How did scientists find out that the earth is just like a magnet?

Question: How did scientists find out that the earth is just like a magnet?
- Keywords:
  earth,
  magnet,
  magnetic field
Asked by kizrarox to Anna, Hayley, Iain, Rebecca on 23 Jun 2015.
- Iain Bethune answered on 23 Jun 2015:
  
  Somebody realised that magnetic rocks (like the iron ore called magnetite), always point in the same direction, even when there are no magnetic materials nearby. Thus, the magnetic field must come from the Earth itself. This is the principle by which a compass needle always points to the North pole.

Question: How did scientists find out that the earth is just like a magnet?