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Accurate information such as may be required for weapon control may best be transmitted over long-distance telegraph or voice communication circuits by using some form of digital representation or pulse code modulation. The paper describes a method by which the position of a scale attached to a datum shaft may be Â¿readÂ¿ photo-electrically as a binary number in much the same way as the scale of a surveying instrument is read in degrees and minutes. Data-transducers have been constructed to represent the angular position of a shaft as a 14-digit binary number, that is to an accuracy of rather better than one minute of arc. Special precautions have been taken in the design of the scale to prevent gross errors due to movement of the scale while it is being read. Such errors as do occur are examined in detail, since in some data-transmission applications they may not be sufficiently serious to justify the more complex equipment required for their complete elimination.