FOLLOWING a request, received early in 1940, by the Scientific Advisory Committee of G. H. Q. Middle East for work to be undertaken on the design and construction of land-mine detectors, Dr. Lawrence Balls, then chairman of the Committee, together with Mr. J. H. Cole, and with the assistance of military personnel, produced, and tried out in actual operations, several successful models. In an article entitled "Land-mine Detectors designed at Giza", which Dr. Balls contributes to the June number of Reme, the journal of the Corps of Royal Electrical and Mechanical Engineers, details are given of the construction, performance and characteristic features of these land-mine detectors. They were of various sizes, the large models suitable for road clearance, and the smallest for surgical applications, such as shrapnel detection in wounds. The Mark IV design Was first used, but was superseded by an original design due to Dr. Balls, consisting of twin search-coils, thus giving double sensitivity. Coils of about 12-in. diameter were found convenient for ordinary field detectors, and of about ⅜-in. for surgical use. Operating on the heterodyne principle, a frequency of one megacycle per second proved most useful, with a frequency ratio of two-thirds to produce the audio-frequency beat-note. Segmented screening, twin oscillators, and a low-loss chassis were the three main characteristics of the ‘twin' design. The earthed screen surrounding a search-coil was cut, usually, into twelve segments, each segment being separately connected by a thin wire to a central earth on the chassis. The two identical search-coils, mounted together in the same plane, Were inductances of separate oscillatory circuits, driven from a single twin-triode valve. Passage of the coils over a metal object caused the beat-note, somewhat lower than middle C, to be altered downward by one coil and upward by the other, thus causing the headphone note to change suddenly from a progressively lowering growl to a falling high shriek as the object passed from one coil to the other. The Weight of a one-man detector is naturally a matter of practical importance, and a prototype, of which two copies were flown to Britain in 1943 but too late to be considered for production, and which was made of wax-boiled wood though designed for production in plastics, weighed only 14 J lb., of which the batteries accounted for more than.half.