Connect an ammeter the wrong way and you short out the circuit; connect a voltmeter the wrong way and it reads almost nothing. The rule, ammeters in series, voltmeters in parallel, is not arbitrary. It follows directly from what each meter measures and how it is built, and once you see why, you will never wire one incorrectly.
You want to measure the current flowing through a globe, and separately the voltage across the same globe.
Before reading: should each meter be placed in the same path as the globe, or across it? Write your prediction and a one-sentence reason.
Why a current meter sits in the path it measures
To measure the current through a component you must make that very current flow through the meter, so the meter has to be inserted into the same path, in series.
Current is the rate of flow of charge along a path. The only way an ammeter can read the current through a component is to have that current pass through it, which means breaking the path and inserting the meter in series. So the meter does not change the current it is reading, it is built with a very low resistance, ideally zero. Using $V = IR$, a tiny meter resistance adds only a tiny extra voltage drop and leaves the series current almost unchanged.
If an ammeter were instead placed in parallel across a component, its very low resistance would create a short circuit, letting a huge current bypass the component, damaging the meter and the circuit.
An ammeter is connected in series with a component so the current being measured flows through it. It is built with very low (ideally zero) resistance so it does not reduce the current it reads.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? True or false: an ideal ammeter has a very large resistance.
Why a voltage meter spans the component instead
We just saw that an ammeter goes in series. That raises a question: why is a voltmeter the complete opposite? This card answers it → voltage is measured between two points, so the meter must connect across them, in parallel.
Voltage is a difference in potential between two points, so to measure it the meter must touch both points at once, connected across the component, in parallel.
Potential difference is measured between the two ends of a component, so a voltmeter is connected in parallel across it. For the meter not to disturb the circuit, it must draw almost no current away from the component, so it is built with a very high resistance, ideally infinite. By $V = IR$ (or the parallel rules), a near-infinite resistance branch carries a negligible current, so the component behaves as if the meter were not there.
A student wants to measure both the current through a resistor and the voltage across it. Where should the ammeter and the voltmeter be connected, and what resistance should each ideally have?
A voltmeter is connected in parallel across a component because voltage is the difference between two points. It has very high (ideally infinite) resistance so it draws negligible current and does not change the voltage it reads.
Add the highlighted rule to your notes before the check below.
Quick check: To correctly measure the voltage across a single resistor, a voltmeter should be connected:
Pattern B, Construct then justify
Draw a circuit of a cell and a single resistor. In your book: (1) add an ammeter to read the current through the resistor and a voltmeter to read the voltage across it, using standard symbols; (2) label which meter is in series and which is in parallel; (3) write one sentence for each meter explaining how its resistance keeps it from disturbing the circuit.
Odd one out: three of these describe an ideal AMMETER. Which one does not?
Dot point: Explain why ammeters are connected in series, and voltmeters are connected in parallel to components, in electric circuits
The resistance rule explains both correct and incorrect wiring
We just learned the correct placements. That raises a question: what actually goes wrong if you swap them? This card answers it → the meter's resistance, which makes it safe in the right spot, makes it harmful in the wrong one.
Each meter is designed around its placement, so putting it in the wrong place turns its helpful resistance into a serious problem.
Put an ammeter in parallel across a component and its near-zero resistance forms a short circuit, nearly all the current rushes through the meter, bypassing the component and potentially burning out the meter. Put a voltmeter in series and its near-infinite resistance blocks the path, so almost no current flows and the circuit barely works, giving a meaningless reading. The single idea, match a low-resistance ammeter to a series path and a high-resistance voltmeter to a parallel connection, keeps every measurement honest.
Each meter is built for its position: a low-resistance ammeter belongs in series, a high-resistance voltmeter in parallel. Swapping them (ammeter in parallel, voltmeter in series) either short-circuits or blocks the path and ruins the reading.
Pause, copy the highlighted summary into your book before the check below.
Last check: Decide if the statement is true or false.
Connecting a low-resistance ammeter in parallel across a component creates a short circuit, because most of the current bypasses the component through the meter.
Match the pairs: Match each meter property to its reason.
Dot point: Explain why ammeters are connected in series, and voltmeters are connected in parallel to components, in electric circuits
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 5(4 marks) 1. Explain why an ammeter is connected in series with a component while a voltmeter is connected in parallel with it. Refer in your answer to the resistance of each meter.
1 mark: ammeter in series so the measured current passes through it · 1 mark: ammeter has very low resistance so it does not reduce that current · 1 mark: voltmeter in parallel across the component because voltage is measured between two points · 1 mark: voltmeter has very high resistance so it draws negligible current and does not change the voltage
AnalyseBand 5(3 marks) 2. A $10\ \text{V}$ source has an internal series resistance of $9.0\ \text{k}\Omega$ and supplies a $100\ \text{k}\Omega$ resistor. A voltmeter with resistance $10\ \text{k}\Omega$ is connected across the $100\ \text{k}\Omega$ resistor. Calculate the voltage across the resistor before connection and the voltmeter reading after connection, then explain the change.
1 mark: before, $V=10\times100/(9+100)=9.17\ \text{V}$ · 1 mark: meter and resistor in parallel give $R_p=(10\times100)/(10+100)=9.09\ \text{k}\Omega$ · 1 mark: after, $V=10\times9.09/(9+9.09)=5.03\ \text{V}$ because the lower parallel resistance shares the source voltage with the nonzero series resistance
Dot point: Explain why ammeters are connected in series, and voltmeters are connected in parallel to components, in electric circuits
AnalyseBand 5(3 marks) 3. A student accidentally connects an ammeter in parallel across a resistor instead of in series. Predict what happens to the current in the circuit and explain why this connection is dangerous.
1 mark: the ammeter's near-zero resistance provides a low-resistance path across the resistor · 1 mark: this short-circuits the resistor, so the total circuit resistance falls sharply and the current rises sharply · 1 mark: the large current can damage the meter or the supply, so it is unsafe
Dot point: Explain why ammeters are connected in series, and voltmeters are connected in parallel to components, in electric circuits
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): An ammeter measures the current through a component, so it must be connected in series with that component, the very current being measured has to flow through the meter. To avoid reducing this current, an ammeter is built with very low (ideally zero) resistance, so it adds almost no extra voltage drop to the series path. A voltmeter measures the potential difference between the two ends of a component, so it must be connected in parallel, spanning those two points. To avoid disturbing the circuit, a voltmeter has very high (ideally infinite) resistance, so it draws a negligible current from the component and the voltage across the component is essentially unchanged by the measurement.
Q2 (3 marks): Before the meter is connected, the $100\ \text{k}\Omega$ resistor is in series with the source's $9.0\ \text{k}\Omega$ internal resistance, so its voltage is $10\times100/(9+100)=9.17\ \text{V}$ (resistances in kΩ). With the meter connected, the branch resistance is $R_p=(10\times100)/(10+100)=9.09\ \text{k}\Omega$. This is now in series with the $9.0\ \text{k}\Omega$ source resistance, so the measured branch voltage is $10\times9.09/(9+9.09)=5.03\ \text{V}$. The meter loading lowers the reading because the source has a specified nonzero series resistance; with an ideal zero-resistance source directly across the branch, the voltage would remain fixed at the source voltage regardless of the parallel load.
Q3 (3 marks): An ammeter is designed to have a resistance as close to zero as possible. Connecting it in parallel across the resistor therefore places a near-zero resistance path directly across that component, so almost all the current bypasses the resistor and flows through the meter instead. The resistor has effectively been short-circuited. The total resistance of the circuit falls sharply, and since $I = V/R$ the current drawn from the supply rises sharply, potentially to many times its intended value. That large current can overheat and destroy the ammeter, damage the supply, and melt insulation, which is why this is a dangerous connection rather than merely a wrong reading.
You can now connect ammeters in series and voltmeters in parallel, and justify each placement by its resistance. Next lesson uses these readings to analyse series circuits with $V_{\text{total}} = \Sigma V$ and $R_{\text{total}} = \Sigma R$.
↩ Back to focus-area hubAt the start you predicted where each meter belongs. The answer: the ammeter goes in series (in the same path, so the current flows through it) and the voltmeter goes in parallel (across the component, between the two points). If you swapped them, revisit Card 3, the meter's resistance is what makes each placement work.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
Tell us what you would like help with. Register interest; support details will be confirmed before enrolment.
Register interest →