Micrometre Screw Gauge ( Physics)

A micrometre screw gauge is used to measure small lengths ranging from 0.10mm and 25.00mm. It can be used to measure diameters of wires and thickness of steel plates to an accuracy of 0.01mm.

 image credit:http://www.phy.uct.ac.za

There are two scales:

a) Main scale : a scale marked on the sleeve.
b) Thimble scale: a scale marked on the thimble.

When taking a measurement reading, the thimble is turned until it gripped the object very gentlybetween the anvil and the spindle.

The ratchet knob is turned until you can here a 'click'. The ratchet knob function is to prevent a user from exerting too pressure on the object. It is not good to put too much pressure on the grip as it will affect the accuracy of the reading.


How to take a reading:

Main scale reading: You have to read the main scale reading at the edge of the thimble.
This reading will be up to the accuracy of 0.1 mm.
 
Next the thimble scale reading where the horizontal reference line of the main scale is in line with the graduation mark on the thimble scale. The reading will be up to the accuracy of 0.01mm.

The reading of the main scale is added to the thimble scale reading.

For video check this one out.