The output from X-ray generating machines tends to fluctuate even over short periods of time. Whenever accurate ion chamber measurements are to be made in the radiation field, it is essential to use two ion chambers and to record the ratio of their outputs. The first or “probe” chamber may be moved during the exposure, while the second or “monitor” chamber must be kept stationary at some point in the radiation field. Charge or current ratios may be recorded. However, recording charge ratios is time-consuming because it means stopping the moving chamber in order to take readings, often lasting several minutes each, at a number of points in the field. Plots of instantaneous current ratios are, therefore, to be preferred. The subject has been discussed by several authors starting with Kemp (1946) and a number of current ratio circuits have since been described in the literature, e.g. those of Mauchel, Epp and Johns (1955), Beasley, Melville and Knight (1961), Greene and Nelson (1963) and King (1969). These ratio circuits formed part of complete automatic isodose plotting systems. However, with the widespread use of computers in radiotherapy, the current ratio trace along a line can suffice as input data for a program which calculates full isodose curves. Work is at present in progress in this department on such a program. In addition, the availability of solid state modules has greatly simplified the design of circuits. Walton (1968) has described a current ratio circuit built up from such modules.