1911 Encyclopædia Britannica/Quadratrix

QUADRATRIX (from Lat. quadrator, squarer), in mathematics, a curve having ordinates which are a measure of the area (or quadrature) of another curve. The two most famous curves of this class are those of Dinostratus and E. W. Tschirnhausen, which are both related to the circle.

Fig. 1

Fig. 2

Fig. 3

The quadratrix of Dinostratus was well known to the ancient Greek geometers, and is mentioned by Proclus, who ascribes the invention of the curve to a contemporary of Socrates, probably Hippias of Elis. Dinostratus, a Greek geometer and disciple of Plato, discussed the curve, and showed how it effected a mechanical solution of squaring the circle. Pappus, in his Collections, treats of its history, and gives two methods by which it can be generated. (1) Let a spiral line be drawn on a right circular cylinder; a screw surface is then obtained by drawing lines from every point of this spiral perpendicular to its axis. The orthogonal projection of a section of this surface by a plane containing one of the perpendiculars and inclined to the axis is the quadratrix. (2) A right cylinder having for its base an Archimedean spiral is intersected by a right circular cone which has the generating line of the cylinder passing through the initial point of the spiral for its axis. From every point of the curve of intersection, perpendiculars are drawn to the axis. Any plane section of the screw (plectoidal of Pappus) surface so obtained is the quadratrix. Another construction is shown in fig. 1. ABC is a quadrant in which the line AB and the arc AC are divided into the same number of equal parts. Radii are drawn from the centre of the quadrant to the points of division of the arc, and these radii are intersected by the lines drawn parallel to BC and through the corresponding points on the radius AB. The locus of these intersections is the quadratrix. A mechanical construction is as follows: Let AMP be a semicircle with centre O (fig. 2). Let PQ be the ordinate of the point P on the circle, and let M be another point on the circle so related to P that the ordinate PQ moves from A to O in the same time as the vector M describes a quadrant. Then the locus of the intersection of PQ and OM is the quadratrix of Dinostratus.

The Cartesian equation to the curve is y = x cot πx/2a, which shows that the curve is symmetrical about the axis of y and that it consists of a central portion flanked by infinite branches (fig. 2). The asymptotes are x = ±2na, n being an integer. The intercept on the axis of y is 2a/π; therefore, if it were possible to accurately construct the curve, the quadrature of the circle would be effected. The curve also permits the solution of the problems of duplicating a cube (q.v.) and trisecting an angle.

The quadratrix of Tschirnhausen is constructed by dividing the arc and radius of a quadrant in the same number of equal parts as before. The mutual intersections of the lines drawn from the points of division of the arc parallel to AB, and the lines drawn parallel to BC through the points of division of AB, are points on the quadratrix (fig. 3). The Cartesian equation is y = a cos πx/2a. The curve is periodic, and cuts the axis of x at the points x = ±(2n−1)a, n being an integer; the maximum values of y are ±a. Its properties are similar to those of the quadratrix of Dinostratus.