Moving Coil Galvanometer
Covers the principle, construction, working, sensitivity, and conversion of a moving coil galvanometer into an ammeter or voltmeter.
What is Moving Coil Galvanometer?
An instrument that uses the magnetic effect of electric current to detect and measure small electric currents, based on the torque experienced by a current-carrying coil in a magnetic field.
Key formula / rule: Torque on a current loop
Key points
- Explain the working principle of a moving coil galvanometer.
- Describe the construction of an MCG.
- Derive the relationship between current and deflection.
- Define and differentiate between current and voltage sensitivity.
Common exam trap
Confusing voltage and current sensitivity.
Definitions
- Term
Moving Coil Galvanometer (MCG)
- Meaning
An instrument that uses the magnetic effect of electric current to detect and measure small electric currents, based on the torque experienced by a current-carrying coil in a magnetic field.
- Term
Current Sensitivity
- Meaning
The deflection produced in the galvanometer per unit current flowing through it (Is = θ/I).
- Term
Voltage Sensitivity
- Meaning
The deflection produced in the galvanometer per unit voltage applied across it (Vs = θ/V).
- Term
Shunt Resistance
- Meaning
A low resistance connected in parallel with a galvanometer to convert it into an ammeter, allowing it to measure larger currents.
- Term
Series Resistance
- Meaning
A high resistance connected in series with a galvanometer to convert it into a voltmeter, allowing it to measure larger potential differences.
Learning objectives
Explain the working principle of a moving coil galvanometer.
Describe the construction of an MCG.
Derive the relationship between current and deflection.
Define and differentiate between current and voltage sensitivity.
Explain how an MCG can be converted into an ammeter and a voltmeter.
Formulae
- Name
Torque on a current loop
- Note
n: number of turns, I: current, A: area of coil, B: magnetic field strength, θ: angle between magnetic field and normal to the coil area.
- Expression
τ = nIAB sinθ
- Name
Restoring Torque
- Note
k: torsional constant of the spring, θ: angle of deflection.
- Expression
τrestore = kθ
- Name
Condition for steady deflection
- Note
Equating deflecting and restoring torques.
- Expression
nIAB = kθ
- Name
Current Sensitivity
- Note
Deflection per unit current.
- Expression
Is = θ/I = nAB/k
- Name
Voltage Sensitivity
- Note
Deflection per unit voltage. V = IRcoil.
- Expression
Vs = θ/V = nAB/(kRcoil)
- Name
Ammeter Conversion
- Note
Rsh: shunt resistance, Ig: full-scale deflection current of galvanometer, I: desired range of ammeter, G: resistance of galvanometer coil.
- Expression
Rsh = (IgG)/(I-Ig)
- Name
Voltmeter Conversion
- Note
Rse: series resistance, V: desired range of voltmeter.
- Expression
Rse = (V/Ig) - G
Prerequisites
Magnetic field due to a current (Biot-Savart Law, Ampere's Law).
Force on a current-carrying conductor in a magnetic field.
Torque on a current loop in a uniform magnetic field.
Basic understanding of springs and restoring force.
Common mistakes
Confusing voltage and current sensitivity.
Incorrectly applying the formula for torque.
Not understanding the role of restoring torque.
Mistaking the function of the soft iron core.
Keywords
Moving Coil Galvanometer
MCG
Torque
Magnetic Field
Current Sensitivity
Voltage Sensitivity
Ammeter
Voltmeter
Shunt Resistance
Series Resistance
Radial Field
Practice preview
In a moving coil galvanometer, the restoring torque that brings the coil back to its original position is primarily provided by which component?…
easy
A moving coil galvanometer is primarily used for which of the following purposes?…
easy
To convert a moving coil galvanometer into an ammeter, a low resistance shunt is connected in which configuration with the galvanometer?…
medium