Micrometre Screw Gauge
A vernier calipers can measure upto 1/100th or 1/200th of a centimeter, For more precise measurement micrometer screw gauge is used. A micrometer screw gauge consists of a U-shaped solid metal frame F. At one end of this frame a stud C with round end and flat face is fixed at A. A fine and accurately cut screw S having flat round end D passes through the other end B. On the outer surface of the nut of the screw a scale is graduated in millimetres. This is parallel to the length of the screw and is called the main scale which is linear. A drum fits on the screw which moves on the nut as it is rotated. This drum has a circular scale at one end with 50 or 100 divisions on it. At the other end the screw has a head R known as ratchet to avoid undue pressure on the object held between the studs C and D.
Pitch of the screw
It is the distance between the two consecutive threads of the linear screw. It is measured by the distance travelled by the circular scale on the main scale during one complete rotation of the circular scale. The least count of the screw gauge is given as,
Least Count = Pitch of the screw/Number of divisions on the circular scale
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