Magnetism and Electromagnetism
Main causes of noisy solenoid are

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

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Magnetism and Electromagnetism
The magnetomotive force is

The voltage across the two ends of exciting coil
The sum of all currents embraced by one line of magnetic field
The flow of an electric current
The passage of magnetic field through an exciting coil

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