Certain galvanometers have a fixed core made of non-magnetic metallic material. The function of this metallic material is:
Options:
To produce large deflecting torque on the coil
To bring the coil to rest quickly
To oscillate the coil in magnetic field for longer period of time
To make the magnetic field radial
Correct answer:B
Standard Method
Given: Certain galvanometers have a fixed core made of non-magnetic metallic material.
Find: The function of this metallic material.
A non-magnetic metallic core produces eddy currents when the coil moves in the magnetic field. These eddy currents provide damping.
Because of this damping force, the coil comes to rest quickly after deflection.
Therefore, the function of the metallic material is to bring the coil to rest quickly. The correct option is B.
Role of Eddy Current Damping
Given: A galvanometer has a fixed core made of non-magnetic metallic material.
Find: Why this metallic core is used.
The metallic core is not used to make the field radial and it does not increase the deflecting torque directly. Its important role is damping.
When the moving coil rotates, the nearby metallic core supports eddy currents. By Lenz's law, these eddy currents oppose the motion responsible for producing them.
So the oscillations die out rapidly and the pointer settles quickly.
Therefore, the coil is brought to rest quickly, so the correct option is B.
Confusing the metallic core with the soft iron core. A non-magnetic metallic core is used for damping through eddy currents, not for making the magnetic field radial. Radial field is associated with the pole-piece arrangement, not this core.
Assuming the metallic core increases deflecting torque. Deflecting torque depends on the magnetic field and current in the coil, whereas the metallic core here mainly helps in reducing oscillations and settling the coil quickly.
Thinking damping means longer oscillation time. Damping does the opposite: it reduces oscillations and helps the coil reach its final position faster.
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