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