Correct Meter Placement: Ammeter in Series, Voltmeter in Parallel

1 · Predict

To correctly measure a resistor's current and voltage, an ammeter must go in series and a voltmeter in parallel. If you set up the meters this way, does the circuit behave exactly as if the meters weren't there?

• Yes — correctly placed ideal meters don't change the circuit's current or voltage at all.
• No — any meter placement changes the circuit somewhat.
• It depends on the resistor's value.

2 · Set Up

1. Open the meters-correct preset and press Reset. The ammeter sits in series with R1; the voltmeter sits in parallel across R1.
2. Enable both the ammeter and voltmeter readouts.
3. Set R1's value for each trial and record the ammeter reading.

3 · Collect Data

Plot ammeter reading I (y-axis) against 1/R1 (x-axis) for your three trials.

4 · Analyze

1. For one trial, compute I = E/R1 using E = 9 V. Compare to the table, and confirm the voltmeter reads 9 V in every trial regardless of R1.
2. Explain, using the ammeter's 0 Ω and the voltmeter's ∞ Ω, why this placement lets you read the resistor's true current and voltage without perturbing the circuit at all.

5 · Extend

1. See the meters-swapped experiment for what happens if the ammeter and voltmeter placements are accidentally reversed. Predict, before looking, whether swapping would make the circuit read zero current, zero voltage, or something else entirely.
2. Modern digital multimeters approximate ideal ammeters and voltmeters very closely (extremely low and extremely high internal resistance respectively). Why might an older, less precise meter give a reading noticeably different from the true circuit value?

The Physics Behind This Experiment

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