Topic: General Physics
Subtopic: Length and time
USE OF VARIOUS MEASURING INSTRUMENTS TO DETERMINE LENGTH
Length is the distance between two points. It tells us how long, wide, tall or thick an object is. In Physics, we choose the measuring instrument according to the size of the object and how accurate the measurement must be.
Important words
| Term | Meaning |
|---|---|
| Length | The distance between two points. |
| Accuracy | How close a measurement is to the true value. |
| Least count | The smallest measurement an instrument can read. |
| Zero error | A reading shown by an instrument when it should read zero. |
Measuring length is important in the home, in school, in workshops and in science practical work. For example, we may measure the length of a desk, the diameter of a bottle top, or the thickness of a copper wire.
Comparing common measuring instruments for length
| Instrument | What it measures | Usual accuracy | Example use |
|---|---|---|---|
| Metre rule | Longer objects such as books, desks and boards | 1 mm | Measuring the length of a textbook |
| Vernier calipers | External diameter, internal diameter and depth | 0.1 mm | Measuring the diameter of a bottle top or a small rod |
| Micrometer screw gauge | Very small thickness or diameter | 0.01 mm | Measuring a thin wire |
Rules for measuring length correctly
- Choose the correct instrument for the object.
- Place the object properly against the scale.
- Read the scale at eye level to avoid parallax error.
- Always check whether the instrument starts at zero.
- Use the smallest possible instrument that gives the needed accuracy.
Example: A school desk can be measured with a metre rule, but the thickness of a wire should be measured with a micrometer screw gauge.
Using a metre rule
A metre rule is used to measure the length of ordinary objects. It has centimetre and millimetre divisions. It is suitable for measuring things such as a ruler, book, table, chalkboard or pencil.
To measure correctly, place one end of the object at the zero mark of the metre rule. Read the other end at eye level. If the object does not start at zero, subtract the starting reading from the final reading.
Formula: Length of object = final reading - initial reading
Example: A pencil starts at 1.4 cm and ends at 15.9 cm on a metre rule.
Working: Length = 15.9 cm - 1.4 cm
Answer: Length = 14.5 cm
Using vernier calipers
Vernier calipers are used to measure small lengths more accurately than a metre rule. They can measure external diameter, internal diameter and depth. They are useful for objects such as a bottle top, a metal rod, a small pipe or a coin.
The main parts of vernier calipers are the main scale, vernier scale, outside jaws, inside jaws and depth rod. The outside jaws measure external size, the inside jaws measure internal size, and the depth rod measures depth.
To take a reading:
- Read the main scale just before the zero of the vernier scale.
- Find the vernier division that lines up exactly with a main scale mark.
- Multiply that vernier division by the least count.
- Add the two readings.
Formula: Reading = main scale reading + vernier coincidence × least count
Example: The main scale reading is 23.0 mm. The 6th vernier division coincides with a main scale mark. The least count is 0.1 mm.
Working: Reading = 23.0 mm + (6 × 0.1 mm)
Answer: Reading = 23.6 mm
If there is zero error, correct the final reading by adding or subtracting the error as instructed by the instrument reading.
Using a micrometer screw gauge
A micrometer screw gauge is used to measure very small thicknesses and diameters. It is more accurate than vernier calipers. It is useful for measuring the thickness of a wire, the diameter of a thin rod or the thickness of a coin.
The main parts are the frame, anvil, spindle, sleeve, thimble and ratchet stop. The object is placed between the anvil and spindle. The ratchet stop is turned gently until the object is held firmly.
To take a reading:
- Read the sleeve scale first.
- Read the thimble scale next.
- Multiply the thimble reading by 0.01 mm.
- Add the two readings.
Formula: Reading = sleeve scale reading + thimble scale reading × 0.01 mm
Example: The sleeve reading is 4.5 mm and the thimble reading is 23 divisions.
Working: Reading = 4.5 mm + (23 × 0.01 mm)
Answer: Reading = 4.73 mm
Common mistakes in measuring length
| Wrong practice | Correct practice |
|---|---|
| Reading the scale from above or from the side | Read at eye level |
| Using a metre rule for a very small wire | Use a micrometer screw gauge |
| Forgetting to check the zero reading | Check for zero error before measuring |
| Starting from a worn-out end of a ruler | Start from a clear zero mark or use another scale mark carefully |
Summary
- Length is the distance between two points.
- A metre rule is used for ordinary lengths.
- Vernier calipers are used for internal diameter, external diameter and depth.
- A micrometer screw gauge is used for very small thicknesses and diameters.
- Accuracy depends on choosing the right instrument and reading it correctly.
- Always check zero error and read scales at eye level.
Revision Questions
- What is length?
- Name the instrument used to measure the length of a school desk.
- Why should a scale be read at eye level?
- State the least count of a metre rule.
- Name three parts of vernier calipers.
- What is the use of the inside jaws of vernier calipers?
- State the least count of a micrometer screw gauge.
- Why is a micrometer screw gauge better than a metre rule for measuring a wire?
Practice Exercise
- Define length.
- Name the instrument used to measure the thickness of a copper wire.
- State one use of vernier calipers.
- A pencil starts at 2.0 cm and ends at 18.5 cm on a metre rule. Find its length.
- A vernier caliper has a main scale reading of 24.0 mm and the 7th vernier division coincides with a main scale mark. The least count is 0.1 mm. Find the reading.
- A micrometer screw gauge has a sleeve reading of 5.5 mm and a thimble reading of 23 divisions. The least count is 0.01 mm. Find the thickness.
Answers to Practice Exercise
1. Length is the distance between two points.
2. A micrometer screw gauge.
3. Vernier calipers are used to measure external diameter, internal diameter and depth.
4. Length = 18.5 cm - 2.0 cm = 16.5 cm
5. Reading = 24.0 mm + (7 × 0.1 mm) = 24.7 mm
6. Reading = 5.5 mm + (23 × 0.01 mm) = 5.73 mm