螺旋测微计的零点读数A0可正可负，当微分筒上的零刻度线在主尺读数基准线之下时A0为负值，反之为正值。
The zero point reading A0 of the screw micrometer can be positive or negative. When the zero scale line on the differential cylinder is below the reading reference line of the main ruler, A0 is a negative value, otherwise it is a positive value.