Class 11 Physics practical Screw Gauge

Aim

To measure diameter of a given wire and thickness of a given sheet using screw gauge.

Apparatus and Material Required

• Screw gauge
• A wire piece
• Sheet
• A magnifying lens (optional, for reading)

Principle

A screw gauge works on the principle of a screw. When a screw is rotated in a nut, the linear distance moved by it is directly proportional to the number of rotations given to it. This converts a small, otherwise immeasurable, linear distance into a measurable rotation, allowing very small lengths (diameter/thickness) to be measured accurately.

A screw gauge consists of a U-shaped metal frame with a fixed stud at one end and a hollow cylinder (sleeve) at the other, through which a screw moves. The pitch scale (linear/main scale) is marked on the sleeve, and the circular scale (head scale) is marked on the rotating thimble. A ratchet is provided to prevent excessive pressure on the object being measured.

Formulae Used

Pitch = Distance moved by the screw / Number of complete rotations

Least Count (L.C.) = Pitch / Number of divisions on the circular scale

Observed Reading = Pitch Scale Reading (P.S.R.) + [Circular Scale Reading (C.S.R.) × L.C.]

Procedure

1. Determine the pitch of the screw gauge by noting the distance moved by the screw for a known number of complete rotations.
2. Determine the least count using the formula above.
3. Find the zero error: bring the flat end of the screw in contact with the stud (fully close the gap) and note which division of the circular scale coincides with the reference line.
   • If it coincides exactly with the base line → zero error = 0.
   • If it is below the reference line → zero error is positive.
   • If it is above the reference line → zero error is negative.
4. Place the wire between the stud and the screw. Rotate the ratchet gently till it gives a “click” sound (this ensures uniform pressure). Do not force the screw.
5. Note the Pitch Scale Reading (P.S.R.) — the last visible division on the main scale.
6. Note the Circular Scale Reading (C.S.R.) — the division of the circular scale coinciding with the reference line.
7. Repeat the measurement at 3 different points along the length of the wire, and at each point take two readings in mutually perpendicular directions (since the wire may not be perfectly circular).
8. Repeat steps 4–7 for the given sheet at different points.
9. Apply the zero error correction to each observed reading and calculate the mean.

Observations

Determination of Pitch and Least Count

Quantity	Value
Distance moved by screw for 5 rotations	5 mm
Pitch = Distance / No. of rotations	1 mm
No. of divisions on circular scale	100
Least Count (L.C.) = Pitch / 100	0.9 mm
Least count = 1 M.S.D. − 1 V.S.D.	0.01 mm
Circular scale division coinciding with reference line (screw fully closed)	12th division (below reference line)
Zero Error = 12 × L.C.	+ 0.12 mm
Zero Correction	– 0.12 mm

Table 2 — Diameter of the wire

S.N.	P.S.R. (mm)	C.S.R. (div)	C.S.R. × L.C. (mm)	Observed Reading (mm)	Corrected Reading (mm)
1.	0	42	0.42	0.42	0.30
2.	0	45	0.45	0.45	0.33
3.	0	40	0.40	0.40	0.28
4.	0	44	0.44	0.44	0.32
5.	0	41	0.41	0.41	0.29
6.	0	43	0.43	0.43	0.31

Mean diameter of wire =sum of corrected readings/6 = 0.40 mm

Table 3 — Thickness of the Sheet

S.N.	P.S.R. (mm)	C.S.R. (div)	C.S.R. × L.C. (mm)	Observed Reading (mm)	Corrected Reading (mm)
1.	1	25	0.25	1.25	1.13
2.	1	22	0.22	1.22	1.10
3.	1	20	0.20	1.20	1.08
4.	1	24	0.24	1.24	1.12
5.	1	21	0.21	1.21	1.09
6.	1	23	0.23	1.23	1.11

Mean thickness of sheet = sum of corrected readings/6 = 1.1 mm

Results

1. The diameter of the given wire = 0.40 mm
2. The thickness of the given sheet = 1.1 mm

Precautions

1. Use the ratchet to advance the screw; never force it directly, as this can damage the threads or give incorrect readings.
2. Note the zero error carefully, with correct sign, before starting the experiment.
3. Take readings at different points and in mutually perpendicular directions, since the wire/sheet may not have a uniform cross-section.
4. Avoid parallax error while reading the scales.
5. Screw gauge should be handled gently and kept free from dust.

Sources of Error

• Backlash error due to wear and tear of threads.
• Wire may not be perfectly cylindrical; sheet may not have uniform thickness.
• Error in estimating zero correction.
• Parallax error while taking readings.

For more practical records in Physics Practical Class 11, Click here

For Viva questions on Physics Practical Class 11, Click here

For more practical records in Physics Practical Class 12, Click here

For Viva questions on Physics Practical Class 12, Click here

Reference: NCERT Lab Manual