An electromagnet is a piece of conductive wire wound in concentric circles, with the ends connected to a power source, usually a direct current battery. The strength of an electromagnetic field is determined by the strength of the current running through it and the number of coils that are wound. In order to increase the strength of an electromagnet, you should run a stronger current through the coil or add more windings. Be careful, however, not to overheat the device as more energy will be lost to heat.
Increase the current running through the coil. Note that due to Ohm's Law, which states that the power dissipated is equal to the square of the current times the resistance, a phenomena called ohmic heating will occur if the current is increased and the resistance is kept constant.
Increase the number of windings in the coil. This is a safer way to increase the strength of the electromagnet. Because the magnetic field is equal to the current multiplied by the number of turns, the field strength increases directly with the increase in coils.
Add an iron bar to the core of the electromagnet. When the ferrous material is placed within a magnetic field, its domains will line up accordingly, adding to the strength of the field. This is the safest way to increase the field strength as it does not interfere with the current in the electromagnet itself.
Substitute enamel-coated copper wire for plastic wire. Because the enamel coating is thinner than plastic insulation, you will be able to wind the wire more times in the same amount of space, and the coils will be smaller.
Be careful not to overheat the electromagnet by providing too much current.