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Which one of the following is essentially a permanent magnet moving coil instrument designed to be sensitive to extremely low current levels?
1. Multimeter
2. Galvanometer
3. Electrodynamic Wattmeter
4. Electrodynamic Voltmeter
5.

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Correct Answer - Option 2 : Galvanometer

Sensitivity Of Moving Coil Galvanometer:

The general definition of the sensitivity experienced by a moving coil galvanometer is given as the ratio of change in deflection of the galvanometer to the change in current in the coil.

S = dθ/dI

The sensitivity of a galvanometer is higher if the instrument shows larger deflection for a small value of current. Sensitivity is of two types, namely current sensitivity and voltage sensitivity.

Current Sensitivity:

The deflection θ per unit current I is known as current sensitivity θ/I

θ/I = nAB/k

Voltage Sensitivity:

The deflection θ per unit voltage is known as Voltage sensitivity θ/V. Dividing both sides by V in the equation θ= (nAB / k)I;

θ/V= (nAB /V k)I = (nAB / k)(I/V) = (nAB /k)(1/R)

R stands for the effective resistance in the circuit.

Voltage sensitivity = Current sensitivity/ Resistance of the coil.

Therefore, under the condition that R remains constant;

Voltage sensitivity ∝ Current sensitivity.

The figure of Merit of a Galvanometer: It is the ratio of the full-scale deflection current and the number of graduations on the scale of the instrument and is designed to be sensitive to extremely low current levels.

It is also the reciprocal of the current sensitivity of a galvanometer.

Factors Affecting Sensitivity Of A Galvanometer:

  • Number of turns in the coil
  • Area of the coil
  • Magnetic field strength B
  • The magnitude of couple per unit twist k/nAB

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