A curve tracer is described which enables the relationship between two, three, or four variables to be observed and recorded. An optical arrangement is made to project an image of an L‐shaped aperture onto photographic paper for recording, onto a mat screen for observation, or onto a larger screen or blackboard for demonstration purposes. By means of the surface mirrors attached to two measuring instruments, the image of the aperture may be moved to any point on the screen depending on the deflection of the instruments. By means of the two light shutters attached to two other instruments the lengths of the vertical and horizontal arms of the L aperture are determined by the instrument deflections. Thus the apparatus provides a two‐coordinate system (the image of the L‐shaped aperture) moving within a second two‐coordinate system (the screen). Thus the value of two, three, or four variables can be seen at a glance, and their interrelations watched while one or more of the variables is altered. The scale on any axis may be altered by the adjustment of a continuously wound shunt. Any small variation that may be of special interest may thus be readily magnified. Full scale deflection for each component is obtained over a range from 20 mv to 2000 v or 2 ma to 2 amp. The maximum size of the photographic record is 13×18 cm. The apparatus is not concerned with rapidly varying phenomena, except insofar as these may be registered as the steady deflection of a measuring instrument. The application of the instrument is described in relation to the characteristics of a split‐anode magnetron.