Among various devices available for measuring magnetic field, we selected an opto-magnetic sensor based on Faraday effect. Its measuring sensitivity is not high compared with other methods, for example, those using a coil, but an opto-magnetic sensor has the advantage of being electrically isolated, not affected by any overvoltages and electrical interferences. Table 1 gives the major characteristics of the field sensor used for the experiment. The sensitivity of the sensor was corrected with a Gauss meter.