To increase the accuracy of the trajectory tracer improved methods of field determination on the resistance network have been investigated. One of these—an interpolation system applicable to axially symmetric fields—is described. This system has been constructed, and some results obtained with it for trajectories through an electron lens are given. These are compared with corresponding results obtained without the interpolation device and with results obtained by other methods.It is seen that the interpolation system provides significantly increased accuracy for this particular type of problem, with little extra complication in the operation of the trajectory plotter.