Micrometers:
The micrometer screw gauge essentially consists of an accurate screw having about 10 or 20 threads per cm and revolves in a fixed nut, see fig. 1.49. The end of the screw forms one measuring up and the other measuring tip is constituted by a stationary anvil in the base of the frame. The screw is threaded for certain length and is plain afterwards. The plain portion is called sleeve and its end is the measuring surface. The spindle is advanced or retraced by turning a thimble connected to the spindle. The spindle is a slide fit over the barrel and barrel is the fixed part attached with the frame. The barrel is graduated in unit of 0.05 cm. i.e. 20 divisions per cm, which is the lead of the screw for one complete revolution. The thimble has got 25 divisions around its periphery on circular portion. Thus it sub- divides each revolution of the screw in 25 equal parts, i.e. each division corresponds to 0.002 cm.