Magnetism and Electromagnetism
Materials subjected to rapid reversal of magnetism should have

Large area of B-H loop
High permeability and low hysteresis loss
High co-ercivity and low density
High co-ercivity and high retentivity

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Magnetism and Electromagnetism
Main causes of noisy solenoid are

uneven bearing surface, caused by dirt or uneven wear between moving and stationary parts
strong tendency of fan out of laminations at the end caused by repulsion among magnetic lines of force
None of these
Both of the listed here

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