When direct current flows through a coil a magnetic field will be built up round the coil.
当直流电流过线圈，就会在线圈的周围产生磁场。
When direct current flows through a coil, a magnetic field will be built up round the coil.
当直流电流过线圈，就会在线圈的周围产生磁场。
When direct current flows through a coil, a magnetic field will be built up round the coil.
当直流电流过线圈，就会在线圈的周围产生磁场。
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