Notes: "The Spirule was designed, made, and sold by Walter R. Evans, an expert in control theory. According to the instructions, which were written by Evans, the Spirule is described thus: ""The Spirule is based on three ideas: direct measurement on plots, addition of angles by successive rotation of the arm with respect to the disk, and conversion of various quantities to angles by means of curves."" While the Spirule would perform many calculations, some of which would have been more quickly and easily accomplished by other means, where it really shone was in analyzing the stability of a control circuit. "
