Ideal meters: ammeter 0 Ω, voltmeter ∞ Ω

01
The tempting model
Placing meters without accounting for their ideal resistance.
02
Why it feels right
Meters seem like passive observers that don't change the circuit.
03
Where it breaks · how to catch it
You connected a voltmeter in series or an ammeter in parallel.
04
The correct model
An ideal ammeter (0 Ω) goes in series so all current passes through it; an ideal voltmeter (∞ Ω) goes in parallel so it draws no current.
05
Repair · see it, then test it
worked example How do you measure the current through a resistor?
Tempting move
Put the ammeter in parallel with it.
Correct move
Put the ammeter in series so the same current flows through both.
Test the repair CIR-02 · transfer

To measure the potential difference across a lamp without substantially changing the circuit, connect an ideal voltmeter:
In series with the lamp In parallel with the lamp Across the battery only In place of the lamp

Make a fresh prediction before referring to the worked example.