An Electromagnetic Log, sometimes called an "EM log", is an electronic sensor which measures the speed of a vessel through sea water.
Like many other technologies, its name derives from the traditional chip log.
It makes use of Faraday's law of induction by measuring the EMF induced in water moving through a magnetic field generated by the sensor.
Some EM logs have three electrodes arranged in a right triangle; this allows the speed to be calculated even if the electrodes are not perfectly aligned with respect to the flow.
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