Meters
High-Yield Summary
- Ammeters measure current, connect in series, and should have near-zero resistance so they don't disturb the circuit.
- Voltmeters measure voltage, connect in parallel, and should have near-infinite resistance so they don't divert current.
- Ohmmeters measure resistance using their own built-in voltage source and R = V/I — the circuit under test must be unpowered.
- A shunt resistor (low resistance, parallel to the ammeter) protects an ammeter from high current.
- Connecting a meter the wrong way (ammeter in parallel, voltmeter in series) disrupts the circuit and can damage the meter.
Ohmmeter Resistance Calculation
R = V/I
- R = Resistance of the component under test
- V = Known voltage the ohmmeter's internal battery applies
- I = Resulting current the ohmmeter measures
Meters at a Glance
| Meter | Connection / Ideal Resistance |
|---|---|
| Ammeter (current) | Series; near-zero resistance; needs active circuit |
| Voltmeter (voltage) | Parallel; near-infinite resistance; needs active circuit |
| Ohmmeter (resistance) | Applies own voltage via R=V/I; circuit must be unpowered |
Common MCAT Trap
- Mixing up ideal resistances is the #1 trap: ammeter → near-zero (so it doesn't add resistance in series); voltmeter → near-infinite (so it doesn't divert current in parallel). Reversing these breaks the circuit.
- Using an ohmmeter on a powered circuit gives inaccurate readings or damages the meter — always de-energize the circuit first.
Quick Recall
Why should an ideal ammeter have near-zero resistance?
Why must a circuit be unpowered before using an ohmmeter?