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TL;DR In a circuit, connect the ammeter in series with the component and the voltmeter in parallel across the component. An ammeter must be connected in series with the component whose current you want to measure; a voltmeter must be connected in parallel across the component whose potential difference you want to measure. This distinction follows from their internal resistances: ammeters have very low resistance (so they do not reduce the current), while voltmeters have very high resistance (so they draw negligible current from the circuit). Most Paper 3 circuit marks depend on getting these connections right, reading meters to the correct precision, and recording values with units.
If you are at Sec 3 and want a step-by-step checklist for how to connect an ammeter and voltmeter in an electric circuit -- covering the series/parallel rule, polarity, resistance reasoning, and the "current gets used up" misconception -- start with the Sec 3 Physics practical checklist for ammeters and voltmeters before returning here for the full Paper 3 treatment.
Exam answer: an ammeter is placed in series because it measures the same current flowing through the component. A voltmeter is placed in parallel because it measures the potential difference across the component.
Ammeter: always in series
An ammeter measures the current flowing through a component. To do this, the ammeter must carry the same current as the component, so it is wired in series - forming part of the single path that the current follows.
Because the ammeter sits inside the current path, its own resistance must be as low as possible. If it had high resistance, it would reduce the current and give a reading that does not reflect the true value. A well-designed ammeter behaves almost like a piece of wire.
Voltmeter: always in parallel
A voltmeter measures the potential difference (p.d.) across a component. To do this, it is connected to the two ends of that component, forming a parallel branch.
Because the voltmeter bridges two points without being part of the main current path, its resistance must be very high. If it had low resistance, a large current would flow through it, altering the circuit behaviour and draining current away from the component under test. A well-designed voltmeter draws almost no current.
2 | Translating a Circuit Diagram to a Physical Circuit
One of the trickiest skills in Paper 3 is turning a printed circuit diagram into real wires and components on the bench. Follow these steps in order.
Step 1 - Identify every component
List what the diagram contains: cells or a power supply, switches, lamps, fixed resistors, variable resistors, ammeters, voltmeters, and any other apparatus. Check the number of each - some circuits use two resistors or two meters.
Step 2 - Start from the positive terminal
Begin at the positive terminal of the cell or power supply. Trace the circuit with your finger, following the diagram in one direction. Every component you pass through on this path is in series with the others along the same path.
Step 3 - Build the main series loop first
Connect the components that form the main loop one after another, using connecting leads. Place the ammeter in this loop at whatever point the diagram shows it - current is the same everywhere in a series loop, so its position along the loop does not matter for the reading, but it must match the diagram.
Step 4 - Add parallel branches
Wherever the diagram shows a branch splitting off and rejoining, that section is in parallel. Create the branch by connecting leads from the junction point to the component in the branch, then back to the point where the branches rejoin. The voltmeter almost always appears as a parallel branch across the component under test.
Step 5 - Check polarity
Ensure the + terminal of the ammeter faces the positive side of the supply, and likewise for the voltmeter. Wrong polarity causes analogue meters to deflect backwards, which can damage the pointer mechanism.
Step 6 - Final trace
Before switching on, trace the circuit one more time from the positive terminal of the supply, through every component, and back to the negative terminal. Confirm the loop is complete and that the voltmeter is not accidentally wired in series.
3 | Common Apparatus in O-Level Practicals
Knowing what each component looks like on the bench prevents confusion during the practical.
Cells and battery holders - cylindrical 1.5 V dry cells in plastic holders. Two cells in series give approximately 3 V.
Switch - a simple knife switch or push-button switch. Always start with the switch open (off) when assembling the circuit.
Lamps (filament bulbs) - small bulbs in holders, used as non-ohmic components in I-V characteristic experiments.
Fixed resistors - colour-coded cylindrical components or wire-wound resistors mounted on boards.
Variable resistor (rheostat) - a large coil of resistance wire wound around a ceramic tube, with a sliding contact. It has three terminals.
Ammeter - analogue or digital meter with low-value scales (typically 0--1 A or 0--5 A). Analogue models have two or three scales printed on the face.
Voltmeter - analogue or digital meter with higher-value scales (typically 0--5 V or 0--15 V).
Connecting leads - insulated wires with crocodile clips or 4 mm plug ends.
4 | Using a Rheostat
A rheostat has three terminals: A (one end of the resistance wire), B (the other end), and S (the sliding contact).
As a variable resistor
Connect terminal A and terminal S only. As you move the slider, the length of resistance wire included in the circuit changes, so the resistance changes. This lets you vary the current flowing through the circuit.
The effective resistance ranges from near zero (slider at end A) to the full resistance of the coil (slider at end B, assuming you connected A and S).
As a potential divider
Connect all three terminals: A and B go to the supply, and the output voltage is taken between S and one end (say B). The slider position sets the fraction of the supply voltage that appears across the output terminals. This arrangement gives finer control of the voltage applied to the circuit under test and is the preferred method in many O-Level practicals because it allows the voltage to be varied from 0 V up to the full supply voltage.
5 | Reading Analogue Meters
Choosing the correct scale
Analogue ammeters and voltmeters often have two or three scales printed on the same dial face. Identify which terminal sockets you have plugged into - these determine the full-scale deflection and therefore which printed scale to read.
Reading to the correct precision
For O-Level Paper 3, you are expected to read to half the smallest division on the scale. If the smallest division is 0.1 A, you should record your reading to the nearest 0.05 A. Similarly, if a voltmeter has smallest divisions of 0.2 V, record to the nearest 0.1 V.
Avoiding parallax error
Read the meter with your eye directly in front of the pointer, perpendicular to the scale. Many analogue meters have a mirror strip behind the scale - align the pointer with its reflection so that you cannot see the reflection beside the pointer. This ensures your line of sight is correct.
6 | Common Circuit Mistakes
Ammeter connected in parallel
Because the ammeter has very low resistance, connecting it in parallel across a component creates a short circuit through the ammeter. A very large current flows through it, which can blow the fuse inside the meter or damage it permanently. This is the single most common mistake and can end your practical early.
Voltmeter connected in series
A voltmeter has very high resistance. If it is accidentally placed in series, almost no current flows through the circuit, so the lamp does not light or the readings are negligibly small. The circuit behaves as though it is open.
Wrong polarity
Connecting the positive terminal of the meter to the negative side of the supply causes the pointer to deflect in the wrong direction. On an analogue meter, the pointer hits the stop pin and the reading is meaningless. Swap the leads to correct this.
Wrong range selected
If the ammeter or voltmeter range is too low, the pointer goes off-scale and you cannot take a reading. If the range is too high, all your readings cluster near the bottom of the scale and precision is poor. Start with the highest range available, take a trial reading, then switch to a lower range if the reading falls safely within it.
7 | How This Appears in Paper 3
The most common electricity practical in O-Level Paper 3 asks you to investigate the I-V characteristics of a component - typically a filament lamp or a fixed resistor. The standard setup requires:
A power supply or battery, a switch, the component under test, and an ammeter all in series.
A voltmeter connected in parallel across the component under test.
A rheostat (used as a variable resistor or potential divider) to vary the voltage or current.
You will be asked to:
Record at least six pairs of I and V values.
Plot a graph of I against V (or V against I).
Determine the resistance of the component from the gradient of the graph. For an ohmic conductor, V=IR, so a plot of V against I has gradient R. Alternatively, a plot of I against V has gradient R1.
For a filament lamp, the graph curves because the resistance increases as the filament heats up - this is a standard discussion point in the ACE section. For two experiments that put these wiring skills into practice, see our resistance of a wire experiment and internal resistance and EMF experiment.
8 | MMO Marks for Circuit Work - What Examiners Look For
MMO (Manipulation, Measurement, and Observation) marks are awarded for how you handle apparatus and record data. In circuit practicals, examiners check the following.
Correct connections
Ammeter in series, voltmeter in parallel.
Correct polarity on both meters.
All connections secure - no loose crocodile clips or dangling leads.
The circuit matches the diagram provided.
Appropriate precision
Readings recorded to half the smallest division on analogue meters.
Consistent number of decimal places throughout your results table.
Units stated in the column headers of the table (e.g. I / A, V / V).
Sensible range of readings
At least five or six data points spread across the available range.
Values not clustered at one end of the scale.
Repeats taken where time allows, to show awareness of reliability.
Safe and methodical practice
Switch opened between readings to prevent overheating of components.
Starting from low voltage and increasing gradually.
Rheostat adjusted smoothly, not jumped between extreme positions.
These habits translate directly into marks. For a complete breakdown of the marking scheme across all sections, see the Paper 3 Marking Guide.
Quick Reference Checklist
Before you switch on the circuit in any practical:
Is the ammeter in series with the component under test?
Is the voltmeter in parallel across the component under test?
Is the polarity correct on both meters (+ to + side)?
Is the switch open (off)?
Is the rheostat set to maximum resistance (slider at the far end)?
Are all connections firm and the circuit matches the diagram?
Have you selected an appropriate meter range?
If all seven answers are yes, close the switch, take your first reading, and record it immediately with the correct number of decimal places and units.
