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Physics · Chapter 2: Physical Quantities and Measurement
Metre Rule and Measuring Tape
Least count, using a metre rule and measuring tape correctly, worn ends and how to avoid parallax error.
Key termRememberExampleWatch out
01Concept
Every measuring instrument has a smallest value it can measure. This is its least count: the smallest change in a quantity that the instrument can measure. An instrument with a smaller least count is more precise.
| Instrument | Range | Least count | Use |
|---|---|---|---|
| Metre rule | 0–100 cm | 1 mm = 0.1 cm | Lengths from a few cm to 1 m: books, tables |
| Measuring tape | up to 50 m or more | 1 mm (some 1 cm) | Long lengths: rooms, plots of land, a cricket pitch; tailors use a flexible tape |
| Vernier callipers | up to about 15 cm | 0.1 mm = 0.01 cm | Small lengths and diameters |
| Screw gauge | up to about 25 mm | 0.01 mm | Very small thicknesses: wire, paper |
Using a metre rule correctly
- Place the rule along the length, touching the object.
- The ends of an old rule are often worn. Start from a clear mark such as 1.0 cm and subtract it from the final reading. Ends at 1.0 cm and 13.6 cm → length = 12.6 cm.
- Keep your eye straight above the mark you are reading.
- Write the reading to the least count: 12.6 cm or 126 mm, not 12.63 cm.
Parallax error
Parallax is the apparent change in the position of an object when it is seen from different directions. If the eye is to one side, the end of the object seems to line up with the wrong mark, giving a parallax error.
To avoid it: keep the eye vertically above the point being read, and keep the object as close to the scale as possible.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Real World Examples
Buying land
Plots in Pakistan are measured with long measuring tapes and given in square feet, marlas or kanals. A small error in each side adds up to a big error in area.
Petrol pump
When you look at a petrol pump's dial from the side, the needle seems to point to a different number. That is parallax: always read it straight on.
Tailor's tape
A tailor uses a flexible tape with 1 cm or 1/8-inch marks to measure the chest, waist and sleeve.
04Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.What is least count? Give the least count of a metre rule.Show answer
Answer
Least count is the smallest value an instrument can measure. The least count of a metre rule is 1 mm (0.1 cm).
Q2.What is parallax error? How can it be avoided?Show answer
Answer
Parallax error is the error caused when the scale is not read with the eye straight in line with the mark. It is avoided by keeping the eye vertically above the mark and the object close to the scale.
Q3.A pencil is measured from the 2.0 cm mark to the 15.4 cm mark of a metre rule. Find its length.Show answer
Answer
- Length = 15.4 − 2.0
- = 13.4 cm (= 134 mm)
Q4.Why should we not start a measurement from the end of an old metre rule?Show answer
Answer
Because the end may be worn or damaged, so the zero mark is not exact and every reading would have an error.
Q5.When would you use a measuring tape instead of a metre rule?Show answer
Answer
For long lengths (more than 1 m), such as a room, a field or a cricket pitch, and for curved surfaces, such as the waist of a person, using a flexible tape.
Long questions
Q6.Describe how to measure the length of a book with a metre rule. Mention two precautions.Show answer
Answer
- Place the metre rule along the length of the book, touching it.
- Line up one end of the book with a clear mark, for example 1.0 cm.
- Read the mark at the other end with your eye straight above it, for example 26.5 cm.
- Length = 26.5 − 1.0 = 25.5 cm.
Precautions:
- Keep the eye vertically above the reading to avoid parallax error.
- Do not use the worn end of the rule; start from a clear mark.
- Repeat the measurement and take the mean.