We say that electric and magnetic fields are "linear" in that the total field from more than one source is the sum of the fields from all the sources. The fields come from all the turns of your electromagnet, and so they can add to be stronger.
This isnít always true, though, and it depends on the geometry of your magnet. A very long solenoid has a magnetic field inside which depends on the current in the wire and the number of turns per unit length of the solenoid. Adding more turns on the end just makes the solenoid longer (and strengthens the field where you put the turns -- the field on the end of a solenoid is weaker than it is in the center), but the turns on the end of a very long solenoid do not strengthen the field much in the middle. If you add the turns to the middle, then you make the field stronger there.
Adding more turns in the coil can also increase the total resistance of the wire. If youíre powering the magnet with a constant voltage source (like a battery), the current will be inversely proportional to the resistance in the wire. In that case adding more turns wonít increase the field, since the field from each turn will go down.
p.s. If you can get enough current, how strong a field you can get from a magnet depends on how big a field it has when the current is as big as you can get it without getting the magnet so hot that itís damaged. It doesnít really matter much for that whether you use fewer coils of thicker wire, that can carry more current, or more coils of thinner wire, that can carry less current. What really matters is making the total thickness of all the coils big enough./ Mike W.
(republished on 08/02/06)