Electric circuits · Investigation
Building and measuring a circuit
A meter in the wrong place does not give you a wrong reading. It gives you a different circuit, and then a perfectly correct reading of that.
Start here
Same diagram. Same box of parts. Two answers.
Two pairs work from the identical circuit diagram with identical apparatus. One pair gets a lit lamp and 0.30 A. The other gets a dark lamp and an ammeter reading zero, and swears they have followed the drawing.
Before you look at anything clever, what is most likely wrong?
Almost always a connection. A loop only works if every joint in it works, and a crocodile clip biting insulation instead of copper looks perfectly convincing. The sequence, incidentally, does not matter at all in a single loop — one current, the same everywhere — so putting the ammeter before or after the lamp changes nothing. What does matter is whether each meter is in the loop or across a component, which is the rest of this lesson.
The two meters are opposites, and it is their own resistance that decides where they go. An ammeter is built to have almost no resistance, so that putting it in the loop barely changes the current it is there to measure. A voltmeter is built to have an enormous resistance, so that hanging it across a component draws almost nothing and barely changes the p.d. it is there to measure.
Swap them over and both of those virtues become disasters. An ammeter across a lamp offers the charge a route with no resistance at all, so the lamp is bypassed and goes out while a dangerous current pours through the meter. A voltmeter in the loop puts a million ohms in series with everything, so almost nothing flows and the lamp is dark.
Which is why building a circuit has an order to it. Follow the diagram, put the loop together with the switch open, check every connection by eye and by hand, close the switch, then read. Take each reading twice, and if one value refuses to sit with the others, go back and take it again rather than writing it down.
At the bench · one lamp, two cells, two meters to place
Wire it wrong on purpose.
Change a control to begin
A 3.0 V battery and a 10 Ω lamp. Choose where each meter goes, and choose whether one connection is properly tightened.
Commit first. Someone connects the ammeter across the lamp instead of in the loop. What happens?
The ammeter
The voltmeter
The connection at the far corner
The ammeter reads
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The voltmeter reads
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The lamp is
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Verdict on this build
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The figure
Six things that go wrong, and what each looks like
Work from the symptom, not from a hunch. Almost every failure in this practical is one of these, and the first two account for most of them.
| What you see | Most likely cause | What to check first |
|---|---|---|
| Nothing at all — both meters on zero, lamp dark | A break somewhere in the loop | Every crocodile clip and terminal, the switch, and that the cells are the right way round in the holder |
| Lamp dark, but the voltmeter reads the whole battery p.d. | The voltmeter is in the loop instead of across the lamp — or the filament is broken | That the voltmeter has one lead on each side of the lamp and is not carrying the loop |
| Lamp goes out and the ammeter slams off the scale | The ammeter is across the lamp, shorting it out | Open the switch at once, then check the ammeter is in the line and not bridging anything |
| Lamp dim, both readings lower than expected | A tired cell, one cell too few, or an extra resistance in the loop | Put the voltmeter across the battery and compare with what it should supply |
| Readings drift downwards while you watch | The cells are running down, or the filament is still heating | Read both meters at the same moment, quickly, and open the switch between settings |
| One value refuses to sit with the rest | A misread scale, or a clip that moved between readings | Repeat that one setting before writing it down — never smooth it over on the graph |
Notice that three of the six are not faults in the apparatus at all. They are faults in the method — reading at the wrong moment, reading the wrong scale, or accepting a value you have not repeated.
Key fact
An ammeter has almost no resistance, so it goes in the loop. A voltmeter has an enormous resistance, so it goes across a component. Swap them and you have not taken a bad reading — you have built a different circuit. Build with the switch open, check the loop, then read, and repeat every reading.
Think again
“The meter reads zero, so the meter must be broken.”
A zero is a reading, and usually a true one. It says there is no current where the meter is sitting, which is exactly what a break in the loop produces — and a break is far more common than a faulty instrument. Swapping the meter for another one is the last thing to try, not the first: it costs a minute, tells you nothing if the fault is a loose clip, and hides the fault from the next pair who use the same box.
“It cannot matter which way round the leads go on a meter.”
On a digital meter it barely does — you get the right number with a minus sign in front. On an analogue meter with a needle it matters a great deal, because the needle is driven backwards off the end of its travel and can be bent. The terminals are marked, red to the side nearer the positive end of the battery, and getting into that habit costs nothing and saves a meter.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the symptom
A group reports: the lamp is dark, the ammeter reads 0.00 A, and the voltmeter reads 3.00 V. Every clip is tight. What have they most likely done?
Rung 2 · The one that catches people
A student wires the voltmeter into the loop, sees it read 3.00 V on a 3.0 V battery, and concludes the circuit is working properly. What is right?
Rung 3 · Explain
Explain why an ammeter is connected in the loop and a voltmeter across a component, referring to the resistance of each meter and to what would happen if they were swapped.
Rung 4 · Take it somewhere new
Plan an investigation into how the current through a lamp depends on the number of cells driving it. Say what you would change, what you would measure, what you would keep the same, and how you would make the result trustworthy.
Key note
An ammeter is made with almost no resistance so that it can sit in the loop without changing the current; a voltmeter is made with an enormous resistance so that it can sit across a component without changing the p.d. Putting either in the other's place builds a different circuit: an ammeter across a lamp shorts it out, and a voltmeter in the loop stops almost all the current. Build from the diagram with the switch open, check the loop, close the switch, take both readings at the same moment, and repeat any value that refuses to sit with the rest.
Going further
Neither meter is quite innocent. An ammeter has a small resistance, so putting it in the loop lowers the current a little; a voltmeter has a large but finite resistance, so hanging it across a component draws a little current away. Both effects are tiny in a school circuit and neither is zero, which is why a careful experimenter reports what the meters were as well as what they read. The art of instrument design is making that disturbance small enough to ignore — and the art of measurement is knowing when it no longer is.
A multimeter is the same instrument with a switch. Turn the dial to amps and it puts a tiny resistance in the circuit; turn it to volts and it puts an enormous one; turn it to ohms and it supplies its own small current and does the division for you. That last setting is exactly the calculation of the resistance lesson, wired into a box — which is why an ohms setting only ever works on a component that has been disconnected from everything else.
Before this lesson
Connects to
At GCSE this becomes
- The required practical on current–p.d. characteristics, systematic and random error, and how a meter's own resistance limits the measurement.
Where to next
- Next: Charging by rubbing
Static electricity
- Previous: Conductors and insulators
Ask Mr Badmus AI
Got a circuit that will not work and a symptom to describe?
Eye protection on. Use only the low-voltage supply your teacher gives you — never mains. The ammeter goes in the circuit, never straight across the cell; that makes a large current and the wires heat up fast. If anything smells hot or feels hot, open the switch straight away and tell your teacher.
The bench is a teaching model. The lamp is treated as a fixed 10 ohms, the battery and leads as having no resistance, the ammeter as having none and the voltmeter as having one megohm; real components differ, a real filament's resistance rises as it heats, and a real ammeter and battery both have a small resistance of their own. The short-circuit reading is shown as off the scale rather than as a figure, because what a real supply and a real meter do in that state depends on the equipment and none of it is a measurement. Readings are rounded to two decimal places.
Lesson content © MrBadmusAI.