Carbon steel

Platinum cobalt

ALNICO-V

All of these

flux

type of material

type of magnetising force

nothing

reluctance

resistance

reactance

conductance

increasing and decreasing the number of turns of the coil

using an exciting coil with number of tappings

varying the current through the exciting coil

varying the core position of the exciting coil

Helix rule

End rule

Right hand grip rule

Fleming's right hand rule