A reversible head for the automatic curve following device
F.C. Williams · Mathematical Proceedings of the Cambridge Philosophical Society · 1939
In the preceding paper, Blackett and Williams described an automatic curve follower for use with the differential analyser. The device would only operate if the x-coordinate was either continuously increasing or continuously decreasing; it could not handle input functions where the x value pulsated, as may often happen when the x-coordinate is not the independent variable.