I saw on Discovery channel that the source of Earth's magnetic field is the molten iron and metals in the earth's core. The spinning of these liquid metals produces the same effect as electric current in a coil which produces a magnetic field. The scientist in the program proved the concept by conducting an experiment where a big metal sphere where winded by a metallic tube in which molten Sodium was pumped so it circulate around the sphere in a similar way that happens inside earth. The experiment showed that a magnetic field was successfully produced.

My question is :

How could a circulating neutrally charged particles produce a magnetic field?

It is like having two electric currents one due to the negative charges (electrons) and the other is due to positive charges (nucleus) with same value and in opposite direction so there should not be a magnetic field. so how does this happen?