Magnetism and Electromagnetism
Main causes of noisy solenoid are

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

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Magnetism and Electromagnetism
The magnetomotive force is

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

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