Ideal Meters: Ammeters and Voltmeters Don't Disturb the Circuit

1 · Predict

An ammeter (in series) and a voltmeter (in parallel) are added to a simple resistor circuit to measure it. Do the meters themselves change the circuit's behavior?

• No — an ideal ammeter has zero resistance and an ideal voltmeter has infinite resistance, so neither disturbs the circuit.
• Yes — adding meters always changes the current and voltage you're trying to measure.
• It depends on which resistor value you're testing.

2 · Set Up

1. Open the metered-resistor 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. Notice the voltmeter reading stays 9 V in every trial — explain why.
2. An ideal ammeter has 0 Ω, so it drops no voltage — the full battery EMF appears across R1 regardless of R1's value. Explain why this means the voltmeter reading never changes even as R1 (and the current) changes.

5 · Extend

1. A real ammeter has a small but nonzero resistance, and a real voltmeter has a large but finite resistance. For very small R1, would a real (non-ideal) ammeter's resistance start to matter more or less compared to R1?
2. This experiment places the ammeter in series and the voltmeter in parallel — the only placement that doesn't disturb the circuit. See the meters-swapped experiment for what goes wrong if they're placed the other way around.

The Physics Behind This Experiment

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