In eddy current damping, the disc of an instrument is made of a material that is:
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In eddy current damping, a conductor that is non-magnetic is used to produce the necessary resistance to motion without the complications of magnetism.
In eddy current damping, the disc of the instrument is made of conductor but non-magnetic material. The non-magnetic property allows the material to conduct eddy currents without the influence of magnetic fields, which generates damping forces to slow down the movement of the disc.
Thus, the correct answer is Conductor but non-magnetic.