bar r magnetic compass 磁杆罗盘
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On the point of an upright needle he laid horizontally a fine steel bar, which could turn to the right and left like the magnetic needle in a compass-box..
他在一根垂直的针头顶点，设置了一根水平的钢制细棍，它可以左右转动，就像罗盘里的磁针那样。
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