Use a digital ammeter and choose a suitable range
Interpret decimal displays and unit prefixes while checking terminals, overload indications and range resolution.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Describe series ammeter connection.
- Convert milli- and micro-prefixes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many milliamperes are in one ampere?
What you will learn
- Describe a digital current measurement.
- Distinguish a display's unit, range and resolution from true accuracy.
Read the unit with the digits
A digital ammeter gives a numerical current reading on its selected range. A display of 0.420 A means 420 mA; a display of 420 μA means 0.000420 A. Digits without the displayed unit and range can differ by factors of a thousand or a million.
Connect the correct current terminals in series in an isolated low-voltage test. Choose a range that includes the expected current. If the instrument shows an overload indication, follow its instructions and use a larger suitable range rather than interpreting the indication as zero current. Different meters use different overload symbols.
Separate resolution and accuracy
A lower suitable range may resolve smaller changes, but it must not be exceeded. If a 2.000 A range has a stated display step of 0.001 A, its resolution is 1 mA. That display step alone does not prove every reading is accurate to 1 mA; the meter's supplied accuracy specification is separate.
A negative d.c. reading can mean the measured current direction is opposite to the meter's chosen polarity. It does not mean a negative quantity of carriers exists. Record the sign and connections, check the zero where relevant and keep the source and sample conditions stable while comparing readings.
Sources for this lesson
Pause and explain
A display step is 0.001 A. What is established by that fact alone?
Worked example
A meter displays 0.420 A on a 2.000 A range. Convert the current to milliamperes.
Show the worked solution
- One ampere is 1000 mA.
- Multiply 0.420 by 1000.
- The displayed current is 420 mA.
Answer 420 mA.
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.
Convert a digital current reading of 0.420 A to mA.
Give me a hint
Use 1000 milliamperes per ampere and retain the measured value.
Compare my reasoning
- The display unit is A.
- 0.420 × 1000 = 420.
- The reading is 420 mA.
420 mA.
Look for these in your work
- Use the displayed unit.
- Keep the conversion direction correct.
Compare displays of 420 μA and 0.420 A in amperes.
Give me a hint
Micro and an unprefixed ampere display use different scales.
Compare my reasoning
- 420 μA = 420 × 10⁻⁶ A = 0.000420 A.
- The second value is already 0.420 A.
- The second is 1000 times the first.
0.000420 A; 0.420 A; factor 1000.
Look for these in your work
- Convert both to one unit.
- Compare magnitudes separately from display digits.
A meter overloads on a low range. A learner records zero current and claims the smallest display step proves perfect accuracy. Correct both claims.
Give me a hint
Distinguish capacity, resolution and accuracy.
Compare my reasoning
- An overload is outside the selected range, not zero.
- Select a larger suitable range following the instrument procedure.
- The display step gives resolution; supplied specifications are needed for accuracy.
Use a larger suitable range; overload is not zero and display resolution is not perfect accuracy.
Look for these in your work
- Correct both interpretations.
- Keep current connection in series.
Common mistakes
- Read the unit as well as the digits.
- Overload is not a zero reading.
- Resolution does not establish accuracy.
What can you explain now?
What does a negative d.c. current reading indicate relative to the chosen meter polarity?
Compare with the explanation
Opposite current direction.
The sign refers to the measurement reference, not a negative number of carriers.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Read a digital current with its unit and range while distinguishing overload, polarity, resolution and accuracy.
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.