In general Vernier calipers can measure accurately up to 0.01 cm and for greater accuracy micrometer screw devices e.g., screw gauge, spherometer are used. These consist of accurately cut screw which can be moved in a closely fitting fixed nut by tuning it axially. In one such type of screw gauge, the linear distance moved by the screw is 2mm in four rotations and there are 50 divisions on its cap. When nothing is put between its jaws, 20th divisions of circular scale coincides with reference line, with zero of circular scale lying above the reference line and zero of main scale is not clearly visible. When a plate is placed between the jaws main scale reads 4 divisions and circular scale reads 20 divisions.