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Summary: Army Instructional film describing the basic principles of the mechanisms used to rotate equipment such as searchlights, radar antennae, guns and gun turrets. The control circuit, including a rheostat, and the system by which a piece of equipment can be accurately moved to a specific bearing, are described in detail. The film also describes the problem of velocity lag in servo mechanism control systems, and ways in which it can be overcome. The practical applications of servo mechanisms are shown in a Centurion tank, Mk 8 Searchlight control gear, No 11 Predictor and the Battle Class destroyer HMS Aisne (D.22).