NATURAL electromagnetic signals of very low frequency have been studied by several investigators during the past few years. In 1948 Willis1 reported a series of measurements of ‘magnetic noise’ in the frequency-range 5–800 cycles/sec. but did not discuss the problem of the physical origin of the observed signals. That some of the signals reported by Willis are due to lightning is strongly indicated by observations of other investigators2. Aarons and Henissart3, working in the frequency-range 1–20 cycles/sec., have reported correlations between low-frequency magnetic fluctuations and geomagnetic disturbance and solar phenomena. Therefore it appears that in the frequency-range 1–100 cycles/sec, there is a transition in the dominant physical cause of natural electromagnetic signals.