The nature of the radiation from a lightning flash is discussed, and it is shown that the main source of inaccuracy in the crossed-loop cathode-ray types of direction-finder at present in use by the Meteorological Office is due to polarization errors. Various methods are then considered for improving the accuracy of thunderstorm location, and the conclusion is drawn that a cathode-ray direction-finder, using a spaced-aerial system and with brilliance modulation applied to the display tube, is likely to be more successful than other systems. A description is then given of a direction-finder, having greatly improved instrumental accuracy, which has been designed as an experimental tool for the development of the improved method of location.