Using more nails will increase the electromagnet's capacity to hold more paper clips, without necessarily increasing the magnetic force. Have them secure one end of the copper wire to the negative pole of the battery. Have their partner hold the looped center of the wire over the compass and they are to observe what happens to the compass needle when the other end of the wire makes contact with the positive pole of the battery. Observe how the compass needle reacts. Repeat if necessary.
Electromagnets have poles which can be reversed when the path of electricity is reversed.
This is not a characteristic of regular magnets. Place them on the overhead projector and place the clear transparency over the electromagnet. Slowly sprinkle iron filings over the transparency and observe what happens to the iron filings. Name 3 ways electromagnets differ from magnets. List 2 ways to increase the strength of electromagnets.