Quick answer: An ammeter measures current and is connected in series, so the whole current flows through it. It has a very low resistance so that it does not change the current it measures. A voltmeter measures potential difference and is connected in parallel across a component. It has a very high resistance so that it draws almost no current from the circuit.
An ammeter and a voltmeter both measure electrical quantities, but they are connected in opposite ways and are designed with opposite resistances. Connecting either one the wrong way gives wrong readings, and an ammeter connected the wrong way can be damaged.
In CBSE Class 10 Science, Chapter 11 (Electricity), how ammeters and voltmeters are connected and why are frequently asked board exam questions.
What You Will Learn
- What an ammeter and a voltmeter measure
- Why an ammeter is connected in series and has low resistance
- Why a voltmeter is connected in parallel and has high resistance
- What happens if either one is connected the wrong way
- How a galvanometer can be turned into an ammeter or voltmeter (extra, beyond Class 10)
1. Ammeter vs Voltmeter
| Feature | Ammeter | Voltmeter |
|---|---|---|
| Measures | Electric current, in amperes (A) | Potential difference, in volts (V) |
| Connection | In series with the component | In parallel across the component |
| Resistance | Very low (ideally zero) | Very high (ideally infinite) |
| If connected the wrong way | In parallel: a very large current flows through it, which can damage it | In series: almost no current flows, so the circuit stops working properly |
2. The Ammeter
Why Is It Connected in Series?
To measure the current through a component, all of that current must also flow through the ammeter. So the ammeter is placed in the same path as the component, i.e. in series.
Why Does It Have Low Resistance?
Step 1: Anything connected in series adds to the total resistance of the circuit.
Step 2: By Ohm's law, , a larger total resistance would reduce the current.
Step 3: An ammeter with a noticeable resistance would therefore change the very current it is measuring.
Key point: An ammeter has a very low resistance, so that connecting it hardly changes the current in the circuit.
What If an Ammeter Is Connected in Parallel?
Because the ammeter's resistance is very low, connecting it across a component gives the current an easy path around the component. A very large current flows through the ammeter. This gives a wrong reading and can damage the ammeter or blow a fuse.
3. The Voltmeter
Why Is It Connected in Parallel?
Potential difference is measured between two points. So the voltmeter's two terminals are connected to the two ends of the component, i.e. in parallel across it.
Why Does It Have High Resistance?
Step 1: A voltmeter in parallel forms a second path for current.
Step 2: If the voltmeter let a lot of current through, it would take current away from the component and change the potential difference across it.
Key point: A voltmeter has a very high resistance, so that it draws almost no current and does not disturb the circuit.
What If a Voltmeter Is Connected in Series?
The voltmeter's very high resistance makes the total resistance very large, so the current becomes almost zero. A bulb in the circuit would not glow. The voltmeter itself is not usually damaged; it reads nearly the full voltage of the battery.
4. Turning a Galvanometer into an Ammeter or Voltmeter (Extra)
Beyond the Class 10 textbook: This is covered in Class 12 Physics.
A galvanometer is a very sensitive instrument that detects small currents.
| To make | Connect | Why |
|---|---|---|
| An ammeter | A low resistance (called a shunt) in parallel with the galvanometer | Most of the current passes through the shunt, so only a small, safe current flows through the galvanometer |
| A voltmeter | A high resistance in series with the galvanometer | The high resistance limits the current, so it can measure large voltages |
5. Summary Table
| Instrument | Measures | Connected | Resistance |
|---|---|---|---|
| Ammeter | Current | In series | Very low |
| Voltmeter | Potential difference | In parallel | Very high |
6. Exam Tips and Common Mistakes
Exam Tip: When explaining the resistances, focus on not disturbing the circuit: the ammeter must not reduce the current; the voltmeter must not draw current.
Exam Tip: In circuit diagrams, draw the ammeter in the main line and the voltmeter across the component.
Common Mistake: Mixing up the connections. Ammeter is in series; voltmeter is in parallel.
Common Mistake: Saying a voltmeter should have low resistance. It should have a high resistance.
Frequently Asked Questions
Why is an ammeter connected in series?
So that the whole current flowing through the component also flows through the ammeter.
Why is a voltmeter connected in parallel?
Because potential difference is measured between two points, so the voltmeter is connected across the two ends of the component.
Why does an ammeter have low resistance?
So that adding it to the circuit does not noticeably reduce the current it is measuring.
Why does a voltmeter have high resistance?
So that it draws almost no current and does not change the potential difference it is measuring.
What happens if an ammeter is connected in parallel?
A very large current flows through it because of its low resistance, giving a wrong reading and possibly damaging it.