This thesis describes the design, operation, and performance of an automatic system which can be used to plot isodose curves such as are used in radiation therapy. The system consists of an apparatus to move an ionization chamber by remote control along rectangular axes so that the position of curves of equal per cent dose relative to any chosen reference point in the field are found and recorded automatically. The apparatus consists of a commerical servo system which operates from the output of an accurate dose comparator to control motion of the ionization chamber along one axis so that it hunts to locate the set per cent dose in the field to be plotted. A second manually-controlled drive system is used to provide a scan motion of the ionization chamber along the second axis. The drive systems are linked by two pairs of synchros to a plotting pen which records the path of the probe. The system is capable of producing an adequate set of isodose curves for a field in two hours or less with an accuracy of better than one-half per cent of the maximum dose in the field. / Science, Faculty of / Physics and Astronomy, Department of / Graduate