* The current flowing through the coil is increased: A higher current means more magnetic field lines are produced, leading to a stronger magnetic field.

* The number of turns in the coil is increased: More turns of wire in the coil mean the magnetic field lines from each turn reinforce each other, creating a stronger overall field.

* The core material is changed to a material with higher permeability: Materials with higher permeability, like iron, concentrate the magnetic field lines, leading to a stronger magnetic field.

* The coil is wound more tightly: A tighter coil means the magnetic field lines are closer together, resulting in a stronger field.

In summary:

The strength of an electromagnet is directly proportional to the current flowing through it, the number of turns in its coil, and the permeability of its core material. A tighter coil also contributes to a stronger magnetic field.