Ehlers, Jürgen Geometric Analysis and Gravitation, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;
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Ehlers, J., & Rindler, W. (1970). A gravitationally induced (machian) magnetic field. Physics Letters A, 32(4), 257-258. doi:10.1016/0375-9601(70)90310-5.