The paper is concerned with the recording of various characteristics of a radio wave reflected from the ionosphere as the frequency is altered rapidlyAn apparatus is described in which a receiver is kept in tune with a sender by means of automatic frequency control circuits operated by the direct pulse from the sender. The tuning is corrected by each pulse and remains fixed for the time between pulses. A frequency range of 1 Mc/s. can be covered in 1 sec. and may be selected anywhere in the range 2-20 Mc/s.A number of applications of the apparatus are described and typical records are reproduced. The irregularities of ionization present in the normal regions are investigated by the observation of the irregular variations of the amplitude of the reflected wave which are produced when the frequency is altered. Irregularities of ionization are sometimes found to be localized in height. Observations of the reflection coefficient of the abnormal region E suggest that there are two distinct types of region, one an irregular region consisting of scattering clouds, the other a coherent layer with a thickness of the order of 5 km. The apparatus is also used to study the behaviour of the subsidiary critical frequencies often present below the main critical frequency of region E. It is found that these are always decreasing in critical frequency whenever they are observed. Records of amplitude and group path near the critical frequency of a region can be used to determine the collision frequencies of electrons. The treatment is mainly descriptive, and a full discussion of those records which require a detailed quantitative analysis is reserved for a later paper.