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To test the possibility of the emission of electromagnetic waves from rocks, experiments have been made to measure the electric field by using mainly granite samples that were struck together or struck by a hammer or a weight and were fractured by a bending moment. The wide‐band (10 Hz to 100 kHz) waveforms of electric signals were digitally recorded. Roughly four kinds of signals have been observed: 30 kHz, 5 kHz, 10 Hz and, in addition, intermittent pulses. Using these measurements of the electric fields, the average electric dipole moment generated was estimated to be 10−14 C m. The possibilities of producing such a dipole moment are discussed in terms of contact electrification (or separating electrification) and piezoelectrification, and both possibilities are shown.

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