An apparatus is described which compares the signals developed by two detectors. An application is the measurement and plotting of the distribution of radiation energy produced by x‐ray generators. Measurement of the radiation energy is made with ionization chambers or scintillation counters as detectors, the signals of which are amplified with dc amplifiers. The noise level of the amplifiers is equivalent to a current of about 5×10−15amperes when used with an input resistor of 1011ohms. The ratio of the outputs of the two detectors is obtained directly, using a servo system which compares a fraction of the larger signal with the smaller one. The accuracy and reproducibility of the balancing circuit is better than 0.2 percent. The minimum detectable signal is less than 0.1 millivolt. The same servo system is also employed to control the motion in space of one of the detectors (a probe) so as to seek a position such that its signal is any preselected portion of the other detector's (monitor) signal. Thus, a path of constant energy is traversed by driving the probe perpendicularly to the direction of the x‐ray beam and controlling the position parallel to the beam through the servo system. The trace of the path of the probe is simultaneously duplicated and recorded on a chart. The mechanical reproducibility of the detector position and plotting mechanism in any direction is better than one millimeter. The application of the apparatus for the automatic determination and recording of isodose contours produced in water by the absorption of x‐ray beams of widely different energies is outlined.