Lets say I have a uniformly charged spherical shell. that is roating with an angular velocity w. Now machs principle says that accleration is relative to the distant stars. If the distant stars are rotatingng with the same w as the spherical shell, there will be no B field inside the shell. This seems strange. Why should the distant stars have anything to do with the B field of the rotating shell? And lets say the stars started moving relative to the shell why would this all of a sudden create a B field to appear in the frame.