Adjusting the distance between the two electrodes is called gapping your spark plugs. You need a feeler gauge to gap your spark plugs properly If you're re-gapping a used plug, make sure that it's clean (gently scrub it with a wire brush)
A field winding is the insulated current-carrying coils on a field magnet that produce the magnetic field needed to excite a generator or motor. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. It is a stationary insulated wire wrapped in a circular shape. It creates a strong magnetic field around the armature.