Galvanometer vs Ammeter — Difference (with Table & FAQs)
Galvanometer vs Ammeter compared side by side — a 6-row comparison table, 4 key points and 7 exam FAQs. Class 10 Physics, free and no sign-up.
TL;DR: Galvanometer vs Ammeter compared side by side — a 6-row comparison table, 4 key points and 7 exam FAQs. Class 10 Physics, free and no sign-up.
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
The main difference between Galvanometer and Ammeter: Galvanometer is a sensitive device detecting small currents. Ammeter measures large currents in electric circuits.
Galvanometer vs Ammeter — Comparison Table
| Basis | Galvanometer | Ammeter |
|---|---|---|
| Function | Detects small currents | Measures large currents |
| Sensitivity | Very sensitive | Less sensitive |
| Current Range | Micro to milliamps | Milliamps to amperes |
| Internal Resistance | Very high | Very low |
| Construction | Simple coil in magnetic field | Galvanometer with shunt |
| Connection | Can be connected carefully | Must be in series |
Key Points
- Ammeter is essentially galvanometer with low-resistance shunt
- Galvanometer would burn out if too much current flows
- Ammeter has negligible internal resistance
- Both work on same principle of current-deflection
Galvanometer — a detector of tiny currents
A galvanometer is an instrument that detects and measures very small electric currents. Its most distinctive feature is the scale: the zero sits at the centre, and the pointer can swing either side of it. That is what lets a galvanometer show the direction of the current as well as its presence.
It works on the magnetic effect of current: a coil carrying current in a magnetic field experiences a torque, and the pointer deflects by an amount proportional to that current. Because it is designed for sensitivity, a galvanometer has a fairly high resistance and can be damaged by any substantial current.
Ammeter — a measurer of ordinary currents
An ammeter measures electric current in amperes, over the range found in ordinary circuits. Its zero sits at the left-hand end of the scale, not the centre, because it is normally used where the current direction is already known.
The defining electrical property of an ammeter is its very low resistance. It is always connected in series with the component whose current is being measured, and if it had appreciable resistance it would reduce the very current it is trying to measure. An ideal ammeter has zero resistance.
Turning a galvanometer into an ammeter
An ammeter is, in fact, a galvanometer that has been modified. Connect a very small resistance — called a shunt — in PARALLEL with the galvanometer coil, and most of the current bypasses the sensitive coil and flows through the shunt instead.
This does two things at once: it protects the delicate galvanometer from large currents, and it brings the combination's overall resistance down to the near-zero value an ammeter needs. The shunt value is given by S = I_g G ÷ (I − I_g), where G is the galvanometer resistance, I_g the current for full-scale deflection and I the maximum current to be measured.
And into a voltmeter — note the reversal
A galvanometer becomes a voltmeter by connecting a HIGH resistance in SERIES with it. This is the exact opposite of the ammeter conversion, and swapping the two is the most common error in the topic.
The logic follows from how each instrument is used. An ammeter goes in series with the circuit, so it must have almost no resistance or it would change the current. A voltmeter goes in parallel across a component, so it must have very high resistance or it would draw current away and change the very voltage it is measuring. An ideal voltmeter has infinite resistance.
The comparison in one table
These are the points examiners ask for by name.
- Purpose — a galvanometer detects and measures very small currents and shows their direction; an ammeter measures current in amperes.
- Zero position — galvanometer at the centre of the scale; ammeter at the left end.
- Resistance — a galvanometer has relatively high resistance; an ammeter has very low resistance.
- Direction — a galvanometer indicates the direction of current by which way the pointer deflects; an ordinary ammeter does not.
- Connection — both are connected in series in the circuit, but only the ammeter is safe for ordinary currents.
- Relationship — an ammeter is a galvanometer with a low-resistance shunt connected in parallel.
Galvanometer vs Ammeter — Worked Examples
- Checking whether a circuit carries any current at all — Use a galvanometer. It responds to currents far too small to move an ammeter needle.
- Measuring the 2 A drawn by a bulb — Use an ammeter, connected in series with the bulb.
- A pointer deflecting to the left instead of the right — A galvanometer showing that the current is flowing in the opposite direction.
- Adding a shunt of a fraction of an ohm across a galvanometer — Converting it into an ammeter — the shunt carries most of the current and drops the overall resistance.
Common Mistakes Students Make
- Saying a shunt is connected in series. The shunt is in PARALLEL with the galvanometer; a series resistance makes a voltmeter instead.
- Connecting an ammeter in parallel with a component. It must always be in series, and connecting it in parallel can short the component and damage the meter.
- Claiming an ideal ammeter has infinite resistance. It has zero resistance; the voltmeter is the one with infinite resistance.
- Forgetting that a galvanometer's zero is at the centre of the scale.
- Assuming a galvanometer can measure large currents directly. It cannot, and will be damaged by them.
Diagrams
Where This Shows Up in Real Life
Checking a circuit for any current at all
A galvanometer detects currents far too small to move an ammeter needle, which is why it is the instrument of choice for null-point experiments such as the Wheatstone bridge and the metre bridge. There the aim is not to measure a value but to find the setting at which the deflection becomes exactly zero, and only a very sensitive centre-zero instrument can show that reliably.
Why a wrongly connected ammeter destroys itself
An ammeter has almost no resistance and must be connected in series. Connect it in parallel across a component and it short-circuits that component: an enormous current flows through the meter, and it is damaged or destroyed. A voltmeter, with very high resistance, fails the opposite way — placed in series it blocks the circuit so almost no current flows. Series for the ammeter, parallel for the voltmeter, and the reason is the resistance in each case.
Reading the direction of a current
Because a galvanometer’s zero sits at the centre of its scale, the pointer can swing either way, showing not only that current is flowing but which way. This matters when demonstrating electromagnetic induction: moving a magnet into a coil deflects the pointer one way and withdrawing it deflects the pointer the other, which is how Lenz’s law is shown experimentally.
One movement, three instruments
The same moving-coil galvanometer becomes an ammeter with a low resistance shunt in parallel, or a voltmeter with a high resistance in series, or an ohmmeter with an added cell. Understanding this makes the whole topic one idea rather than three, and it explains why a multimeter can switch between functions using a single movement.
Solved Numericals — Step by Step
Problem 1. A galvanometer of resistance 50 Ω gives full-scale deflection for 2 mA. What shunt is needed to convert it into an ammeter reading up to 1 A?
- G = 50 Ω, Ig = 2 mA = 0.002 A, I = 1 A.
- S = Ig G ÷ (I − Ig)
- = (0.002 × 50) ÷ (1 − 0.002)
- = 0.1 ÷ 0.998.
Answer: S ≈ 0.1002 Ω, about 0.1 Ω. The shunt is very small, which is what drops the combination’s overall resistance to near zero as an ammeter requires.
Problem 2. The same galvanometer (G = 50 Ω, Ig = 2 mA) is to be converted into a voltmeter reading up to 10 V. Find the series resistance.
- For a voltmeter, R = (V ÷ Ig) − G.
- = (10 ÷ 0.002) − 50
- = 5000 − 50.
Answer: 4950 Ω in SERIES. Contrast this with the previous answer: a huge series resistance for a voltmeter, a tiny parallel one for an ammeter.
Board-Style Questions with Model Answers
Distinguish between a galvanometer and an ammeter. (3 marks)
A galvanometer is an instrument used to detect and measure very small electric currents. Its zero is placed at the centre of the scale so the pointer can deflect either side, indicating the direction of the current as well as its presence, and it has a comparatively high resistance, which means a large current would damage it. An ammeter measures current in amperes over the range found in ordinary circuits. Its zero is at the left-hand end of the scale, it has a very low resistance, and an ideal ammeter would have zero resistance. Both are connected in series in a circuit. The relationship between them is that an ammeter is simply a galvanometer fitted with a low resistance, called a shunt, connected in parallel with its coil.
Explain how a galvanometer is converted into (a) an ammeter and (b) a voltmeter. (5 marks)
To convert a galvanometer into an ammeter, a very small resistance called a shunt is connected in PARALLEL with the galvanometer coil. Most of the current then bypasses the sensitive coil and flows through the shunt, which protects the galvanometer and simultaneously reduces the overall resistance of the combination to nearly zero, as an ammeter requires since it is connected in series. The shunt value is given by S = Ig G ÷ (I − Ig), where G is the galvanometer resistance, Ig the current for full-scale deflection and I the maximum current to be measured. To convert a galvanometer into a voltmeter, a high resistance is connected in SERIES with it. This limits the current drawn to a safe value and raises the overall resistance to a large value, as a voltmeter requires since it is connected in parallel across a component and must not draw appreciable current. The series resistance is R = (V ÷ Ig) − G. Noting that the two conversions are opposite in both the size of the resistance and its mode of connection is the key to the topic.
Why is an ammeter connected in series and a voltmeter in parallel? (3 marks)
An ammeter measures the current flowing through a component, and current can only be measured if it actually passes through the meter, which requires a series connection. For this not to disturb the circuit, the ammeter must have a very low resistance, since any appreciable resistance added in series would reduce the very current it is measuring; an ideal ammeter has zero resistance. A voltmeter measures the potential difference across a component, so it must be connected across it, that is in parallel. For this not to disturb the circuit, the voltmeter must have a very high resistance, since a low-resistance path in parallel would draw current away and reduce the potential difference it is measuring; an ideal voltmeter has infinite resistance.
What is a shunt? State two functions it performs. (2 marks)
A shunt is a resistance of very small value connected in parallel with a galvanometer in order to convert it into an ammeter. It performs two functions. First, it protects the galvanometer: because it offers a much easier path, the greater part of the current passes through the shunt and only a small safe fraction flows through the delicate coil. Second, it lowers the effective resistance of the combination to a very small value, which is essential because the instrument is to be connected in series and must not alter the current in the circuit. Its value is calculated from S = Ig G ÷ (I − Ig).
Assertion–Reason Questions (CBSE Format)
For each question choose: (a) both A and R are true and R is the correct explanation of A; (b) both are true but R is not the correct explanation; (c) A is true but R is false; (d) A is false but R is true.
1. Assertion (A): An ammeter must have a very low resistance.
Reason (R): It is connected in series and should not alter the current being measured.
Answer: (a) — Both are true and the reason explains the assertion. Any appreciable series resistance would reduce the current the meter is trying to report.
2. Assertion (A): A shunt is connected in series with a galvanometer to convert it into an ammeter.
Reason (R): The shunt protects the galvanometer from large currents.
Answer: (d) — The assertion is false — the shunt goes in PARALLEL. The reason is true (it does protect the coil), which is exactly why this pairing traps students who remember the purpose but not the connection.
3. Assertion (A): A galvanometer has its zero at the centre of the scale.
Reason (R): It is designed to indicate the direction of the current as well as its magnitude.
Answer: (a) — Both are true and correctly linked. A centre zero lets the pointer deflect either way.
4. Assertion (A): An ideal voltmeter has infinite resistance.
Reason (R): It is connected in parallel and must draw no current from the circuit.
Answer: (a) — Both are true and the reason explains the assertion. Drawing current would change the potential difference being measured.
Quick Self-Check
Q1. A galvanometer is converted into an ammeter by connecting:
- A. A high resistance in series
- B. A low resistance in parallel
- C. A high resistance in parallel
- D. A low resistance in series
Answer: B. A low resistance in parallel — A low resistance shunt in parallel carries most of the current, protecting the coil and lowering the overall resistance as an ammeter requires.
Q2. An ammeter is always connected:
- A. In parallel with the component
- B. In series with the component
- C. Across the battery
- D. In any convenient way
Answer: B. In series with the component — Current must pass through the ammeter for it to be measured, so it goes in series.
Q3. The zero of a galvanometer scale is usually located:
- A. At the left end
- B. At the right end
- C. At the centre
- D. It has no zero
Answer: C. At the centre — A centre zero lets the pointer deflect either way, showing the direction of the current as well as its magnitude.
Q4. An ideal voltmeter has:
- A. Zero resistance
- B. Infinite resistance
- C. The same resistance as the circuit
- D. Negative resistance
Answer: B. Infinite resistance — A voltmeter is connected in parallel, so infinite resistance ensures it draws no current and does not alter the voltage it measures.
Key Terms Glossary
- Galvanometer
- An instrument detecting and measuring very small currents, with a CENTRE zero showing direction.
- Ammeter
- An instrument measuring current in amperes. Very LOW resistance, connected in SERIES.
- Voltmeter
- An instrument measuring potential difference. Very HIGH resistance, connected in PARALLEL.
- Shunt
- A very small resistance connected in PARALLEL with a galvanometer to make an ammeter. S = Ig G/(I − Ig).
- Series connection
- Components joined end to end so the same current passes through each. Used for an ammeter.
- Parallel connection
- Components joined across the same two points so each has the same potential difference. Used for a voltmeter.
- Ideal ammeter
- One with ZERO resistance, so it does not change the circuit current.
- Ideal voltmeter
- One with INFINITE resistance, so it draws no current.
- Full-scale deflection (Ig)
- The current that moves the galvanometer pointer to the end of its scale.
- Moving-coil galvanometer
- A galvanometer working on the torque experienced by a current-carrying coil in a magnetic field.
- Wheatstone bridge
- A circuit using a galvanometer at NULL deflection to compare resistances.
- Ohmmeter
- A galvanometer with an added cell and resistance, used to measure resistance directly.
One-Minute Revision
- GALVANOMETER: detects TINY currents, CENTRE zero (shows direction), HIGH resistance.
- AMMETER: measures amperes, LEFT zero, VERY LOW resistance, always in SERIES.
- Galvanometer → AMMETER: LOW resistance shunt in PARALLEL. S = Ig G/(I − Ig).
- Galvanometer → VOLTMETER: HIGH resistance in SERIES. R = V/Ig − G.
- The two conversions are OPPOSITE in both size and connection. This is the exam trap.
- Ideal ammeter = ZERO resistance. Ideal voltmeter = INFINITE resistance.
- Ammeter in SERIES (current must pass through it). Voltmeter in PARALLEL (across the component).
- An ammeter wrongly placed in parallel short-circuits the component and is destroyed.
Syllabus reference: NCERT Class 10 Science, Chapter 11 — Electricity; conversion using a shunt in Class 12 Physics, Chapter 4 — Moving Charges and Magnetism.
Frequently Asked Questions
What is the difference between a galvanometer and an ammeter?
A galvanometer detects and measures very small currents and can show the direction of current. An ammeter measures larger currents in amperes and is made by connecting a low-resistance shunt across a galvanometer.
How is an ammeter connected in a circuit?
An ammeter is always connected in series with the circuit, and it has very low resistance so it does not change the current being measured.
Can a galvanometer be converted into an ammeter?
Yes — by connecting a small resistance (called a shunt) in parallel with the galvanometer.
What is the main difference between a galvanometer and an ammeter?
A galvanometer detects and measures very small currents and shows their direction, with the zero at the centre of its scale and a relatively high resistance. An ammeter measures ordinary currents in amperes, has its zero at the left end and a very low resistance. An ammeter is essentially a galvanometer fitted with a low-resistance shunt in parallel.
Why must an ammeter have very low resistance?
Because it is connected in series with the circuit. Any appreciable resistance would add to the circuit's total resistance and reduce the current, meaning the meter would alter the very quantity it is measuring. An ideal ammeter therefore has zero resistance.
What is a shunt?
A shunt is a very small resistance connected in parallel with a galvanometer to convert it into an ammeter. It provides an alternative path for most of the current, protecting the sensitive coil and bringing the overall resistance down. Its value is S = I_g G ÷ (I − I_g).
Can a galvanometer be used as a voltmeter?
Yes. Connecting a high resistance in series with a galvanometer converts it into a voltmeter. Note the contrast with the ammeter conversion, which uses a low resistance in parallel — the series-versus-parallel distinction is the point examiners test.