If naked wire is used then the iron rod also acts as a conductor and short circuit takes place instead of showing magnetic effect of electricity to the molecules of the rod. Hence, naked wire is not used while making electromagnet.
Electromagnets can be created by wrapping a wire around an iron nail and running current through the wire. The electric field in the wire coil creates a magnetic field around the nail. In some cases, the nail will remain magnetised even when removed from within the wire coil.
It will short between the windings; the current rather than passing around and around and around the core will just pass across the surface and create no magnetic field (which is proportional to the number of turns it takes around the core).
Solid copper wire is better because it can usually carry more current. It is best to have a large amount of copper to keep the resistance down. It is also good to have a lot of turns to make better use of the available current. Copper has the lowest resistance at room temperature, so its a great choice. Steel is not used in electromagnets as steel when used will make electromagnet a permanent magnet which cannot be stopped if we stop the electric current flowing.
The copper wire around an electromagnet is insulated to avoid current flow between the wires. If the wire is uninsulated the current will take a short cut and will not flow multiple times around the core.To make an electromagnet, you'll need either a 6- or 9-volt battery, 10 feet or more of insulated wire and an iron bolt or nail.