An ammeter goes in series with the component whose current you want. A voltmeter goes in parallel across the component whose p.d. you want.
Everything else about meter placement follows from that pair of rules and from the fact that the two meters have very different resistances.
This lesson uses the circuit rules from current in series and parallel branches and belongs to potential difference, resistance and circuits.
Why do the meters behave differently?
An ammeter must let the current pass through, so an ideal ammeter has a very small resistance. A voltmeter compares the potential at two points, and it should not draw current away, so an ideal voltmeter has a very large resistance.
| Meter | Measures | Connected | Ideal resistance |
|---|---|---|---|
| Ammeter | current (A) | in series | very low |
| Voltmeter | p.d. (V) | in parallel | very high |
How do I work through a question?
- Name the quantity and the component: “the current in the lamp” or “the p.d. across the resistor”.
- Choose the meter and decide series or parallel.
- Describe the connection in words: “in series with the lamp” or “across the lamp, one terminal on each side”.
- Check the polarity: the positive terminal of the meter faces the side of the circuit nearer the positive terminal of the cell.
- Use the readings in R = V ÷ I if asked for resistance.
Worked example
A circuit has a cell, a switch, a resistor and a lamp, all in series. (Invented example data.) A student wants the resistance of the lamp. The voltmeter reads 2.5 V and the ammeter reads 0.30 A.
Step 1, ammeter: place it in series with the lamp, anywhere in the single loop, so the lamp current flows through it.
Step 2, voltmeter: connect it across the lamp only, one terminal on each side of the lamp, and not across the resistor.
Step 3, calculate: R = V ÷ I = 2.5 ÷ 0.30 = 8.3 Ω (2 significant figures).
Step 4, check: 0.30 × 8.3 = 2.49, which rounds to 2.5 V, matching the reading.
The resistance of the lamp at that moment is 8.3 Ω.
The mistake to watch for
A common slip is to swap the meters, or to connect a voltmeter in series.
Mistaken answer: “Put the voltmeter in the line with the lamp so the current goes through it.”
The student applied the ammeter rule to the voltmeter. A voltmeter in series has such a high resistance that the current almost stops, the lamp stays off, and the voltmeter reads close to the supply p.d.
The correction is to ask “does this meter need the current to pass through it?” Yes for the ammeter, so series. No for the voltmeter, so across the component. An ammeter placed across a component does the opposite damage: it gives the current a near-zero-resistance path, creating a short circuit.
Check yourself
Try these, then open each answer.
1. In a circuit with two parallel branches, where must an ammeter go to measure the current in one branch only?
Show answer
In series with that branch, in the wire of that branch. An ammeter in the main wire would read the total current for both branches.
2. Where does a voltmeter go to read the p.d. of the supply?
Show answer
Across the terminals of the cell or battery, in parallel with it, and not in the main loop.
3. An ammeter reads 0.20 A and a voltmeter across the same resistor reads 1.5 V. Find the resistance.
Show answer
R = V ÷ I = 1.5 ÷ 0.20 = 7.5 Ω. Check: 0.20 × 7.5 = 1.5 V.
Where this leads next
The practice set brings all five skills together. If you want to see a meter error, the circuit reasoning simulator lets you test a placement before you write an explanation.
Students who can place meters but struggle to explain why can gain from a teacher listening to the explanation in real time. That is part of our online one-to-one Physics tuition.