Micrometer screw-gauge is another instrument used for measuring accurately the diameter of a thin wire or the thickness of a sheet of metal.
It consists of a U-shaped frame fitted with a screwed spindle which is attached to a thimble.
The screw has a known pitch such as 0.5 mm. Pitch of the screw is the distance moved by the spindle per revolution. Hence in this case, for one revolution of the screw the spindle moves forward or backward 0.5 mm. This movement of the spindle is shown on an engraved linear millimeter scale on the sleeve. On the thimble there is a circular scale which is divided into 50 or 100 equal parts.
When the anvil and spindle end are brought in contact, the edge of the circular scale should be at the zero of the sleeve (linear scale) and the zero of the circular scale should be opposite to the datum line of the sleeve. If the zero is not coinciding with the datum line, there will be a positive or negative zero error as shown in figure below.
Determination of Diameter of a Wire
The wire whose thickness is to be determined is placed between the anvil and spindle end, the thimble is rotated till the wire is firmly held between the anvil and the spindle. The rachet is provided to avoid excessive pressure on the wire. It prevents the spindle from further movement. The thickness of the wire could be determined from the reading as shown in figure below.
شاهد الرابط اعلاه
Reading = Linear scale reading + (Coinciding circular scale x Least count)
= 2.5 mm + (46 x 0.01)
= (2.5 + 0.46) mm
= 2.96 mm
Relationship in the Metric system of length
1 kilometer (km) = 103 m
1 centimeter (cm) = 10-2 m
1 millimeter (mm) = 10-3 m