An Overview of Class 11 Physics To Measure Thickness Of A Given Sheet Using Screw Gauge Experiment
Introduction
Measurement is a common problem in both physics and engineering. Many times we are required to accurately and precisely measure extremely small lengths such as the thickness of a sheet, the diameter of a thin wire, etc. in construction and blueprint services. Any small miscalculation might lead to large errors in the measurement.
Therefore, it becomes essential to get a simple hands-on way to measure the thickness of a given sheet using a screw gauge.
Table of Contents
Aim
Apparatus required
Theory
Procedure
Observations
Result
Precautions
Lab manual questions
Viva questions
Practical based questions
Conclusion
Frequently asked questions
Aim
To measure the thickness of a given sheet using a screw gauge.
Apparatus Required
Given sheet
Screw gauge
Magnifying glass
Theory
A screw gauge works using a main and a circular scale. The distance covered on the main scale by the circular scale after completing one full rotation is known as the pitch of the circular scale. In this experiment, we hold the sheet gently between two rods of the main scale and tighten the rods using the screw of the circular scale. The corresponding readings on the main and circular scales give the measurement of the thickness of the sheet.
Procedure
Rotate the circular scale so that it traverses towards the fixed end of the gauge, till it comes in contact with the end.
Check if the zero of circular scale coincides with the zero of main scale. If not, note the circular scale division that coincides best with the main engraved scale.
Now rotate the circular scale exactly once and note the division on the main scale with which the circular scale coincides to get the pitch.
Divide this distance by the number of divisions on a circular scale to get the least count.
A Screw Gauge with its Key Components
Now rotate the circular scale using screw in the opposite direction and hold the sheet between the two rods of the main scale. Hold the sheet gently between the rods till the screw stops rotating.
Note the main scale reading where the circular scale lies using magnifying glass.
Note the circular scale reading which coincides with the main scale.
Loose the circular scale screw and note both the readings in a similar fashion two other times. Note the observations.
Observations
Pitch of the screw gauge = 1 mm
Number of divisions on circular scale = 100
Least count = \[\dfrac{{Pitch}}{{Number\;of\;divisions\;on\;circular\,scale}} = \dfrac{1}{{100}}mm = 0.01\;mm\]
Observation Table
Result
Average thickness of the sheet = \[\dfrac{{Sum\;of\;the\;total\;readings}}{3}mm\]
Precautions
Note the zero error very carefully.
Do not hold the sheet using the gauge too tightly or too loosely.
Only note the circular scale division which coincides best with the main scale using magnifying glass. If no division is coinciding, note the preceding division.
Lab Manual Questions
1. How is the accuracy of a screw gauge related to its least count?
Ans: A screw gauge with lesser least count has more accuracy. The accuracy of a screw gauge is determined directly by the number of divisions on a circular scale. Hence, the one having a circular scale with more number of divisions is more accurate in measurement as it can provide more details for a given length.
2. What is the type of error caused when the linear distance moved by the screw is not proportional to the rotation provided to it?
Ans: This type of error is called backlash error. It is usually caused by prolonged use of the instrument due to wear and tear.
3. What is the zero error of a screw gauge? How is it avoided?
Ans: The zero error in a screw gauge arises when the zeroes of the main scale and circular scale do not coincide with one another at the full loosening of the circular scale. Usually it arises due to more than enough loosening/ tightening of the screws and hence the observer needs to lubricate the gauge before adding/ subtracting the zero error from the final reading.
4. What makes a screw gauge far more accurate than a vernier caliper of the same number of divisions?
Ans: A screw gauge works on the principle of a nut and a screw. Its circular scale contains 100 divisions that linearly move 1mm on the main scale while rotating. These more number of divisions on the circular scale give screw gauge much more accuracy than a vernier scale which is a linear instrument with lesser divisions on its small scale.
Viva Questions
1. How many divisions are there on the circular scale?
Ans: There are 100 divisions on the circular scale in a screw gauge.
2. Explain the working principle of a screw gauge.
Ans: A screw gauge is based on the principle of a rotating screw. As the screw rotates, it covers a linear distance on the main scale. This gives us the pitch and least count and hence the required accuracy for measurement.
3. What are the orders of length a screw gauge is suitable for measuring?
Ans: A screw gauge is capable of precisely measuring lengths up to 0.01 mm.
4. Write three commercial applications of a screw gauge.
Ans: Screw gauge finds commercial applications in precisely determining the diameter of thin wires, thickness of a paper sheet and the thickness of a given experimental metallic chip used in communication devices.
5. Explain the formula for the calculation of least count.
Ans: Least count is calculated by dividing the distance covered by the circular scale on its one full rotation on the main scale (pitch) by the number of divisions on the circular scale.
6. Explain zero error. How to determine zero error?
Ans: A zero error is the error which arises in the final result when the zeroes of main scale and circular scale do not coincide. It is determined by noting the main scale division with which the circular scale coincides at the full range of rotation of the circular scale.
7. How is a screw gauge better than a vernier caliper?
Ans: A screw gauge has a greater precision of 0.01 mm of measuring the lengths as compared to that of vernier calipers (0.1 mm). Hence, the lengths measured by a screw gauge are more accurate to a second decimal than a vernier caliper.
8. Give the working formula of a screw gauge.
Ans: The final reading is given by the sum of main scale and circular scale readings, which is given by the product of the number of coinciding circular scale divisions and the least count of the instrument.
9. What is pitch?
Ans: The length covered by the circular scale over the main scale on one complete rotation of the circular scale is known as the pitch of the instrument. It is roughly equal to 1 mm.
10. Name two other examples that can be used alternatively in the above experiment.
Ans: The thickness of the plastic ruler and the diameter of a thin stick can also be used in the above experiment.
Practical Based Questions
1. Least count of screw gauge is:
1 mm
0.1 mm
0.01 mm
0.001 mm
Ans: (C) The least count of a screw gauge is 0.01 mm.
2. How many screws are there in a screw gauge?
1
2
3
4
Ans: (A) A screw gauge has 1 screw for the movable hinge.
3. How many scales does a screw gauge have?
2
3
5
6
Ans: (A) A screw gauge has 2 scales.
4. The ratio of the least counts of the centimeter scale to that of a screw gauge is:
1
10
100
1000
Ans: (C) The least count of a centimeter scale is 1 mm, while that of a screw gauge is 0.01 mm. Hence the ratio is 100.
5. Screw gauge is ideal for measuring:
Edge of a dice
Thickness of a wire
Radius of curvature
Length of a notebook
Ans: (B) A screw gauge can be used to accurately measure the thickness of a wire.
6. A screw gauge can be manufactured by a combination of:
Copper and tin
Aluminum
Stainless steel and iron
None of the above
Ans: (A) A screw gauge can be manufactured by an alloy named Gun-metal which is a combination of Copper and Tin.
7. The error that arises when the zeroes of the main scale and the circular scale do not coincide is called:
Systematic error
Random error
Backlash error
Zero error
Ans: (D) The error that arises when the zeroes of main and circular scales do not coincide is called zero error.
8. Accuracy of measurement can be increased by:
Increasing the number of divisions on circular scale
Using a magnifying glass
Using a thick wire for measuring diameter
Using more circular scales
Ans: (A) The accuracy of measurement can be increased by increasing the number of divisions on a circular scale.
9. How many types of zero errors are there?
One
Two
Three
Four
Ans: (B) There are two types of zero errors - positive and negative.
10. Which of the following is not suitable for measurement by a screw gauge?
Diameter of a thin wire
Thickness of a fine slab
Depth of a vessel
Thickness of a semiconductor chip
Ans: (C) Depth of a vessel is not suitable for measurement by a screw gauge.
Conclusion
From this experiment, we can conclude that screw gauge finds immense applications in the field of experimental physics to accurately measure the diameter of thin wires, the thickness of a sheet etc. They find various commercial applications in numerous engineering and medical fields in society.
Throughout this experiment, we have taken utmost care in teaching the novice about the very idea, and the basic parameters should have been crystal clear in his/ her mind.
We hope that the reader is now enlightened regarding the very concept of the same and is motivated to explore the field in the coming time.
FAQs on Class 11 Physics To Measure Thickness Of A Given Sheet Using Screw Gauge Experiment
1. What are the pitch and least count of a screw gauge, which are frequently asked in Class 11 viva exams?
The pitch of a screw gauge is the linear distance the screw moves on the main scale during one complete rotation of the circular scale. The least count (LC) is the smallest measurement that can be accurately made with the instrument. It is a crucial parameter for determining the precision of your measurement and is calculated by the formula:
Least Count = Pitch / Total number of divisions on the circular scale.
2. What is the standard formula to calculate the observed thickness from a screw gauge reading?
The observed thickness is calculated using the formula: Total Reading = Main Scale Reading (MSR) + (Coinciding Circular Scale Division (CSD) × Least Count (LC)). MSR is the reading on the main linear scale, while CSD is the division on the circular scale that aligns with the main scale's baseline.
3. How do you identify and correct positive and negative zero errors in a screw gauge for an accurate measurement?
A zero error is an important factor for accurate results. To identify it, bring the jaws of the screw gauge together.
- Positive Zero Error: If the zero mark of the circular scale is below the main line, the error is positive. The correction is negative (subtract the error from the observed reading).
- Negative Zero Error: If the zero mark of the circular scale is above the main line, the error is negative. The correction is positive (add the error's magnitude to the observed reading).
4. What is backlash error, and what is the correct procedure to minimise it during the Class 11 practical exam for the 2025-26 session?
Backlash error is a common mechanical error that occurs due to wear and tear in the screw's threads, causing a lag when the direction of rotation is reversed. To minimise this for any important measurement, you should always rotate the screw in a single direction (preferably clockwise) when approaching the final reading. If you overshoot the mark, move back well behind it and approach it again from the same direction.
5. Why is a screw gauge the preferred instrument over a Vernier caliper for an important task like measuring a thin sheet's thickness?
A screw gauge is preferred over a Vernier caliper for this task due to its higher precision. A typical screw gauge has a least count of 0.01 mm, whereas a Vernier caliper has a least count of 0.1 mm. For measuring very small dimensions like the thickness of a sheet or the diameter of a thin wire, the greater precision of the screw gauge is essential for obtaining a meaningful and accurate result.
6. As per CBSE experimental guidelines, why is it important to take measurements at several different locations on the sheet and then calculate a mean value?
Taking multiple readings across the sheet is crucial because a given sheet may not have a perfectly uniform thickness. There can be minor variations due to manufacturing processes. By measuring at different points and calculating the mean (average) thickness, you get a more representative and accurate value for the overall thickness of the sheet, minimising the impact of local irregularities.
7. What is the function of the ratchet mechanism in a screw gauge, and what type of experimental error does it help prevent?
The ratchet is a safety mechanism at the end of the thimble that clicks when the stud touches the object with a specific, optimal pressure. Its primary function is to prevent over-tightening of the screw. This avoids deforming the object being measured (like a thin sheet) and ensures that a uniform pressure is applied for every measurement, thereby reducing personal or random errors.
8. An important question for 3 marks: A student finds the Main Scale Reading (MSR) is 0 mm and the 68th Circular Scale Division (CSD) coincides with the reference line. If the instrument has a negative zero error of -0.04 mm, what is the correct thickness of the sheet? (LC = 0.01 mm).
Here is the step-by-step calculation:
1. Calculate the Observed Reading:
Observed Reading = MSR + (CSD × LC)
= 0 mm + (68 × 0.01 mm) = 0.68 mm.
2. Apply the Zero Correction:
The zero error is negative (-0.04 mm), so the correction is positive.
Correct Reading = Observed Reading - (Zero Error)
= 0.68 mm - (-0.04 mm) = 0.68 + 0.04 = 0.72 mm.
The correct thickness of the sheet is 0.72 mm.
9. Besides zero error and backlash error, what are other potential sources of error that could be important to consider when using a screw gauge?
Other important sources of error include:
- Parallax Error: This occurs if the observer's eye is not positioned directly perpendicular to the scale while taking the reading.
- Non-uniform Screw Pitch: If the screw threads are worn out or manufactured imperfectly, the pitch may not be consistent, leading to inaccuracies.
- External Factors: Temperature variations can cause expansion or contraction of the metallic instrument, affecting the reading.
- Personal Error: Inconsistent pressure application (if the ratchet is not used) or errors in reading the scales can also affect the final result.
