The authors of the presented paper deal with investigations concerning applications of the optical fiber sensor of the magnetic field in electric DC engines with permanent magnets. The aim of the sensor is to measure the state of magnetization of permanent magnets (basing on the measurements of magnetic field induction). The knowledge of the state of magnetization helps to decide whether permanent magnets need to be additionally magnetized in order to prevent damages of the whole engine. This

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... n be done automatically, even when the engine is running. For the purpose the Faraday magnetooptic effect is used. The presented investigations concern an optical fiber sensor of the magnetic field provided with a glass rod characterized a high value of the Verdet constant as the sensing element. Thanks to the application of optical fibers the connection of the sensor head with the whole measuring system is constructed of insulating material without using of electrical connections.