,000 feet (9,000 meters), about 9 inches (226millimeters). The changes in atmospheric pressure causedby the weather are much smaller. At sea level weatherchanges will cause the air pressure to change only withinthe range of about 27 to 31 inches (686 to 787millimeters) of mercury.
Modern mercury barometers have an enclosed reservoir forthe mercury with only a tiny hole to let in the outsideair. Usually a scale on the barometer tube allows themercury column to be measured directly. Since the levelof mercury at the bottom rises or falls as mercury flowsout of or into the tube, some allowance must be made forthe change in this level. In the Fortin barometer, thereservoir can be raised or lowered by an adjusting screw.This allows the surface of the mercury to be put at thezero mark on the scale for easier reading. In the Kew, ormarine, barometer, the reservoir cannot be moved, but thescale is graduated to allow for mercury-level changes.
Any liquid could be used in a barometer. However, becauseall other liquids are lighter than mercury, they wouldrequire much longer columns. Water, for example, wouldneed a column more than 30 feet (9 meters) tall. Thecolumn needed for mercury is about three feet (one meter)high.
Aneroid BarometerThe mercury barometer is cumbersome and fragile, and itdoes not work properly if it is disturbed by outsidemotion. The aneroid barometer is more convenient. Theword aneroid comes from the Greek words meaning "notliquid," and this barometer contains no liquid.Light and sturdy, it works well under most conditions.Although not as accurate as the mercury barometer, it issufficient for most uses. Airplane altimeters are basedon the aneroid barometer.
The heart of the aneroid barometer is an airtight metalbox from which most of the air has been removed to form avacuum. The top and bottom of the box are thin andflexible. Springs counteract the vacuum, helping to holdthe top and bottom apart. When the atmospheric pressureincreases, the top and bottom are forced closer together.When atmospheric pressure decreases, the springs are ableto push the top and bottom apart. The movement of the topof the box is transmitted by small levers to a pointerthat indicates the atmospheric pressure on a graduatedscale.
History of the BarometerThe first barometer was made in 1643 by EvangelistaTorricelli, an Italian physicist. Torricelli wasexperimenting to determine the nature of air and vacuums.He wondered why water pumps were unable to raise watermore than about 33 feet (10 meters) from its source.Vacuum pumps, in which a tight-fitting piston was raisedin a pipe to suck water upward, were used then. It waswidely believed that water rushed up the pipe because"nature abhors a vacuum." Torricelli, whothought the operation was misunderstood, showed that airexerts a pressure on the water. Whenever that pressure islowered in one place, water is pushed into that place bythe higher pressure exerted elsewhere on the water. Thisprinciple is illustrated by using a straw to drink water.Sucking on the straw lowers the internal air pressure.The greater outside pressure pushing down on the water inthe glass forces some of the water to move up into thestraw. Torricelli's barometer and the principles that ituncovered were soon accepted by other scientists. The newunderstanding of air pressure and vacuums gained from thebarometer was important in the eventual development ofthe steam engine, one of the main inventions thatsignaled the beginning of the Industrial Revolution.
