An apparatus is described by means of which plots of the equipotentials of two-dimensional fields may be made automatically from models of the electrodes. The usual electrolytic tank method has been extended by the addition of a servomechanism which causes the probe to traverse an equipotential automatically. A stepping relay operated by a limit switch effects automatic plotting of successive equipotentials and limits the plot to a desired area. A number of plots made with such an apparatus are reproduced.