名词
中文翻译：Unknown
同义词：
['principal plane', 'principal axis', 'principal line']
例句：
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1. The compass is used to determine the direction of a magnetic needle.
指南针用于确定磁针的方向。
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2. The principal plane of the compass is perpendicular to the plane of the card.
指南针的主平面与卡片的平面垂直。
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3.The principal axis of the compass is parallel to the Earth's magnetic field.
指南针的主轴线平行于地球的磁场。