Background Paragraph: Our knowledge of electromagnets developed from a series of observations. In 1820, Hans Oersted discovered that a current-carrying wire produced a magnetic field. Later in the same year, André-Marie Ampere discovered that a coil of wire acted like a permanent magnet and François Arago found that an iron bar could be magnetized by putting it inside of a coil of current-carrying wire. Finally, William Sturgeon found that leaving the iron bar inside the coil greatly increased the magnetic field. 
Story: Mark is doing an experiment creating a magnet. He takes a piece of iron named iron bar A and wraps it in a coil of copper wire. After that he puts an electric current through the copper for about an hour.
Given Mark disconnects and unwraps the copper wire, will the magnetic strength of the iron bar have increased or decreased? Given Mark repeates his experiment with a second iron bar, Iron bar B but leaves it in the coil for 3 hours, which bar has a stronger magnetic field, Iron bar A or Iron bar B? Will leaving an iron bar wrapped in copper wire carrying a current cause the bars magnetic field to increase or decrease?