In 1820, Hans Christian Oersted discovered that an electric current deflects a compass needle. In the decades that followed, this discovery was refined into one of the most sensitive and precise electrical instruments ever built: the galvanometer. Named after Luigi Galvani, the galvanometer is a device that detects and measures tiny electric currents by exploiting the torque exerted on a current-carrying coil in a magnetic field.
The galvanometer is not just a historical curiosity — it is the foundational instrument from which modern ammeters, voltmeters, and multimeters are derived. Converting a galvanometer into an ammeter (by adding a low-resistance shunt in parallel) or into a voltmeter (by adding a high-resistance series resistor) is one of the most important and most frequently asked topics in CBSE Class 12 Physics, Chapter 4: Moving Charges and Magnetism.
This guide covers the galvanometer completely: its definition, the construction of the moving coil galvanometer, the working principle and torque equation, the deflection formula (I = kθ/NBA), current sensitivity and voltage sensitivity, the figure of merit, conversion to ammeter and voltmeter with worked numerical examples, types of galvanometers, uses, and five CBSE exam-ready FAQs. All instruments described are manufactured and supplied by AJKANT Overseas from Ambala, India.
- 1. What is a Galvanometer?
- 2. Moving Coil Galvanometer — Construction
- 3. Working Principle & Torque Equation
- 4. Sensitivity and Figure of Merit
- 5. Converting Galvanometer to Ammeter
- 6. Converting Galvanometer to Voltmeter
- 7. Types of Galvanometers
- 8. Uses of Galvanometer
- 9. Galvanometer vs Ammeter vs Voltmeter
- 10. Frequently Asked Questions (FAQ)
1. What is a Galvanometer?
A galvanometer is a sensitive electromagnetic instrument used to detect and measure small electric currents (typically in the microampere to milliampere range). It works on the principle that a current-carrying conductor placed in a magnetic field experiences a mechanical force (torque), which is used to deflect a pointer over a calibrated scale.
Key characteristics of a galvanometer:
- It detects current direction: The pointer deflects to the left for current in one direction and to the right for the opposite direction. The scale is symmetric with zero at the centre.
- It measures very small currents: A standard school galvanometer has a full-scale deflection current (Ig) of about 1–10 milliamperes. High-sensitivity galvanometers can detect currents as small as 10²&sup7; A.
- It has a finite internal resistance (G): The resistance of the galvanometer coil is typically 10–100 ohms. This internal resistance is crucial in the conversion calculations.
- It cannot directly measure large currents or voltages: Without modification, a galvanometer would be damaged by large currents or high voltages. Conversion to ammeter or voltmeter (by adding appropriate resistors) extends its range.
2. Moving Coil Galvanometer — Construction
The most widely used type is the Permanent Magnet Moving Coil (PMMC) Galvanometer. Its construction consists of these key components:
3. Working Principle and Torque Equation
The working principle of the moving coil galvanometer is based on the interaction between a current-carrying conductor and a magnetic field, described by the motor effect (F = BIL for a straight wire).
When current I flows through the N-turn rectangular coil (dimensions L × B) placed in a magnetic field of strength B:
- The magnetic force on the two long sides of the coil (length L) creates a torque that tends to rotate the coil.
- The magnitude of this deflecting torque is: τᵇₜᵑ = NBIA, where A = LB is the area of the coil.
- The two phosphor bronze spiral springs provide a restoring torque proportional to the deflection angle θ: τᵣₜᵀ = kθ, where k is the spring constant (torsional restoring torque per unit angle).
- At equilibrium (when the pointer is steady): Deflecting torque = Restoring torque
The equation I ∝ θ confirms that the deflection of the pointer is directly proportional to the current through the coil. This is why the galvanometer scale is linear and uniform — each equal division represents the same increment of current.
4. Sensitivity and Figure of Merit
5. Converting a Galvanometer to an Ammeter
A galvanometer measures only very small currents (milliamperes or microamperes). To measure large currents, a low-resistance shunt (S) is connected in parallel with the galvanometer. Most of the large current flows through the shunt (bypassing the galvanometer), while only a small fraction Ig flows through the galvanometer coil.
Connect shunt resistance S in parallel with the galvanometer. Since both are in parallel, voltage across both is equal:
S = Shunt resistance required (Ω)
Iᵌ = Full-scale deflection current of galvanometer (A)
G = Galvanometer coil resistance (Ω)
I = Desired full-scale range of ammeter (A)
Worked Example: Galvanometer: Iᵌ = 5 mA = 0.005 A, G = 20 Ω. Convert to ammeter of range 5 A.
S = (0.005 × 20) / (5 − 0.005) = 0.1 / 4.995 = 0.02002 Ω ≈ 0.02 Ω
Properties of the converted ammeter:
• Very low total resistance (G and S in parallel) → Negligible voltage drop
• Connected in series in the circuit
• Shunt must be able to carry (I − Iᵌ) current without overheating
Connect high resistance R in series with the galvanometer. The large series resistance limits current so voltage can be measured safely.
R = Series resistance required (Ω)
V = Desired full-scale range of voltmeter (V)
Iᵌ = Full-scale deflection current of galvanometer (A)
G = Galvanometer coil resistance (Ω)
Worked Example: Galvanometer: Iᵌ = 5 mA = 0.005 A, G = 20 Ω. Convert to voltmeter of range 10 V.
R = (10 / 0.005) − 20 = 2000 − 20 = 1980 Ω
Properties of the converted voltmeter:
• Very high total resistance (G + R) → Draws negligible current
• Connected in parallel across the component
• Larger voltage range requires larger R
6. Key Differences: Ammeter vs Voltmeter (from Galvanometer)
7. Types of Galvanometers
| Type | Working Principle | Key Feature | Applications |
|---|---|---|---|
| Moving Coil (PMMC) | Torque on current-carrying coil in magnetic field (motor effect) | Linear scale, high accuracy, most common school/lab type. Damped by eddy currents in aluminium frame. | School physics labs, ammeters, voltmeters, multimeters, Wheatstone bridge, metre bridge, potentiometer |
| Tangent Galvanometer | Earth’s magnetic field deflects compass needle in the plane of a current-carrying coil. Tan law: I = K tanθ | Measures absolute current using Earth’s field as reference. Scale is non-linear (tangent scale). | Measuring currents using Earth’s field, calibration of other instruments, historical demonstrations |
| Ballistic Galvanometer | Measures the total charge (not steady current) in a brief pulse. The first deflection (throw) is proportional to total charge Q. | Very light coil, long period, minimal damping. Used only for transient currents (pulses, discharges). | Measurement of magnetic flux, mutual inductance, high-resistance measurement, capacitor charge measurement |
| Mirror (Light-Beam) Galvanometer | A tiny mirror on the coil reflects a light beam onto a distant scale. Angular deflection is amplified by the beam distance. | Extremely sensitive (currents down to 10²³ A). No friction since no mechanical pointer contacts the scale. | Submarine telegraph, seismographs, electrocardiogram (ECG), very sensitive laboratory measurements |
8. Uses of Galvanometer
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10. Frequently Asked Questions (FAQ)
The moving coil galvanometer works on the principle that a current-carrying coil placed in a magnetic field experiences a torque. When current I flows through the N-turn coil of area A in a field of strength B, the deflecting torque is τ = NBIA. This torque rotates the coil and its pointer. The rotation is opposed by the restoring torque of the phosphor bronze spiral springs: τᵣₜᵀ = kθ. At equilibrium: NBIA = kθ, giving I = (k/NBA) × θ. Since k, N, B, A are all constants, I ∝ θ — the deflection is directly proportional to the current. The permanent magnet creates a radial field (via concave pole pieces and soft iron core) so the torque is constant for all positions of the coil, giving a linear (uniform) scale.
A galvanometer is converted to an ammeter by connecting a low-resistance shunt S in parallel with it. The value of the shunt S is: S = IᵌG / (I − Iᵌ), where Iᵌ is the galvanometer’s full-scale deflection current, G is its coil resistance, and I is the desired ammeter range. The shunt allows most of the current (I − Iᵌ) to bypass through itself, while only Iᵌ flows through the galvanometer coil. Since S is very small, the combined parallel resistance (G||S) is very small, ensuring the ammeter does not significantly reduce the circuit current. The ammeter is then connected in series in the circuit to measure current.
A galvanometer is converted to a voltmeter by connecting a high-resistance series resistor R in series with it. The value of R is: R = V/Iᵌ − G, where V is the desired voltage range, Iᵌ is the galvanometer’s full-scale deflection current, and G is its coil resistance. When the voltmeter (galvanometer + R) is connected across a component, the full-scale deflection current Iᵌ flows when the voltage across the component equals V. Since R is very large, the total resistance (G + R) is very high, ensuring the voltmeter draws negligible current and does not disturb the circuit voltage. The voltmeter is connected in parallel across the component.
The figure of merit (k) of a galvanometer is defined as the current required to produce a deflection of one scale division: k = I/θ amperes per division (A/div). It is the reciprocal of current sensitivity. A smaller figure of merit means a more sensitive galvanometer (less current produces the same deflection). The figure of merit is determined experimentally by connecting a known EMF (E) with a known large resistance R in series with the galvanometer and measuring the deflection θ: k = E/(R+G)/θ ≈ E/(Rθ) when R ≫ G. Using k, the current for any deflection can be found: I = kθ.
The galvanometer scale is linear and uniform because the torque equation gives I ∝ θ (deflection directly proportional to current). This linearity is achieved by the radial magnetic field created by the concave pole pieces and the soft iron cylindrical core. In a radial field, the plane of the coil is always parallel to the field lines regardless of the coil’s angular position, making sinθ = 1 always. Therefore, torque = NBIA × sin(90°) = NBIA (constant) for all positions. If the field were not radial (e.g., uniform in one direction), torque would be proportional to sinθ of the coil angle, making the deflection non-linear and the scale non-uniform (crowded at one end, spread out at the other).
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