A moving coil galvanometer is a fundamental instrument used in physics to detect and measure small electric currents. It operates on the principle of electromagnetic induction and is widely studied in Class 12 Physics under the topic of electromagnetic devices.
Understanding the diagram working principle and construction of a moving coil galvanometer is crucial for students preparing for board exams and competitive tests like JEE and NEET.
This topic provides a detailed explanation of the moving coil galvanometer including a labeled diagram working mechanism mathematical expressions and practical applications.
Diagram of a Moving Coil Galvanometer
The moving coil galvanometer diagram consists of several essential components that contribute to its functioning. Below are the main parts labeled in the diagram:
- Coil (C): A rectangular loop of wire that moves when current passes through it.
- Magnet (N and S): A strong permanent magnet that provides a uniform magnetic field.
- Soft Iron Core (I): Increases the strength of the magnetic field and ensures uniformity.
- Suspension (T): A thin wire or strip that holds the coil and provides restoring torque.
- Spring (S): Restores the coil to its original position when the current stops flowing.
- Pointer (P): A needle that moves over a scale to indicate the current.
- Scale (D): A calibrated scale that shows the deflection of the pointer.
The diagram of a moving coil galvanometer typically shows the coil placed between two magnetic poles with a pointer attached to indicate the deflection.
Construction of a Moving Coil Galvanometer
A moving coil galvanometer is constructed using the following components:
1. Rectangular Coil
The coil is made of a thin copper wire wound around a lightweight frame. It is suspended in a radial magnetic field produced by a strong permanent magnet.
2. Permanent Magnet
A curved horseshoe magnet creates a uniform magnetic field that allows the coil to move freely when current flows through it.
3. Soft Iron Core
A cylindrical soft iron core is placed inside the coil to concentrate and strengthen the magnetic field making the instrument more sensitive.
4. Suspension System
The coil is suspended by a thin phosphor bronze strip that allows free movement while also providing restoring torque to bring the coil back to its initial position.
5. Pointer and Scale
A pointer is attached to the coil and moves over a calibrated scale indicating the amount of current flowing through the galvanometer.
Working Principle of a Moving Coil Galvanometer
The moving coil galvanometer works on the principle of torque acting on a current-carrying coil in a magnetic field. This is based on Ampere’s force law which states that a conductor carrying current in a magnetic field experiences a force.
Step-by-Step Working Mechanism:
- Current Flow: When an electric current passes through the coil it experiences a force due to the magnetic field.
- Torque Generation: The force on opposite sides of the coil creates a torque that causes the coil to rotate.
- Deflection of Pointer: As the coil rotates the pointer attached to it moves over the scale indicating the current.
- Restoring Torque: The suspension system generates a restoring torque that opposes the motion of the coil.
- Equilibrium: The coil stops rotating when the restoring torque equals the applied torque.
The angle of deflection (θ) is directly proportional to the current (I) flowing through the coil given by:
where:
- N = Number of turns in the coil
- B = Magnetic field strength
- A = Area of the coil
- I = Current flowing through the coil
- k = Torsional constant of the suspension wire
Sensitivity of a Moving Coil Galvanometer
The sensitivity of a galvanometer is defined by how well it can detect small currents. It depends on:
- Number of Turns (N): More turns in the coil increase sensitivity.
- Magnetic Field Strength (B): A stronger field improves sensitivity.
- Coil Area (A): A larger coil area results in greater torque.
- Suspension Wire Stiffness (k): A softer suspension increases sensitivity.
How to Increase Sensitivity?
- Using a stronger magnet
- Increasing the number of turns in the coil
- Reducing the stiffness of the suspension wire
Uses of a Moving Coil Galvanometer
1. Detection of Electric Current
A moving coil galvanometer is primarily used to detect small electric currents in a circuit.
2. Measurement of Small Voltages
When combined with a high resistance it acts as a voltmeter to measure small voltages.
3. Conversion into an Ammeter
By connecting a low resistance (shunt) in parallel it can be converted into an ammeter to measure current.
4. Use in Laboratory Experiments
It is used in physics laboratories for various experiments involving electromagnetic induction and circuit analysis.
Advantages of a Moving Coil Galvanometer
- High Sensitivity: Detects very small currents.
- Linear Scale: The deflection is directly proportional to the current.
- Reduced Damping Error: Uses air damping for smoother operation.
- Accurate Readings: Provides precise and reliable measurements.
Limitations of a Moving Coil Galvanometer
- Cannot Measure Large Currents: Limited to small current detection.
- Delicate Construction: The suspension system is fragile.
- Polarized Instrument: Works only with DC (Direct Current) and not AC (Alternating Current).
Difference Between Moving Coil Galvanometer and Ammeter
| Feature | Moving Coil Galvanometer | Ammeter |
|---|---|---|
| Purpose | Detects small currents | Measures large currents |
| Resistance | High internal resistance | Low internal resistance |
| Shunt Required | No | Yes |
| Deflection | Proportional to current | Proportional to applied voltage |
| Use | Laboratory experiments | Electrical circuits |
The moving coil galvanometer is an essential instrument in physics used to detect small electric currents. Understanding its diagram working principle and construction is crucial for Class 12 students and anyone studying electromagnetism.
By carefully analyzing its sensitivity advantages and applications students can appreciate its significance in both theoretical and practical electrical studies.
