The principle of operation of an instrument designed for automatic long-term measurement of the cloud flash to ground flash ratio in thunderstorms is described. The instrument uses a simple antenna to detect the electric field of lightning and is designed to have an effective range of about 14 km and a maximum range of about 20 km. Wave-shape feature measurement is used to distinguish between the electric field wave-shapes corresponding to cloud and ground flashes and is based principally on the presence of relatively large, well-spaced, positive-going changes (R changes) in the field changes caused by ground flashes. From observations with the instrument over two thunderstorm seasons from September 1982 to May 1984 in Brisbane, Australia, the cloud flash to ground flash ratio was 3.0. Large variations were observed in the ratio of cloud flashes to ground flashes for individual thunderstorm days. For days on which at least 100 total registrations occurred, the range was 0.9 to 24.7. Possible applications of an instrument of this type in atmospheric electricity studies are discussed.