Gravimeter - Gravimeter

Japanese: 重力計 - じゅうりょくけい

A machine that measures gravity. There are two types of gravimeters: absolute gravimeters, which measure the gravity value at one point itself, and gravimeters that measure the gravity difference between two points or the change in gravity at one point over time. Generally, gravimeters refer to the latter. There are two absolute measurement methods: using a pendulum and using free-fall motion. The period T of a simple pendulum with length l is given by (Equation 1), so the value of gravitational acceleration g can be calculated by measuring l and T. Using this relationship, a reversible pendulum (solid pendulum) is actually used instead of a simple pendulum. The relationship between the fall distance s and fall time t of an object falling freely from a height s(/0) with an initial velocity v(/0) is s = 1/2gt ² + v(/0)t + s(/0). g can be calculated by measuring s and t. t is measured using an atomic frequency standard, and s is measured using optical interference. In this case, the wavelength of laser light is used as the measurement standard, and measurements with a high accuracy of 0.01 milligals can be made. A string gravimeter is a device that measures gravity by vibrating a string under tension with a weight attached, and changing the tension and frequency of the vibration due to gravity. It is used to measure gravity in the ocean. There is an instrument that measures gravity by comparing the tension of a spring with a weight attached, a type of precision spring scale, and this instrument is what is meant by the term gravimeter. It is widely used in gravity exploration for oil, weighing 2 to 3 kg, taking about 5 minutes to measure, and has a sensitivity of 0.001 milligal. In recent years, superconducting gravimeters have been developed that balance stable electromagnetic force and gravity through superconductivity, and are attracting attention as gravimeters with long-term stability and ultra-high sensitivity.
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Source : Heibonsha Encyclopedia About MyPedia Information

Japanese:

重力を測定する機械。１地点の重力値そのものを測定する絶対重力計と，２地点間の重力差あるいは１地点の重力の時間変化を測定する重力計とに分けられ，一般に重力計といえば後者をさす。絶対測定法は振子を用いる方法と自由落下運動を利用する方法がある。長さlの単振子の周期Tは（式１）で与えられるから，lとTを測れば重力加速度gの値が計算できる。この関係を利用し，実際には単振子の代りに可逆振子（実体振子）を用いる。高さs(/0)から初速v(/0)で自由落下する物体の落下距離s，落下時間tの間にはs＝1/2gt²＋v(/0)t＋s(/0)の関係がなりたつ。s，tを測定すればgが計算できる。tは原子周波数標準により計測され，sは光波干渉で測定される。その際レーザー光の波長が測定基準に使われ，0.01ミリガルの高精度の測定ができる。おもりをつけて張力をかけられた弦を振動させ，重力により張力が変化し，振動数が変化することから重力値を測定するものを弦重力計といい，海洋の重力測定に使用される。おもりをつるしたばねの伸縮により重力を比較測定する計器，つまり一種の精密ばねばかりがあり，単に重力計といえばこの計器を指す。石油の重力探査に多用され，重量２〜3kg，測定時間５分程度で，0.001ミリガルの感度をもつ。近年，超伝導による安定な電磁力と重力をつりあわせる超伝導重力計が開発され，長期の安定性と超高感度をもつ重力計として注目されている。
→関連項目重力

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