Barometer - Attention

An instrument used to measure atmospheric pressure. Because changes in atmospheric pressure are closely related to the weather, ordinary barometers are also called barometers. The old ones are broadly divided into Torricelli's mercury barometer, which uses a vacuum, and the method of measuring the deformation of an evacuated metal container according to the outside air pressure. The latter is also called an aneroid barometer because it does not use liquid. Great care must be taken when measuring with a mercury barometer. The barometer is placed in a small room or container, and fixed vertically so that the temperature of the mercury and the scale does not change suddenly. When measuring, first tap the mercury level lightly with a finger to stabilize it, set the zero point of the scale, and read the height of the top of the mercury column. The scale is usually in hectopascals, but there are also millimeter and inch scales. The value is corrected for temperature and recalculated to the height at 0 °C, and then corrected to the value at standard gravity (acceleration 980.665 centimeters per second per second). This is also called local air pressure. There are various types of ingenious instruments, such as station types that do not require zero-point adjustment, marine types that can withstand the motion of a ship, and types that convert the height of the mercury column into electrical resistance. Sprung's barometer measures the weight of mercury on a balance and records it continuously. Although precise, it is difficult to manufacture and maintain, so it has gradually fallen out of use. In addition to indicating types that read the scale indicated by a needle, there are also self-recording barometers that transmit the movement of the sensor to a recording pen and draw a continuous graph. In both types, temperature compensation is automatically performed by a bimetal. The readings are adjusted in advance to the local air pressure. The vibration type barometer, which has recently been put into practical use, divides the change in the aneroid sensor into forces in opposite directions and applies them to two quartz crystal oscillators. The difference in the oscillation frequency of the quartz crystals is proportional to the change in air pressure, so the air pressure can be determined by counting the frequency. There is also a type in which a metal cylinder is placed in a vacuum case and vibrates it with a piezoelectric element. Aneroid barometers are widely used because they are easy to measure, but their measurement accuracy is generally not as good as that of mercury barometers. To maintain accuracy, it is a good idea to compare the readings with those of a mercury barometer from time to time and make corrections. Microbarometers magnify and record changes in air pressure. If a very small hole is made in a sealed container such as a drum, the air pressure inside the container will not change for a while even if the outside air pressure changes suddenly. This internal pressure is directed to a float floating on oil. If the up and down movement of the float in response to changes in air pressure is magnified, changes of up to about one thousandth of a hectopascal can be recorded.

[Takeji Shinohara]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

気圧を測定する際に用いられる計器。気圧の変化は天気と密接な関係があるので、普通の気圧計は晴雨計ともよばれる。古くから用いられているものは、トリチェリの真空を利用した水銀気圧計と、排気した金属容器の、外気圧に応じた変形を測る方式とに大別される。後者は、液体を使用しないとの意味からアネロイド気圧計ともいう。水銀気圧計での測定には細心の注意がいる。気圧計を小さい部屋あるいは収容箱に入れて、水銀とスケールの温度が急に変化しないようにし、鉛直に固定する。測定の際は、まず軽く指でたたいて水銀面を安定させ、スケールのゼロ点をあわせ、水銀柱の上面の高さを読み取る。目盛りはヘクトパスカルのものが普通であるが、ミリメートルやインチ目盛りもある。温度補正をして0℃での高さに引き直し、次に標準重力（加速度980.665センチメートル・毎秒・毎秒）での値に補正する。これは現地気圧ともよばれる。ゼロ点あわせのいらないステーション型、船上での動揺にも耐えるマリン型、水銀柱の高さを電気抵抗に変換する方式など、いろいろくふうした器械もある。スプルングの気圧計は、水銀の重量を天秤(てんびん)で測り、これを連続して記録する。精密ではあるが製作や保守に難点があるので、しだいに使用されないようになった。アネロイド気圧計には、指針が示す目盛りを読み取る指示型のほかに、感部の動きを記録ペンに伝えて、連続したグラフのように描く自記気圧計がある。いずれも、バイメタルで自動的に温度補正がなされる。その示度は、あらかじめ現地気圧にあわせておく。近年実用化されつつある振動式気圧計は、アネロイド型の感部の変化を互いに逆方向の力に分けて、二つの水晶振動子に加える。水晶の発振周波数の差は気圧の変化に比例するから、周波数をカウントすれば気圧がわかる。真空ケースに金属製円筒を入れ、これを圧電素子で振動させる方式もある。アネロイド気圧計は測定が簡単なため広く用いられるが、測定精度は、一般には水銀気圧計に及ばない。精度を保つには、ときどき水銀気圧計による値と比較し、修正するとよい。微気圧計は、気圧の変化を拡大して記録する。ドラム缶のような密閉容器にごく細い穴をあけると、外気圧が急に変化しても容器内の気圧はしばらく変わらない。この内圧を油に浮かべた浮きに導く。気圧変化に応じた浮きの上下の動きを拡大すると、1000分の1ヘクトパスカルくらいの変化まで記録できる。

［篠原武次］

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