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 an electromagnet produce a stronger magnetic force is to count how many paperclips the different number of coils can pick up.
NOTE: This is also a great test to judge the relative strengths of two magnets.
Above, the 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: As another method of determining strength, you can check the increased distance at which a compass is affected by an electromagnet as you add more coils.
NOTE: The strength of an electromagnet is directly proportional to not only the current but the number of windings. Doubling the number of windings doubles the strength of the magnet.