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Measure length and angle · Biology · Physics

Ruler and metre rule

Ruler and metre rule, with labels: zero mark, scale in cm and mm.

What it is for

A ruler or metre rule measures length. Put the 0 mark at one end of the object and read the mark level with the other end; the smallest divisions are millimetres.

Range
30 cm ruler, 50 cm half-metre rule, 100 cm metre rule
Smallest division
1 mm
Record to
the nearest 1 mm, e.g. 47 mm (4.7 cm)

Why choose it

For lengths from a few millimetres to a metre: the side of a block, a spring’s extension, a pendulum’s length, a lamp’s distance from a plant. A tape measure suits lengths over a metre; for something too thin to read well, such as a sheet of paper, measure a stack of 100 and divide by 100.

How to read it

0123456cmeye to one side:parallax errorfrom the 0 mark: 47 mm (4.7 cm)
  1. Lay the rule flat against the object, or upright right beside it.
  2. Line the 0 mark up with one end; do not measure from the worn end of the rule.
  3. Put your eye straight above the other end, not to one side.
  4. Read the nearest millimetre mark and write the unit: 47 mm.

Exam-style question

A spring hangs beside an upright metre rule. Its pointer is level with 12.0 cm with no load, and with 16.7 cm when a 2.0 N weight hangs on it.

  1. Calculate the extension of the spring.
  2. Calculate the spring constant.
  3. How should the student read the pointer to avoid parallax error?
Show what a good answer says
  1. 16.7 − 12.0 = 4.7 cm, which is 47 mm.
  2. k = F ÷ x = 2.0 ÷ 4.7 = 0.43 N/cm (43 N/m).
  3. With the eye level with the pointer, looking straight at the rule; a pointer that almost touches the rule helps.

Teaching points for checking your own answer, not official Cambridge marks.

Common errors

  • Measuring from the end of the rule instead of the 0 mark, when the end is worn or the 0 is set in (zero error).
  • Looking at the mark from one side, not straight on (parallax error).
  • Holding the rule at a slant to the object, or not upright beside a spring, so the length read is wrong.
  • Mixing units or leaving them out: 4.7 cm is 47 mm, and the table heading must say which.

Where you use it here

From the Practical Test apparatus lists of the Cambridge IGCSE Biology 0610, Chemistry 0620 and Physics 0625 syllabuses for 2026–2028, and the apparatus the experiments on this site use.

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