The document describes a micrometer or screw gauge, which is a precision measuring tool invented in 1638. It has two scales - a main linear scale graduated in 0.500 mm marks and a rotating scale with 50 divisions each equal to 0.010 mm. To take a measurement, the spindle is turned until it contacts the object being measured. The reading is the sum of the values indicated on the two scales. The precision or least count is 0.01 mm, determined by dividing the pitch (distance moved per rotation) by the number of divisions on the rotating scale. Potential sources of error include non-removal of zero error and improper reading.

1. MICROMETER
OR
SCREW GAUGE
Talha Javed
Comsats University Islamabad

2. Screw Gauge
•Invented in 1638 by William Gascoigne, an English astronomer.
•Precision upto 0.01mm

3. Parts of screw gauge
• Anvil: The shiny part that the spindle moves
toward, and that the sample rests against
• Sleeves: The stationary round component with
the linear scale on it, sometimes with vernier
markings.
• lock : The lever that one can tighten to hold
the spindle stationary, such as when
momentarily holding a measurement.

4. Continued…
• Spindle :The shiny cylindrical component that the
thimble causes to move toward the anvil.
• Ratchet knob : Device on end of handle that
limits applied pressure by slipping at a calibrated
torque
• Thimble : The component that one's thumb
turns. Graduated markings.
• Frame : The C-shaped body that holds the anvil
and barrel in constant relation to each other.

5. Two Scales Are Better Than One
The micrometer has two scales: the main or linear
scale and the rotating scale.
The main scale or linear scale is graduated into
0.500 mm marks.
The rotating scale is composed of 50 divisions,
each graduated into 0.010 mm marks.

6. Linear scale divided
into 0.500 mm marks

7. Whole Marks: 0.000, 1.000, 2.000, 3.000, etc
Half Marks: 0.500, 1.500, 2.500, 3.500 etc

8. Rotating scale with
50 divisions, each
equal to 0.010 mm

9. Pitch of screw gauge
Pitch = Distance moved on Linear Scale(S)
Number of Rotation(N)

10. Least Count
Least Count = Pitch
Number of Division on circular scale

11. Micrometer closed = 0.000 mm
means no zero error

12. After first rotation = 0.500 mm

13. After second rotation = 1.000 mm

14. After third rotation =1.500 mm

15. Calculating Pitch
Number of Rotations = 3
Distance moved on Linear Scale= 1.500 mm
So,
Pitch=S/N
Pitch = 1.500/3
Pitch= 0.5 mm

16. Calculating L.C
Now we are going to find Least Count (L.C)
Pitch= 0.5 mm
No. of div. on circular scale= 50
L.C =
L.C= 0.5/50
L.C = 0.01mm
Pitch
Number of Division on circular scale

17. Linear Scale: 3.000
Rotating Scale: 0.270
Total Reading=3+0.270mm
= 3.270mm
To get the final reading, simply
add the readings on the linear
and rotating scales

18. Linear scale: 6.500
Rotating scale: 0.170
Total Reading: 6.670
mm

19. Linear scale: 1.000
Rotating scale: 0.205
Total Reading: 1.205
mm

20. Reading no. Main Scale
Reading M
(mm)
Rotating
Scale
Reading N
(mm)
R = N *
Least count
(mm)
Final
Reading =
M + R (mm)
1
2
3
Observations And Calculations
Sources of Errors :
1. Zero error may not be removed
2. Reading may not be noted properly
3. Thimble may be rotated hardly beyond its limits

21. Zero Error
• If the zero mark of the circular scale comes on
the reference line. In this case, the zero error
and the zero correction, both are nil.
• If zero error is positive then subtract it from
total or measured reading
• If zero error is negative then add it to the total
or measured reading.

22. Zero Error