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The Core
An electromagnet requires a magnetic core for current to pass through. The core renders the conductor magnetic enough to attract objects. For a simple school experiment, almost any soft iron or steel nail, screw, tent peg or bolt will serve well. Ideal core diameters should be at least 0.31 inch and core length should be about 1.2 to 2 inches. Using cast iron cores or smaller ones do not release magnetized materials right away when disconnected from the power source.
The Conductor
The conducting material is a long thin wire, preferably a copper wire. Rubber insulated wires are not good conductors because their insulation is thicker than the copper wire diameter. Enamel-coated wires are ideal magnetic wires because of thinner insulation, conducting stronger magnetic force through the iron core.
The Power Source
The power source can be a single battery or a battery pack. More elaborate experiments involve resistors to restrict electricity flow. Single dry cells allow intermittent electromagnetic use; double dry cells permit longer periods. Testing how strongly different voltage ratings produce electromagnetism is also a good point of study.
Constructing the Electromagnet
Leaving 2 inches free on both ends, wind the wire smoothly around the nail or bolt. If using a nail, start from the flat point and wrap towards the pointed end; with a bolt, winding is easier, as the thumb guides the wire through the grooves.
Afterwards, scrape off insulation from the free ends. Connect one end to the positive battery pole, and the other to the negative pole. Bring the contraption near some paper clips, and observe as the magnet picks them up. Disconnecting the wires from the battery poles will cause the clips to drop. Experiment with several voltages and see how many paper clips can be picked up.