Log

Japanese: 測程儀 - そくていぎ（英語表記）log

Also called a log. An instrument that measures and indicates a ship's speed (speed through the water) and distance traveled. The word log means "log," and is said to have originated from the fact that ship speeds were measured using pieces of wood floating in the sea. Historically, there was a hand log, which was a device that attached a fan-shaped plate to the end of a long knotted rope (log line) and let it float for a certain period of time, and calculated the ship's speed by dividing the length of the rope that was let out by the time. Currently, the following types are used: (1) Towing log: A log line is towed from the stern or side of the ship, and a rotator with rotating blades is attached to the end of the log line to measure the ship's speed from its number of rotations (called a pea log), or a cone-shaped device is attached to measure the water pressure that hits it and the frequency of the vortex that is created afterwards. (2) Bottom gauges: These include those that measure ship speed by the number of rotations of a small rotor attached to the bottom of the ship, those that measure ship speed by dynamic pressure proportional to the ship's speed using a pitot tube protruding from the bottom of the ship (dynamic pressure gauge), those that determine ship speed by measuring the induced electromotive force generated by a water current crossing a magnetic field created underwater using a device protruding from the bottom of the ship (electromagnetic gauge), and those that measure ship speed by using the Doppler effect when ultrasonic waves emitted from a device on the bottom of the ship are reflected off the seabed and return (Doppler sonar, Doppler log).

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

ログともいう。船の速力 (対水速度) と航程を測定指示する計器。 logは「丸太」の意味で，海中に浮べた木片で船速をはかったことから出たという。歴史的には，結び目をつけた長い紐 (測程索) の先に扇形の板をつけて一定時間流し，繰出された紐の長さを時間で割って船速を算出した手用測程儀 (ハンドログ) があった。現在では次のようなものが用いられている。 (1) 曳航測程儀　船尾または船側から引索 (ログライン) を曳航し，その先端に回転翼のついたローテータをつけてその回転数から船速をはかるもの (ピーログ) や，円錐形のものをつけてそれに当る水圧やそのあとにできる渦の振動数から測定するものなど。 (2) 船底測定儀　船底に小さな回転翼を出してその回転数からはかるもの，船底にピトー管を突き出させて船の速度に比例した動圧によって船速を知るもの (動圧式測程儀) ，船底から突き出した装置で水中につくった磁場を横切る水流が起す誘導起電力を測定して船速を求めるもの (電磁測程儀) ，船底の装置から発射した超音波が海底で反射して戻ってくる際のドップラー効果を利用して船速をはかるもの (ドップラーソーナー，ドップラーログ) など。

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