Use a digital ammeter and choose a suitable range
Interpret decimal displays and unit prefixes while checking terminals, overload indications and range resolution.
Section 4.2.2 · Electric current
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
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.
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.
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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.