Difference Between Galvanometer and Ammeter: Explained Simple
The Galvanometer is a sensitive instrument used to detect and measure small electric currents in a circuit. In JEE Main, understanding its working principle, construction, and differences from related devices is essential for scoring in current electricity and circuit-based questions. This device forms the foundation for practical measurements and appears often in theory and numerical questions. Mastery of the galvanometer brings significant advantages in tackling laboratory experiments and real-world circuit analysis, as seen in typical JEE problems.
What is a Galvanometer? Principle and Simple Explanation
A Galvanometer is an electrical instrument that detects and indicates the presence, direction, and relative magnitude of a small electric current. It works on the principle that a current-carrying coil experiences a torque when placed in a magnetic field, causing a pointer attached to it to deflect. Imagine a magnetic compass needle that moves due to electric current—this is the basic idea behind a galvanometer’s function.
The main law at work is the Motor Principle, which says a force acts on a conductor carrying current in a magnetic field. The resulting torque moves the coil, which is visible as the needle’s deflection. This movement is proportional to the current flowing through the coil, allowing sensitive detection of tiny currents.
Galvanometers can be of several types, but the moving coil galvanometer is most commonly discussed in the JEE syllabus. Other variants like ballistic galvanometers are less common and mainly for advanced physics.
Construction and Working of Galvanometer
A typical moving coil galvanometer consists of a rectangular wire coil suspended or pivoted between the poles of a strong permanent magnet. The coil is wound on a frame, often of aluminum. The ends of the coil connect to fine spiral springs, providing both electrical connection and a restoring force. A lightweight pointer is attached to the coil to indicate the current on a calibrated scale.
When current passes through the coil, it interacts with the magnetic field, producing a torque. The pointer attached to the coil shows the deflection. The restoring torque from the attached spring balances this, and the final deflection is proportional to the current. This mechanism helps explain linear readings for small currents, an important property in lab and circuit analysis questions.
- Coil: Thin insulated copper wire wound on an aluminum frame
- Magnet: Provides a strong, uniform radial magnetic field
- Soft iron core: Concentrates and strengthens the field
- Pointer: Indicates deflection along a scale
- Springs: Offer restoring force and supply current path
For moving coil types, the deflection angle (θ) relates to current (I) as I = kθ, where k is the galvanometer constant. Keeping the torque proportional for small angles is a common pitfall—linear approximation holds only for small deflections.
Galvanometer vs Ammeter: Key Differences
A frequent JEE question is distinguishing between a galvanometer and an ammeter. While they appear similar, their usage and construction differ.
| Aspect | Galvanometer | Ammeter |
|---|---|---|
| Function | Detects small currents | Measures larger currents |
| Sensitivity | Very high, detects microamperes | Lower, measures up to amperes |
| Internal resistance | High | Low (designed for minimal drop) |
| Use in circuit | Detect presence/direction | Quantitative current measurement |
| Scale type | Typically central zero | Generally starts from zero |
For more on direct quantitative measurement devices, see ammeter construction and uses. When comparing, remember both can be derived from similar moving coil mechanisms, but an ammeter always includes a low resistance shunt in parallel with the galvanometer.
Converting Galvanometer to Ammeter or Voltmeter
In JEE Main, you may be asked how to convert a Galvanometer into an ammeter or voltmeter using simple circuit modifications. This is a common board and entrance exam topic.
- To make an ammeter, connect a low-resistance shunt (Rsh) in parallel with the galvanometer. It allows most of the current to bypass the sensitive device, protecting it.
- Formula for shunt: Rsh = (Itotal / Ig - 1) × Rg, where Ig is full-scale galvanometer current, Rg is internal resistance.
- To make a voltmeter, connect a large resistance (Rseries) in series. This limits current through the galvanometer to safe levels across large voltages.
- Formula for series resistance: Rseries = V / Ig - Rg.
- Carefully calculate resistance values to avoid device damage and ensure desired range.
Practice problems on this conversion abound. For stepwise procedures, refer to galvanometer conversion guide for worked examples and diagrams.
Always connect in the right orientation—polarity matters due to deflection conventions. If confused between galvanometer and ammeter roles, review their scale, resistance, and function in lab set-ups.
Applications and Uses of Galvanometer in JEE Physics
Galvanometers go far beyond textbook devices, playing roles in laboratory, practical, and modern technological contexts covered by JEE.
- Detecting and balancing currents in Wheatstone bridge circuits
- Measuring small EMFs using the potentiometer method
- Null-point detection in meter bridge and resistance trials
- Serving as the basic mechanism in voltmeters and ammeters Modern laser scanners and mirror galvanometers (seen in physics labs and advanced optics)
- Current direction identification in electricity experiments
- Practical verification of Ohm’s law and temperature-resistance effects
As per the Vedantu Physics team, learning galvanometer-based principle helps students grasp the working of almost all basic electrical measurement devices. Applying this in lab and circuit analysis, including magnetism and EMI sections, is critical.
Key Formula, Pitfalls, and Example Using Galvanometer
The core formula relating current (I) to deflection (θ) in a moving coil galvanometer is:
I = (C / nAB) × θ
where C: restoring torque per unit twist, n: number of coil turns, A: area of the coil, B: magnetic field strength.
A common pitfall is ignoring the small angle assumption—proportionality breaks for large θ. Also, do not use galvanometers for AC detection unless specified types, as most are DC sensitive.
Example:
A galvanometer with Rg = 50 Ω gives full-scale deflection at Ig = 2 mA. To convert it into an ammeter of range 10 A, what shunt is needed?
Rsh = (Itotal / Ig - 1) × Rg
= (10 / 0.002 - 1) × 50 = 499,950 × 50 = 0.01 Ω
So, the required shunt is 0.01 Ω.
Galvanometer in JEE: Practice, Connected Topics, and Revision Tips
To fully master the Galvanometer for JEE Main, revise related topics often paired in exam questions.
- Explore moving coil application in voltmeter construction and usage
- Solve practice problems from current electricity mock tests
- Review Ohm’s law experiment set-ups using galvanometer readings
- Apply concepts in Kirchhoff’s circuit laws and calculation questions
- Practice conversion derivations as per conversion procedures page
- Connect with current electricity and resistance key ideas to avoid common exam errors
- Use experimental skills tests for practical applications
Regularly update your concept map with connected ideas from Vedantu’s curated resources, as topics like magnetism and induction often overlap in multi-concept JEE Main questions.
Summary: The galvanometer is the gateway to understanding current detection and measurement in physics. Its principle, construction, and transformation underpin the design of nearly every current-measuring instrument tackled in JEE Main. Master its formula, small angle regime, and circuit conversions to succeed in lab and theory papers.
FAQs on Galvanometer – Working, Diagram, Principle & Applications
1. What is galvanometer in simple words?
A galvanometer is a sensitive instrument used to detect and measure small electric currents.
Key points:
- It is mainly used in laboratories and electrical experiments
- Works by showing needle movement when current passes through it
- Helps indicate the presence and direction of current
- Often forms the basis for meters like ammeters and voltmeters
2. How does a galvanometer work?
A galvanometer works by converting a small electric current into mechanical movement.
Here's how:
- A current-carrying coil is placed in a strong magnetic field
- When current flows, the coil experiences a torque
- This torque turns the coil, moving the pointer over a scale
- The amount the pointer moves shows the magnitude and direction of the current
3. What is the difference between galvanometer and ammeter?
A galvanometer detects small currents, while an ammeter measures the actual value of larger currents.
Main differences:
- Galvanometer: Sensitive, shows direction, used for detection (small currents, lab work)
- Ammeter: Measures large currents, always connected in series, shows magnitude only
- Galvanometer can be converted to an ammeter by adding a low resistance (shunt) in parallel
4. Is a galvanometer used for AC or DC?
A galvanometer is generally used for detecting and measuring small DC (Direct Current).
Key points:
- Most common type: Moving coil galvanometer (works only for DC)
- Special AC galvanometers exist but are less common in student labs
5. What are the uses of a galvanometer?
A galvanometer has several important uses in physics and electronics.
Main uses:
- Detecting and measuring small electric currents
- Forming the core part of ammeters and voltmeters
- Null point detection in Wheatstone bridges and potentiometers
- Current direction and circuit continuity tests in labs
- Applications in laser scanning and optical positioning (modern uses)
6. What is the symbol and unit of a galvanometer?
The galvanometer does not have a standard SI unit as it detects current (measured in ampere, A) but has a specific symbol.
- Symbol: Usually represented by a circle with the letter G inside in circuit diagrams
- Unit: Shows current (ampere, A), but the device itself does not define a new unit
- Can measure very small currents (microamperes or milliamperes)
7. Why can't we use a galvanometer directly as an ammeter for large currents?
You cannot use a galvanometer directly as an ammeter because it is designed to detect only small currents.
Main reasons:
- Large currents could damage its delicate coil and mechanism
- It has high resistance, which would reduce current in the circuit
- To measure larger currents, a low resistance shunt is connected in parallel, converting it into an ammeter
8. What is meant by the sensitivity of a galvanometer?
The sensitivity of a galvanometer refers to how small a current it can detect and how much the pointer deflects per unit current.
Details:
- High sensitivity: Detects very tiny currents, shows larger deflection for small currents
- Increased by using strong magnets, many turns in the coil, and low moment of inertia
- Important for precise laboratory experiments and measurements
9. How is a galvanometer converted into a voltmeter?
A galvanometer is converted into a voltmeter by connecting a high resistance in series with it.
Main facts:
- The high resistance limits current, allowing the device to measure voltage across a component
- The value of the series resistance is chosen so that full scale deflection corresponds to the desired voltage range
- This setup lets the galvanometer safely read voltages without damage
10. What is the principle of a moving coil galvanometer?
The principle of a moving coil galvanometer is that a current-carrying coil placed in a magnetic field experiences a torque.
Key points:
- The torque turns the coil, causing the pointer to move
- The movement is proportional to the amount of current flowing
- The device uses this effect to measure or detect small electric currents in labs
