A screw gauge is used to measure small lengths and diameters more accurately than a Vernier caliper. It works on the principle of a screw, where rotating the thimble causes the threaded spindle to move linearly. The spindle is connected to a main scale with millimeter divisions. Each division on the thimble's circular scale corresponds to 0.01mm movement. The least count is therefore 0.01mm. To take a measurement, the object is placed between the spindle and stud. Its diameter can be read where the thimble's zero aligns with the main scale.

1. SCREW GAUGE

2. SCREW GAUGE
 A screw gauge is a precision instrument that is used
to measure small lengths with accuracy greater than
a Vernier Caliper.
 It is used for measuring diameter of circular objects
mostly wires, with an accuracy of 0.001cm.
 It is also called as micrometer screw gauge.
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3. Principle
• A screw gauge works on the principle of the screw. By
rotating the screw head , we get the linear movement of the
main scale. This linear movement is used to calculate the
diameter of wire or thickness of the metal plate.
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4. Construction
• A simple screw gauge consists of a U-shaped metal frame with a
metal stud at its one end.
• A hollow cylinder (or sleeve) has a millimeter scale over it along a
line called index line parallel to its axis. The hollow cylinder acts
as a nut. It is fixed at the end of U-shaped frame opposite to the
stud.
• A Thimble has a threaded spindle inside it. As the thimble
completes one rotation, the spindle moves 1 mm along the index
line.
• It is because the distance between consecutive threads on the
spindle is 1 mm. This distance is called the pitch of screw on the
spindle.
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5. Construction
 The thimble has 100 divisions around its one end. It is the
circular scale of the screw gauge.
 As thimble completes one rotation, 100 divisions pass the
index line and the thimble moves 1 mm along the main scale.
 Thus each division of circular scale crossing the index line
moves the thimble through 1/100 mm or 0.01 mm on the main
scale.
 Least count of a screw gauge can also be found as given below:
L.C. = (
Pitch of the screw
No. of circular scale divisions
)
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Least Count

6. Working ZERO-ERROR
• To find the zero error, close the gap between the spindle and
the stud of the screw gauge by rotating the ratchet in the
clockwise direction. If zero of circular scale coincides with
the index line, then the zero error will be zero as shown in
figure (a).
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INSIGHTS OF PHYSICSZERO-ERROR = 0

7. ZERO-ERROR
• Zero error will be positive if zero of circular scale is behind
the index line. In this case, multiply the number of divisions
of the circular scale that has not crossed the index line with
the least count of screw gauge to find zero error as shown in
figure (b).
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ZERO-ERROR = +0.18mm

8. ZERO-ERROR
• Zero error will be negative if zero of circular scale has crossed
the index line. In this case, multiply the number of divisions
of the circular scale that has crossed the index line with the
least count of screw gauge to find the negative zero error as
shown in figure (c).
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Zero error = -0.05mm

9. Find the diameter of a wire using a screw gauge.
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10. Quiz
1. What is the least count of a screw gauge?
Least count is 0.01mm for micrometer screw gauge.
2. What is the pitch of your laboratory screw gauge?
The pitch of the screw is the distance moved by the spindle per
revolution. The micrometers in our laboratory have a pitch of 0.50 mm
(two full turns are required to close the jaws by 1.00 mm).
3. What is the range of your laboratory screw gauge?
The range of screw gauge is 0.01 mm to 100mm.
4. Which one of the two instruments is more precise and why?
Vernier caliper and screw gauge because of their least count as 0.1 mm for
Vernier Calipers and 0.01mm for micrometer screw gauge.
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11. INSIGHTS OF PHYSICS
Thank you
Physical Quantities and Measurement UNIT 1