CBSE Physics Experiment Thickness of a Given Sheet Using Screw Gauge

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

1. Rotate the circular scale so that it traverses towards the fixed end of the gauge, till it comes in contact with the end.

2. 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.

3. 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.

4. Divide this distance by the number of divisions on a circular scale to get the least count.

A Screw Gauge with its Key Components

5. 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.

6. Note the main scale reading where the circular scale lies using magnifying glass.

7. Note the circular scale reading which coincides with the main scale.

8. 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

1. Note the zero error very carefully.

2. Do not hold the sheet using the gauge too tightly or too loosely.

3. 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. 1 mm

2. 0.1 mm

3. 0.01 mm

4. 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. 1

2. 2

3. 3

4. 4

Ans: (A) A screw gauge has 1 screw for the movable hinge.

3. How many scales does a screw gauge have?

1. 2

2. 3

3. 5

4. 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. 1

2. 10

3. 100

4. 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:

1. Edge of a dice

2. Thickness of a wire

3. Radius of curvature

4. 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:

1. Copper and tin

2. Aluminum

3. Stainless steel and iron

4. 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:

1. Systematic error

2. Random error

3. Backlash error

4. 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:

1. Increasing the number of divisions on circular scale

2. Using a magnifying glass

3. Using a thick wire for measuring diameter

4. 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?

1. One

2. Two

3. Three

4. 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?

1. Diameter of a thin wire

2. Thickness of a fine slab

3. Depth of a vessel

4. 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.