central knob meaning in Chinese

核膜孔
中央小球

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。
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。