Strength of an electromagnet (Number of coils)
The strength of an electromagnet changes with the number number of coils of the wire.
The way to judge if more turns on a electromagnet produces a stronger magnetic force is to count how many paperclips the different number of coils pick up.
NOTE: This is also a great test to judge between the strength of two magnets.
Here this electromagnet can only pick up three paperclips. If you add a fourth paperclip they all fall off.
Here, with more turns of wire on the nail, the electromagnet can pick up five paperclips.
More turns = More strength
NOTE: You could also demonstrate how much further away a compass is affected by an electromagnet with more coils as another method to determine strength.
NOTE: The strength of an electromagnet is directly proportional to not only the current but the number of windings. Double the number of windings doubles the strength of the magnet.