Many investigations of Region E of the ionosphere have been described, in the more detailed of which the critical-frequency technique of Appleton has been used to determine the penetration frequency of the region. In the latest work this has generally involved continuous variation of frequency and photographic recording so that the finer details of the P'-f curve, relating equivalent path (P') to frequency (f), could be obtained. It has usually happened, however, that these records have been obtained at considerable time intervals, such as half an hour or an hour, so that the finer details of the temporal variations in the behaviour of the region could not be followed. The present paper describes observations made in such a way as to exhibit the finer details of the temporal variations. The Breit and Tuve pulse method was used, in conjunction with a cathode-ray oscillograph employing a timesweep in the usual way at the receiver. The oscillograph, was, however, observed visually, and the observer could alter the frequency of the transmitter (situated about 1 mile away) by means of a remote control. In this way it was possible to keep a continuous detailed watch on the temporal variations of any particular feature of the P'-f curve. The observations were made in Cambridge during the second half of 1936.
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Best et al. (1938) studied this question.