牛顿环实验中，测量干涉环的直径是本实验的重点与难点．本文将证实一个结论：弦长代替直径，测量牛顿环曲率半径的方法会影响结果的B类不确定度．当线性拟合的相关系数非常接近1时，B类不确定度不可以被忽略．另外，从理论上分析了弦长代替直径的测量方法所具备的优势，给出了因变量B类不确定度的计算方法．

In the experiment of Newton's ring， measuring the diameter of interference ring is the key and difficult step．It is confirmed in this paper that the type B uncertainty of the curvature radius of Newton's rings is affected when the parameter measured is the chord length rather than the diameter.The type B uncertainty can not be ignored when the linear correlation coefficient approaches to 1． In addition，the advantages are analyzed when measuring the chord length instead of the diameter on the basis of theoretical analysis，the method of calculating the type B uncertainty is also given．