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Electric polarization induced by mechanical loading of Solnhofen limestone

Electromagnetic radiation was detected while loading a sample of Solnhofen limestone, which is not a piezoelectric material. The sample was subjected to mixed tensile and shear loading. The signals observed are associated with a fall-off in electric polarization induced by the loading, and represent the second time derivative of the polarization. The polarization is ascribed to the complicated type of loading, leading to the existence of a chosen direction, which makes polarization possible.