The central knob has fewer positions and all you need to do is to switch it to the quantity you want to measure.

中间的旋钮只有几个位置，您所需要做的，就是选择所需要的测量功能。

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The central knob has lots of positions and you must choose which one is appropriate for the measurement you want to make.

中间的旋钮可以旋转很多位置，你必须选择一个所需要的量程来作为测量量程。

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Aboveground is a gorgeously symmetrical crown of green, celery-like growth radiating from the central knob to about 12 inches.

NPR: The Vegetable World's Ugly Duckling: Celeriac