(Physics Ch-4) 8. Moving Coil Galvanometer


Its working is based on the fact that when a current carrying coil is placed in a magnetic field, it experiences a torque.


Suppose the coil PQRS is suspended freely in the magnetic field.


l = Length PQ or RS of the coil

b = Breadth QR or SP of the coil

n = Number of turns in the coil

Area of each turn of the coil, A = l × b

Let B = Strength of the magnetic field in which coil is suspended

= Current passing through the coil in the direction PQRS

Let, at any instant, α be the angle which the normal drawn on the plane of the coil makes with the direction of magnetic field.

The rectangular coil carrying current when placed in the magnetic field experiences a torque whose magnitude is given by,

τ = nIBA sinα

Due to deflecting torque, the coil rotates and suspension wire gets twisted. A restoring torque is set up in the suspension wire.

Let θbe the twist produced in the phosphor bronze strip due to rotation of the coil and K be the restoring torque per unit twist of the phosphor bronze strip. Then,

Total restoring torque produced = kθ

In equilibrium position of the coil,

Deflecting torque = Restoring torque

∴ NIBA = kθ


Where, [constant for a galvanometer]

It is known as galvanometer constant.

  • Current sensitivity of the galvanometer is the deflection per unit current. 

  • Voltage sensitivity is the deflection per unit voltage.


Conversion of a galvanometer to ammeter

  • A shunt (low resistance) is connected in parallel with the galvanometer.


I → Total current in circuit

G → Resistance of the galvanometer

Resistance of the shunt

I→ Current through galvanometer

Conversion of galvanometer to voltmeter

  • A high resistance is connected in series with the galvanometer.


→ Potential difference across the terminal A and B

Ig → Current through the galvanometer

R → High resistance

G → Resistance of galvanometer


  1. good it is nice to recap before examination .... in fact tomorrow is my final 12th board

  2. very good for any competitive exam

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    2. Variable Resistors: Potentiometer, Preset & LDR: Sequential glowing of LEDs, Internal structure of potentiometer/preset, Working of LDR
    3. Capacitors-Charging and Discharging With Resistors & Time Constants
    4. Relay & Electromagnetism- Burglar Alarm, Alternate Switching, Oscillator
    5. Diode- VI Characteristics & Working, Low Resistance Path, Protection Device
    6. OR and AND Digital Logic Gates using Diodes
    7. NOR and NAND Digital Logic Gates using Diodes
    8. Zener Diode- Working & Reverse Breakdown Voltage, Voltage Regulator
    9. Transistor as Amplifier & Switch: Cut-off, Active & Saturation Regions
    10. Touch Activated Switch (Passing current through body) using a Transistor
    11. Darlington Pair (Multi-stage amplifier using transistors)
    12. Automatic Night Lamp using transistor and LDR
    13. OR and AND Logic Gates using Transistors
    14. NOR and NAND Logic Gates using Transistors
    15. NOT Gate (Transistor as an Inverter)
    16. LED Blinker (or Flasher) using a Transistor
    17. H-Bridge (Motor-driving circuit used in robotics) using transistors
    18. DC Motor: Studying relation between RPM and Voltage
    19. Wheatstone Bridge: Calculating the unknown resistance using two fixed resistors and a variable resistor (potentiometer)
    20. Full-wave Bridge Rectifier (Using diodes and a 12V AC transformer)
    21. Full-wave rectifier using diodes and a center-tapped 12V AC transformer
    22. Joule Thief: Drawing energy from a dead cell using an inductor
    23. IR (Infrared) Security or Intrusion Alarm System
    24. Temperature Sensor using a thermistor