| 摘要 |
A permanent magnet has a D-shaped cross section including an arcuate top surface ( 22 ), a flat bottom surface ( 24 ), and side surfaces ( 26, 28 ). Provided that a plurality of permanent magnets are circumferentially arranged so that a great circle (S) circumscribes the apexes (P) on the arcuate top surfaces ( 22 ), the top surface ( 22 ) includes a central region which delineates an arc of a small circle (T) off-centered from the great circle, and end regions ( 22 a, 22 b) which are positioned outside the small circle (T) and inside the great circle (S).
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