Concept:The magnetic field strength of a solenoid increases when a ferromagnetic material is used as its core.
Explanation:Soft iron, steel, and nickel are ferromagnetic materials.
Inserting any of them into the solenoid concentrates the magnetic field lines and increases the field strength.
Brass is not a ferromagnetic material.
Inserting a brass rod does not concentrate the magnetic field lines.
Therefore, a brass rod will not increase the strength of the magnetic field of the solenoid.
Answer:Inserting a brass rod in the test tube will not increase the strength of the magnetic field of the solenoid.