20.4.2—The magnetic field due to the current in a solenoid is increased by a

For a long solenoid, field strength is approximately proportional to current and turns per unit length; a high-permeability core can increase it further.

Use the right-hand grip rule for polarity and distinguish ideal uniform interior field from fringing at the ends.

Increasing turns per metre or current strengthens the field, while reversing current reverses the poles.

A solenoid is not a permanent magnet by default; its field depends on current and core conditions.