The cause of the Earth's magnetism

In this Journal for June, 1900, it was shown that a sphere of radius a charged with electricity of volume density ρ, and revolving round an axis with angular velocity Ω, would have a magnetic potential at a point r , θ where θ is the colatitude, (1) If an equal amount of the opposite sort of electricity were distributed through a sphere of radius a ‐ da , its volume density ρ′ would be determined by the condition, (2) and its magnetic potential would be, The magnetic potential of the two distributions together would be If ρ is assumed to be not a function of R , this becomes (3) This is the right form of expression for the average magnetic potential round parallels of latitude on the Earth's surface.