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TL;DR An ammeter must be wired in series with the component under test; a voltmeter must be wired in parallel across it. The reason is resistance: an ideal ammeter has zero resistance so it does not steal voltage from the circuit, and an ideal voltmeter has infinite resistance so it does not draw current away from the main loop. The single biggest misconception at Sec 3 is that current gets "used up" as it travels around a circuit -- it does not. Energy is transferred, but charge is conserved.
Before placing any meter, you need a clear picture of how series and parallel connections differ.
Series connection -- every component is part of a single, unbroken loop. The same current flows through every component in that loop, one after another. If you break the loop at any point, no current flows anywhere in the circuit.
Parallel connection -- the circuit splits into two or more branches that share the same two junction points. Each branch carries its own current, but the voltage across every branch between those two junctions is the same.
This distinction is the key to placing both meters correctly:
The ammeter must join the series loop because it measures the current that passes through the component.
The voltmeter must form a parallel branch because it measures the voltage between two points.
A memory hook many students find useful: the letter V forms a shape that spreads across two points, just like a voltmeter spans two terminals in parallel. The ammeter, by contrast, is inserted into the wire -- it is part of the single path, not branching off to the side.
2 | Where the ammeter goes (in series, low resistance)
The placement rule
To measure the current through a resistor, you break the wire at one point and insert the ammeter into that gap. The ammeter becomes part of the series loop. Every coulomb of charge that flows through the resistor must also flow through the ammeter, so the ammeter reads the true current through that component.
Once the ammeter is in series, it is inside the main current path. Any resistance it adds subtracts from the current you are trying to measure. An ideal ammeter would have zero resistance -- it would be indistinguishable from a piece of wire. Real ammeters come as close as practical to this ideal: their internal resistance is very small (typically a fraction of an ohm).
If someone handed you an ammeter with a 100 ohm internal resistance and asked you to measure the current through a 10 ohm resistor connected to a 3 V battery, the series total would jump from 10 ohm to 110 ohm. The ammeter would report roughly 27 mA, but the "true" current (without the ammeter) would be 300 mA. The meter would distort the very thing it is supposed to measure. Low internal resistance prevents this.
Worked example
A 6 V battery is connected to a 30 ohm resistor. You want to measure the current through the resistor.
Without an ammeter: I=RV=306=0.20 A
With a well-designed ammeter (internal resistance 0.1 ohm) in series:
I=30.16≈0.199 A
The ammeter introduces negligible error -- the reading is still effectively 0.20 A. A high-resistance ammeter would give a completely different and incorrect result.
3 | Where the voltmeter goes (in parallel, high resistance)
The placement rule
To measure the voltage across a resistor, you connect one lead of the voltmeter to one end of the resistor and the other lead to the other end. The voltmeter bridges those two terminals, creating a parallel branch. It does not interrupt the main current path.
Why the voltmeter needs very high resistance
Because the voltmeter forms a parallel branch, it provides an alternative path for current. If the voltmeter had low resistance, significant current would divert through it, reducing the current through the component you are testing and altering the voltage across it. An ideal voltmeter would have infinite resistance -- it would draw zero current and have no effect on the circuit at all. Real voltmeters aim to be as close to this ideal as possible: their internal resistance is very high (tens of kilohms to megaohms in typical school voltmeters).
Consider a 1000 ohm resistor connected across a battery. If you attach a voltmeter with only 100 ohm of internal resistance in parallel, the combined parallel resistance of the branch drops to about 91 ohm. The voltmeter massively changes the circuit. A voltmeter with 1 megaohm of resistance in parallel with a 1000 ohm resistor barely changes the effective resistance at all.
Worked example
A 6 V battery is connected to a 30 ohm resistor. You want to measure the voltage across the resistor.
With a voltmeter whose internal resistance is 100 kilohms connected in parallel with the 30 ohm resistor:
Parallel resistance of the branch: 1/R_parallel = 1/30 + 1/100000, which gives approximately 0.03334, so R_parallel works out to roughly 29.99 ohm.
The voltmeter draws negligible current and the reading is essentially 6 V -- accurate. If the voltmeter had only 30 ohm of internal resistance, the parallel combination would be 15 ohm and the voltage reading would be halved, giving an incorrect 3 V.
4 | Wiring diagram walkthrough
This section walks you through building a basic circuit that measures both the current through a resistor and the voltage across it. Follow the steps in order.
Step 1: draw the main series loop
Start from the positive terminal of the battery. Connect a switch in series, then the resistor, then return to the negative terminal of the battery. You now have a closed series loop: battery -- switch -- resistor -- battery. This is the main current path.
Step 2: insert the ammeter in series
Choose one point on the wire between the switch and the resistor (or anywhere else in the series loop -- it does not matter for the ammeter reading, since current is the same everywhere in a series loop). Break the wire at that point and connect the ammeter across the gap. The ammeter is now part of the main loop.
Step 3: branch the voltmeter across the resistor
Connect a lead from one terminal of the voltmeter to the junction where the wire meets one end of the resistor. Connect the other lead from the voltmeter's other terminal to the junction at the other end of the resistor. The voltmeter now forms a parallel branch, spanning only the resistor.
Step 4: check polarity
On analogue meters, the positive terminal (marked + or red) must connect to the more positive side of the circuit -- the side closer to the positive terminal of the battery. The negative terminal (marked -- or black) connects to the more negative side.
For the ammeter: the + terminal faces the battery positive side; the -- terminal faces the component.
For the voltmeter: the + terminal connects to the wire entering the high-potential side of the resistor; the -- terminal connects to the wire leaving the low-potential side.
Getting polarity wrong on an analogue meter causes the pointer to deflect in the wrong direction, hitting the stop pin. On a digital meter, reversed polarity shows a negative reading. Either way, the result is unusable.
Pre-switch checklist
Before you close the switch:
Is the ammeter in series with the resistor (not branched off to the side)?
Is the voltmeter in parallel across the resistor (not inserted into the main loop)?
Is the + terminal of the ammeter facing the battery positive side?
Is the + terminal of the voltmeter on the higher-potential side of the resistor?
Is the switch open (off)?
If a rheostat is present, is the slider set to maximum resistance?
Are all connections secure?
If all seven answers are yes, close the switch and take your first reading.
5 | The "current gets used up" misconception
One of the most persistent errors in Sec 3 Physics is the idea that current is consumed as it travels around a circuit -- that by the time current reaches the far end of a resistor, there is less of it than when it entered.
This is not what happens. Current is the rate of flow of charge. Charge is conserved: every coulomb that enters one end of a resistor must leave the other end. The current through a component is the same on both sides. In a series circuit, the ammeter reads the same value whether you place it before or after the resistor.
What the resistor does consume is energy. Positive charge loses electrical potential energy as it passes through the resistor. That energy is transferred to the resistor as heat (or light, in the case of a lamp filament). The charge carriers themselves continue around the circuit at the same rate.
This distinction matters practically for two reasons.
First, it means the position of the ammeter in the series loop is irrelevant to the reading. You can place it before the switch, between the switch and the resistor, or after the resistor -- the current reading will be the same in each position (assuming ideal components). Some students believe they must place the ammeter immediately before the resistor to "catch the current before it's used up." That belief is false, and it produces confused wiring.
Second, it means that when a lamp glows more dimly as battery voltage drops, the cause is falling current (and falling voltage) -- not current being "used up." Understanding this correctly is required for the ACE (Analysis, Conclusion, and Evaluation) section of Paper 3.
6 | Three wiring mistakes that lose practical marks
Mistake 1: voltmeter wired in series
A student connects the voltmeter into the main loop instead of branching it across the resistor. Because the voltmeter has very high resistance, almost no current flows through the circuit. A lamp will not light; an ammeter in the same loop will read near zero. The voltmeter will read close to the full battery voltage (since almost all the voltage appears across the high-resistance voltmeter), but the circuit is effectively broken for any practical purpose.
Examiners deduct marks for this because the circuit diagram produced is incorrect and the readings obtained in any subsequent step will be meaningless.
Mistake 2: reversed polarity on the ammeter
The student connects the + terminal of the ammeter to the negative side of the circuit. On an analogue ammeter, the pointer swings backward, hitting the mechanical stop. On a digital ammeter, the reading appears as a negative number. In either case, no valid data is collected, and the meter may be damaged.
The fix is simply to swap the two leads on the ammeter. Examiners look for correct polarity as a mark point before the student even records any data.
Mistake 3: both voltmeter terminals at the same potential
This happens when both leads are connected to the same junction in the circuit -- for example, both to the same wire that runs from the battery to the resistor. The voltage difference between two points on the same conductor is zero (assuming ideal wires), so the voltmeter reads exactly zero.
This mistake is easy to make when the circuit is physically complex and the student loses track of which junction is which. The fix is to trace from the voltmeter leads and confirm that one lead connects to the wire on the left side of the resistor and the other connects to the wire on the right side.
7 | Sample Sec 3 Physics circuit question
Question: A student connects a 10 ohm resistor to a 6 V battery. Draw a circuit diagram to measure both the current through the resistor and the voltage across it. State where each meter is placed and explain why.
What the mark scheme expects:
Circuit diagram shows: battery (with correct symbol), switch, ammeter, and resistor all connected in a single series loop.
Voltmeter branches off in parallel across the resistor only (not the whole circuit, not the battery).
Polarity is indicated correctly: + terminals on the positive-potential side.
The explanation identifies the ammeter as being in series so that the full current through the resistor passes through it.
The explanation identifies the voltmeter as being in parallel so that it measures the potential difference between the two terminals of the resistor.
If the question asks for a predicted ammeter reading: I=RV=106=0.6 A. The voltmeter should read 6 V (assuming a battery with negligible internal resistance and ideal connecting wires).
A common examiner note: students who correctly draw the circuit but describe the voltmeter as "measuring the current" instead of "measuring the voltage" lose explanation marks even if the diagram is correct. Be precise about what each meter measures and how the placement achieves that.
