物理量Hodgson, 2nd Edition, 1978, John Murray, ISBN 0-7195-3382-1 Encyclopedia of Physics, R.G. Lerner(英语:Rita G. Lerner), G.L. Trigg, 2nd Edition, VHC Publishers
引力場quantity which takes on different values at different points in space. Lerner, R. G.; Trigg, G. L. (编). Encyclopaedia of Physics 2nd. Wiley-VCH. 1991