Gyromagnetic Ratio of a Massive Body

It is well known that the gyromagnetic ratio ($g$ factor) of a classical, slowly rotating body whose charge density is proportional to its mass density must be equal to unity. However, if the body is very massive the spacetime curvature effects of general relativity become important and the result $g=1\mathrm{}$ is no longer valid. We calculate here the gyromagnetic ratio of a slowly rotating, massive shell with uniform charge density. When the shell is large compared with the Schwarzschild radius we have $g=1\mathrm{}$, but as the shell becomes more massive the $g$ factor increases. In the limit as the shell approaches its Schwarzschild radius we obtain $g\to 2$ (the same value as for an electron).