What causes the Earth to have a magnetic field?

Dave - They're not sure but they think its to do with the centre of the Earth being a liquid metal ball and you get convection currents in that. There's a strange thing where it excites itself and turns itself into a dynamo, you get electric currents which make an electro magnet and that's what they reckon is making the magnetic field. Chris - The field does flip from time to time doesn't it? I think the average is about every 100, 000 years, but in fact its less than that.

Dave - Yes, they reckon we're due for one fairly soon as the Earth's magnetic field is getting weaker very fast.

Chris - So does the North pole become the South Pole? Dave - Yes.