what is galvanometer

In the world of physics and electrical engineering, instruments play a vital role in understanding, measuring, and analyzing different quantities. One such important instrument is the galvanometer — a device used to detect and measure small electric currents.

Though simple in appearance, a galvanometer is a cornerstone of experimental electrical science and has paved the way for modern measuring devices such as ammeters and voltmeters. In this blog, we will explore what a galvanometer is, its history, construction, working principle, types, uses, and how it differs from other instruments.

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Introduction to Galvanometer

A galvanometer is an electromechanical instrument used to detect and measure small electric currents by deflecting a pointer in response to the flow of current through a coil placed in a magnetic field.

In simple words, when current flows through a coil placed in a magnetic field, a torque is generated that causes the coil (and hence the pointer attached to it) to move. The amount of deflection of the pointer is proportional to the current passing through the coil.

The galvanometer is a very sensitive instrument, meaning it can detect even minute currents that ordinary ammeters cannot. This sensitivity makes it invaluable in laboratory experiments and precision measurements.

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History and Origin of the Galvanometer

The term “galvanometer” comes from the name of Italian scientist Luigi Galvani (1737–1798), who discovered that electric currents could cause muscle contractions in frogs — an early observation of bioelectricity.

Later, in 1820, Hans Christian Ørsted discovered that an electric current in a wire produces a magnetic field around it, leading to the development of instruments capable of detecting this magnetic effect.

Early versions of the galvanometer were quite primitive — they simply consisted of a compass needle placed near a current-carrying wire. The needle deflected when current flowed, indicating the presence of an electric current.

Over time, with the contributions of scientists such as Johann Schweigger, André-Marie Ampère, and William Thomson (Lord Kelvin), the galvanometer evolved into a more sophisticated, precise, and practical instrument.

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Principle of a Galvanometer

The working of a galvanometer is based on the electromagnetic deflection principle — when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force.

The basic principle can be explained as:

“When an electric current passes through a coil placed in a magnetic field, a torque acts on the coil, causing it to rotate. The angle of rotation (deflection) is proportional to the current flowing through the coil.”

This principle is derived from Lorentz’s force law, which states that a current-carrying conductor experiences a force in a magnetic field given by:

$$F=BIL\sin \theta$$

Where:

• $F$ = Force on the conductor

• $B$ = Magnetic flux density

• $I$ = Current through the conductor

• $L$ = Length of the conductor

• $\theta$ = Angle between conductor and magnetic field

This force produces a torque on the coil, which leads to deflection of the pointer. The deflection is balanced by a restoring torque provided by a spring, and the final equilibrium position gives a measure of the current.

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Construction of a Galvanometer

A moving coil galvanometer is the most common type used in laboratories. Its basic construction consists of the following parts:

(a) Moving Coil

A rectangular coil made of thin copper wire is wound on a non-metallic frame. The coil is suspended between the poles of a powerful magnet so that it can freely rotate. The number of turns of the coil increases the sensitivity of the instrument.

(b) Magnetic Field

The coil is placed between two concave pole pieces of a permanent magnet (usually made of a horseshoe magnet). These concave poles create a radial magnetic field, ensuring that the plane of the coil always remains parallel to the magnetic field lines during rotation. This uniform field ensures linear deflection.

(c) Soft Iron Core

A soft iron cylinder is placed inside the coil to strengthen the magnetic field and make it more uniform. It also increases the instrument’s sensitivity.

(d) Suspension System

The coil is suspended by a thin phosphor bronze wire that carries current into and out of the coil. The lower end of the coil is connected through another fine wire or a spiral spring, which provides the restoring torque.

(e) Mirror and Pointer

A small mirror is attached to the suspension wire or the coil to reflect a light beam onto a calibrated scale. This mirror system helps measure tiny deflections accurately without friction.

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Working of a Galvanometer

Let’s understand step-by-step how a galvanometer works.

1. Current Flow
   When an electric current passes through the coil, magnetic forces act on the coil because of the interaction between the magnetic field of the coil and the field of the permanent magnet.

2. Deflecting Torque
   This interaction produces a deflecting torque on the coil, causing it to rotate.

   Td=nBIAT_d = n B I ATd​=nBIA

   Where:

   • TdT_dTd​ = Deflecting torque

   • $n$ = Number of turns of the coil

   • $B$ = Magnetic field strength

   • $I$ = Current through the coil

   • $A$ = Area of the coil

3. Restoring Torque
   As the coil turns, the suspension wire twists, producing a restoring torque Tr=kθT_r = k \thetaTr​=kθ, where $k$ is the torsional constant and $\theta$ is the deflection angle.

4. Equilibrium
   The coil comes to rest when both torques balance:

   $$nBIA=k\theta$$

   Therefore:

   θ=nBAkI\theta = \frac{n B A}{k} Iθ=knBA​I

   This shows that deflection (θ) is directly proportional to current (I) — a key property for measurement.

5. Reading the Deflection
   The deflection is measured using the mirror and scale arrangement or a pointer. The scale can then be calibrated to show the current directly.

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Sensitivity of a Galvanometer

The sensitivity of a galvanometer refers to how small a current it can detect.

It depends on:

• Magnetic field strength (B): Stronger magnetic fields produce greater deflection.

• Number of turns (n): More turns increase torque.

• Area of the coil (A): Larger area increases torque.

• Suspension constant (k): A thinner suspension wire (lower $k$) increases sensitivity.

A highly sensitive galvanometer can detect currents as low as 10⁻⁹ amperes (nanoamperes).

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Types of Galvanometers

There are several types of galvanometers, depending on design and application.

1. Moving Coil Galvanometer (MCG)

The most common type, used in laboratories and analog instruments. It provides accurate and linear measurements.

2. Ballistic Galvanometer

Used to measure the total charge that passes through a circuit in a very short time — such as in capacitor discharge experiments.

3. Mirror Galvanometer

Used in optical and telecommunication systems, where deflection of a light beam indicates current flow (historically used in submarine telegraphy).

4. Tangent Galvanometer

An older type that uses a circular coil and compass needle. It measures current using the tangent law of magnetism. Though less common today, it remains a valuable educational tool.

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Conversion of Galvanometer into Ammeter and Voltmeter

A galvanometer can be easily converted into other measuring instruments:

(a) Conversion to Ammeter

To measure larger currents, a low resistance (shunt) is connected in parallel with the galvanometer. This allows most of the current to bypass the galvanometer, preventing damage.

IgRg=(I−Ig)RsI_g R_g = (I – I_g) R_sIg​Rg​=(I−Ig​)Rs​

Where RsR_sRs​ is the shunt resistance.

(b) Conversion to Voltmeter

To measure potential difference, a high resistance is connected in series with the galvanometer. This limits the current passing through it.

V=Ig(Rg+R)V = I_g (R_g + R)V=Ig​(Rg​+R)

By proper calibration, the galvanometer scale can then indicate voltage directly.

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Advantages of Galvanometers

• High sensitivity: Detects even tiny currents.

• Linear scale: Deflection is proportional to current.

• Compact and reliable: Simple mechanical structure.

• Can be converted easily: Serves as the basis for ammeters and voltmeters.

• Accurate: Especially when properly calibrated and shielded from vibrations.

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Limitations of Galvanometers

Despite its usefulness, a galvanometer has certain drawbacks:

• Cannot measure large currents directly — may get damaged.

• Affected by temperature — as resistance changes.

• Friction and vibration errors can affect readings.

• Delicate suspension system — requires careful handling.

• Cannot measure alternating current (AC) because the pointer would oscillate about zero without giving a steady reading.

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Applications of Galvanometers

Galvanometers are widely used in both educational and practical settings:

1. Detection of current flow in circuits.

2. Measurement of small currents in research laboratories.

3. Bridge and potentiometer circuits for detecting null points.

4. Conversion to ammeters and voltmeters in analog instruments.

5. Telecommunication systems (historically).

6. Calibration and testing of other electrical measuring devices.

7. Electrochemical experiments, such as detecting end points in titrations.

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Difference Between Galvanometer, Ammeter, and Voltmeter

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Modern Developments

While traditional galvanometers are less common in modern electronics, their principle lives on in digital measuring instruments.

Today’s digital ammeters, voltmeters, and oscilloscopes all stem from the galvanometer concept — using electromagnetic or electronic deflection to detect and display current or voltage.

In advanced systems, optical and microelectromechanical (MEMS) galvanometers are used for precise beam steering in laser scanners, 3D printing, and medical imaging systems.

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Conclusion

The galvanometer is more than just a laboratory instrument — it’s a symbol of the transition from classical physics to modern electrical engineering.

By detecting even the faintest currents, galvanometers allowed scientists to uncover the laws of electromagnetism, develop accurate measurement systems, and design the electrical technologies we use today.

From Ørsted’s compass needle to MEMS laser scanners, the basic idea remains the same: a current produces motion. This simple yet powerful concept continues to drive innovation, making the galvanometer an enduring icon of scientific progress.

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