concepts of differential geometry. The locus m of the centers of curvature of a plane curve l is called the evolute of l, and l is said to be an involute of m (see Figure 1).
An involute l of a curve m is the curve described by the end B of the thread AB as it is unwound from m or wound on m. This construction of the involute makes clear the following properties of the evolute and involute: (1) The tangent CD at an arbitrary point C on the evolute is normal to the involute at the corresponding point D; consequently, an involute is an orthogonal trajectory of the tangents to the evolute. (2) Any orthogonal trajectory of the tangents to a curve m is an involute of m; m therefore has infinitely many involutes. (3) The difference between the radii of curvature AB and CD at the points B and D of an involute is equal to the length of the arc AC of the evolute. (4) The evolute is the envelope of the family of normals of an involute.
An involute of a space curve is an orthogonal trajectory of the tangents of the curve.
Rashevskii, P. K. Kurs differentsial'noi geometrii, 4th ed. Moscow, 1956.