What Is a Galvanometer? Definition, Working, and Uses

A galvanometer is a highly sensitive electrical instrument used to detect and measure small electric currents. It operates based on electromagnetic principles and plays a crucial role in laboratory experiments and the development of electrical measuring devices. Understanding the galvanometer's working, construction, and applications is essential for students studying current electricity in physics.

Principle of a Galvanometer

A galvanometer works on the principle that a current-carrying coil placed in a uniform magnetic field experiences a torque. This torque causes deflection of a pointer, which is proportional to the current passing through the coil.

The main equation governing the functioning of a moving coil galvanometer is $N B I A = k \theta$, where $N$ is the number of turns, $B$ is the magnetic field strength, $I$ is the current, $A$ is the area of the coil, $k$ is the spring constant, and $\theta$ is the angular deflection.

Construction and Components

A moving coil galvanometer consists of a rectangular coil of thin insulated copper wire wound on a light aluminum frame and suspended or pivoted between the poles of a strong permanent magnet, which provides a uniform radial magnetic field.

The core is often made of soft iron to enhance and concentrate the magnetic field lines. Two control springs offer necessary restoring torque and also serve as the current path to the coil. The pointer attached to the coil moves over a calibrated scale to indicate deflection. For detailed descriptions and mechanisms, refer to Understanding the Galvanometer.

Working Mechanism

When a small current passes through the coil, the interaction between the current and the magnetic field generates a torque on the coil. This torque is opposed by the restoring torque from the control springs, and equilibrium is reached when both torques are equal.

The deflection of the pointer is directly proportional to the current, making it possible to measure or detect very small current values accurately. This process adheres to the equation $I = \dfrac{k\theta}{N B A}$.

Key Differences: Galvanometer vs Ammeter

Both galvanometers and ammeters are used in electric circuits but have different roles. These differences are central for understanding circuit measurements and are covered in depth in Difference Between Ammeter and Galvanometer.

Galvanometer Conversion: To Ammeter and Voltmeter

A galvanometer can be converted into an ammeter by connecting a low resistance, called a shunt, in parallel with it. This allows most of the current to bypass the sensitive coil, enabling accurate measurement of higher currents.

To convert a galvanometer into a voltmeter, a high resistance is connected in series. This limits the current so the instrument measures the potential difference across two points in a circuit. Details about this transformation are found in Galvanometer Basics.

Sensitivity and Calibration

The sensitivity of a galvanometer is defined as the deflection per unit current. It can be increased by using a strong magnetic field, a coil with a large number of turns, and a core of low moment of inertia.

• High sensitivity allows detection of very small currents
• Critical for null-point methods in laboratory circuits
• Calibration ensures accuracy in readings

Applications in Physics

Galvanometers are widely used in laboratory measurements, particularly in experimental setups like Wheatstone bridges and potentiometers, where they help in achieving balance for precise readings.

They also serve as the underlying mechanism in voltmeters and ammeters, enabling measurement of potential difference and current. For more JEE-relevant uses, see Electric Field Intensity Explained.

• Null-point detection in bridge circuits
• Measurement of small EMFs using potentiometers
• Ohm’s law experimental verification
• Current direction identification in practical circuits

Key Formulae Involving Galvanometer

The primary equation for a moving coil galvanometer is $N B I A = k \theta$, where the right side represents the restoring torque. For conversion to ammeter and voltmeter:

• Shunt resistance: $S = \dfrac{I_g R_g}{I - I_g}$
• Series resistance for voltmeter: $R = \dfrac{V}{I_g} - R_g$

Here, $I_g$ is the maximum current for full-scale deflection, $R_g$ is the resistance of the galvanometer, and $V$ is the voltage to be measured.

Example: Calculating Shunt Resistance

Suppose a galvanometer with $R_g = 20\,\Omega$ provides full-scale deflection at $I_g = 5\,\mathrm{mA}$ and must be converted into an ammeter to measure $1\,\mathrm{A}$. The shunt resistance is calculated as follows:

$S = \dfrac{I_g R_g}{I - I_g} = \dfrac{5 \times 10^{-3} \times 20}{1 - 5 \times 10^{-3}} \approx 0.1005\,\Omega$

Galvanometer in JEE Main: Strategy and Practice

Understanding the principle, construction, and conversions of a galvanometer is essential for solving circuit-based and measurement questions in JEE Main. Regular practice with mock tests and derivation problems is effective.

Linking galvanometer use to other concepts such as Ohm’s law, Kirchhoff’s laws, and current electricity enhances problem-solving skills. Explore related strategies at Circuit Solving Techniques.