Read an analogue ammeter using its selected range
Connect an ammeter in series and convert a pointer position on a shared scale using the selected full-scale range.
Section P4.2.2 · Current
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Recognise a single complete circuit path.
- Use fractions of a labelled scale.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
On a 0–30 scale, what fraction of full scale is the mark 15?
What you will learn
- Describe series connection and range selection for an ammeter.
- Read a stated analogue scale without confusing divisions and amperes.
Put the current meter in the path
An ammeter measures current and is connected in series with the path being measured, so that path's current passes through the meter. An ideal ammeter has very small resistance. Connecting it directly across a source is not a suitable measurement arrangement and can make a large unwanted current.
In a supervised isolated low-voltage circuit, use the correct current terminals and a range above the expected current. Start with the highest suitable range when the current is unknown, then select a lower suitable range for a clearer reading. For a d.c. measurement, the indicated polarity must agree with the meter connections. Check the zero before use.
Read the selected range
A shared pointer scale can represent different full-scale current ranges. On a 0–30 scale, a pointer at 18 represents 18/30 of the selected full-scale value. If the range is 3.0 A, the reading is 1.8 A; if the range is 0.30 A, the same pointer position means 0.18 A. The scale number is not automatically the current in amperes.
Identify labelled intervals and count spaces rather than assuming every printed tick means one ampere. View the pointer square-on to avoid parallax, using a mirror scale if supplied. Report only the precision supported by the divisions; a fine-looking decimal does not create additional measurement accuracy.
Sources for this lesson
Pause and explain
A pointer is at 12 on a 0–30 scale with a 3 A full-scale range. What is the reading?
Worked example
An analogue scale runs from 0 to 30. Its pointer is at 18 and the selected range is 3.0 A. Find the current.
Show the worked solution
- The pointer is 18/30 of full scale.
- Full scale represents 3.0 A.
- 18/30 × 3.0 = 1.8 A.
Answer 1.8 A.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Read a pointer at 18 on a 0–30 scale when full scale is 3.0 A.
Give me a hint
Find the fraction of full scale before multiplying by the selected range.
Compare my reasoning
- Pointer fraction = 18/30.
- Selected full scale = 3.0 A.
- Current = 1.8 A.
1.8 A.
Look for these in your work
- Use the selected range.
- State amperes.
A pointer is at 24 on a 0–30 scale. Compare readings on the 3.0 A and 0.30 A ranges.
Give me a hint
The position is the same fraction, but the full-scale values differ.
Compare my reasoning
- 24/30 = 0.8 of full scale.
- 0.8 × 3.0 = 2.4 A.
- 0.8 × 0.30 = 0.24 A.
2.4 A; 0.24 A.
Look for these in your work
- Calculate both ranges.
- Do not equate pointer position with a fixed ampere value.
An unknown current may be near 1 A. A learner chooses a 0.1 A range and connects the meter across the cell. Identify two problems.
Give me a hint
Check both range capacity and whether the current path passes through the meter correctly.
Compare my reasoning
- The likely current exceeds the chosen range.
- An ammeter belongs in series rather than across the cell.
- Use a suitable larger range and series connection in the supervised low-voltage test.
The range is too small and the connection is wrong; use a suitable higher range in series.
Look for these in your work
- Address both errors.
- Keep the low-voltage supervised procedure explicit.
Common mistakes
- Connect in series rather than across the source.
- Use the selected range, not just the printed scale number.
- Avoid parallax and unsupported extra digits.
What can you explain now?
How is an ammeter placed to measure a path's current?
Compare with the explanation
In series in that path.
The current being measured must pass through the meter.
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Make it stick
Connect an ammeter in series, choose a suitable range and translate an analogue pointer using its scale.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.