An ammeter goes in series in the circuit, because all the charge you measure must pass through it; that is why it has almost no resistance. A voltmeter goes in parallel across the component, because voltage is a difference between two points; it has a huge resistance instead. Swap them, and you wreck the ammeter or the lamp stays off.
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Two meters, one rule: the ammeter goes into the circuit, the voltmeter beside it. Swap them, and you don't just measure the wrong thing. You wreck your meter.
Before you measure, you draw what you build, and you do that as a circuit diagram. Every wire is a straight line, every corner a right angle, and every component a fixed symbol. Two setups that look completely different on the table can have the same diagram, and then they also behave the same.
Current is charge passing a point. To measure it, all the charge has to go through your meter: you cut the circuit open and put the meter in between, in series. That is why an ammeter has almost no resistance, around a hundredth of an ohm, so that it changes the circuit it measures as little as possible.
Voltage is something else: a difference between two points, how much energy a coulomb loses on its way from here to there. You measure a difference by connecting to both sides, so you put the voltmeter in parallel across the component. And it is built exactly the other way round, with a gigantic resistance of millions of ohms, so that practically no current goes through it.
Then the question from the start. Connect an ammeter across the terminals of a battery, and that is a wire across the source: a short circuit, and the whole current goes through your meter. Put a voltmeter in the circuit, and suddenly there are millions of ohms in your circuit and your lamp stays off. One mistake goes bang, the other does nothing.
For upper secondary physics, on electricity.
In this video
The full explanation, in writing
A rule that seems to make no sense
0:00Two meters, one rule: the ammeter goes into the circuit, the voltmeter across it. Swap them, and you don't just measure the wrong thing. You wreck your meter.
Drawing a circuit
0:14Before you measure, you draw what you build, and you don't do that as a picture but as a circuit diagram. In a circuit diagram every wire is a straight line, every corner a right angle, and every component a fixed symbol. A source, a lamp, a resistor, a switch: those symbols are fixed and the same all over the world.
0:36That's not to make it look neat. Two circuits that look completely different on the table can have the same circuit diagram, and then you know straight away they behave the same. A photo shows you how the wires lie; a circuit diagram shows you what happens.
The ammeter goes in the circuit
0:55Current is charge passing a point. To measure it, all the charge you want to count really has to go through your meter. So you cut the circuit open and put the meter in between. In series. And that's why an ammeter is built to have almost no resistance: it must change as little as possible in the circuit it measures.
1:20A good ammeter is around a hundredth of an ohm.
The voltmeter goes beside it
1:24Voltage is something else. Voltage is a difference between two points: how much energy a coulomb loses on its way from here to there. You measure a difference by hooking on at both sides, not by sitting in between. So you put the voltmeter across the component whose voltage you want to know. In parallel.
1:44And it is built exactly the other way round: a voltmeter has a gigantic resistance, millions of ohms. Because practically no current may go through it, or it would change the circuit it measures. Now the question from the start. You connect an ammeter of a hundredth of an ohm, straight across the terminals of a battery. What happens?
What goes wrong when you swap them
2:12Connecting an ammeter across the source is connecting a wire across the source: almost no resistance between plus and minus. That's a short circuit. The current gets as large as the battery can deliver, and all of it goes through your meter.
2:25It doesn't survive that. The other way round is less dramatic but just as wrong: put a voltmeter in the circuit, and suddenly there are millions of ohms in your circuit. Almost no current flows any more and your lamp stays off. So the two mistakes look very different: one goes bang, the other does nothing.
Three things to remember
2:48Three things. In a circuit diagram every wire is straight and every component a fixed symbol, so the circuit diagram shows you what the circuit does. The ammeter goes in the circuit, because all the charge must go through it, and it has almost no resistance.
3:03The voltmeter goes across it, because voltage is a difference between two points, and it has a huge resistance instead.
Two questions
3:13Two questions. One. You want to know the voltage across a lamp. Where does your meter go, and which of the two is it? Two. Someone measures with an ammeter and reads zero, while the lamp is simply on. What is probably going on?
The answers
3:32The first: the voltmeter, and you connect it in parallel across the lamp, so with one lead on each side of it. The second: that meter isn't in the circuit but off to the side, in a branch where no current flows. If the lamp is on, current is flowing, so if your meter says zero, it is measuring in the wrong place.
That's how you measure what happens in a circuit
3:53That's how you measure what happens in a circuit. On the channel there's a separate video about components that let current through in only one direction, and about the difference between direct and alternating voltage.
Frequently asked questions
How do you connect an ammeter and a voltmeter?
You connect an ammeter in series: you cut the circuit open and put the meter in between, so all the charge goes through it. You connect a voltmeter in parallel, with one connection on each side of the component whose voltage you want to know, because voltage is a difference between two points.
What happens if you connect an ammeter across the terminals of a battery?
Then you make a short circuit. An ammeter has almost no resistance, around a hundredth of an ohm, so it acts like a wire between plus and minus. The current gets as large as the battery can deliver and all of it goes through the meter, and the meter does not survive that.
Why does a voltmeter have such a large resistance?
Because practically no current may go through it, otherwise it changes the circuit it measures. That is why a voltmeter has a resistance of millions of ohms. If you accidentally put it in the circuit, almost no current flows any more and your lamp stays off.
Why does an ammeter read zero while the lamp is on?
Then the meter is not in the circuit but somewhere beside it, in a branch where no current flows. If the lamp is on, current is flowing, so if the meter says zero, it is measuring in the wrong place.
Before this came voltage and capacity: why two batteries with the same number still last a different amount of time. Next: direct voltage, alternating voltage, and why an LED only works one way.
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Physics with Thomas: physics explained with pictures where they add something, and without them where they distract.
In this series
Electric currentthe electrons crawl, your lamp is on at once6:12
Voltage and capacitytwo batteries, the same number4:43
Connecting ammeter and voltmeterswapping them wrecks your meter4:10
About me
My name is Thomas Schuurmans, and I have a passion for physics and for understanding why things work the way they do. What I want is to get young people asking “why is that?” and “how does that work?” more often, and to hand them the tools of physics in a way that is simple and that you can see.
I graduated in applied physics at Delft University of Technology in 2003. After that I spent more than twenty years outside education: first at TNO, the Dutch applied research institute, then at a design agency, and eventually founding Proportion Global, through which I work on innovation questions in Africa, Latin America and South Asia. That work resembles physics more than you would expect: don't start from a solution, first understand what is going on, try something, be wrong, and look again.
Since 2026 I have been teaching physics to both lower and upper secondary classes, and I started a master's at the University of Amsterdam for my full teaching qualification. That is where I learned that a secondary school pupil's real attention for new material lasts about seven minutes. And my own weakness happens to be telling too many side stories.
That is where the idea came from: videos that explain one topic sharply and visually, inside those seven minutes. I make them for my own pupils. Then I publish them, because good explanation should be within reach of anyone who needs it, wherever you live and whatever language you think and speak in.