Geom. Pl. tractrices. [mod.L. (Huygens) fem. of tractor: see TRACTOR, and cf. DIRECTRIX.] A curve such that the intercept on the tangent between its point of contact and a fixed straight line is constant; so called as being traced by the center of gyration of a rigid rod of which one end is moved along the fixed straight line, or as being the form assumed by an inextensible string which is first laid straight upon a plane surface, and one end of it then drawn in a direction at right angles to that in which the string was laid. Also, a class of curves similarly traced, e.g., by movement along a fixed curve.
1727–41. Chambers, Cycl., Tractrix, in geometry, a curve line, called also catenaria. [Error: the tractrix is the involute of the catenary, not the catenary itself.]
1843. Penny Cycl., XXV. 109/2. Tractrix, or Tractory, the name given to a curve described by a heavy point attached to a string, the other end of which is moved along a given straight line or curve.
1852. Salmon, Higher Plane Curves, vii. (1879), 289. The involute of the catenary is therefore a curve such that the intercept SN, on its tangent between the point of contact and a fixed right line, is constant. Such a curve is called the tractrix.
1877. B. Williamson, Int. Calc. (ed. 2), vii. Ex. 9.
