THIS PAPER describes a desk-type, autographically recording instrument built especially for precise stress-strain measurements of single fibers, which are tested in a horizontal plane to facilitate measurements in solvent media.' The instrument was designed primarily for constant-rate-of-load (stress) testing, but provision for constant-rate-of-elongation testing over a limited range of rates is also incorporated. The mechanical and optical principles used are similar to those used in earlier instruments described by Osumi and Kato [7] and Montcrieff [5].The present report will facilitate comparison with various other laboratory-constructed and commercial instruments [1,2,3,8 ] , and describes in detail an apparatus that has been used over a period of several years, with uniformly satisfactory performance, in research on the influence of chemical environment on wool fibers [6]. Principles and DesignThe principal mechanical components are shown schematically in Figure 1, and the basic electrical circuit is illustrated in Figure 2. Load is applied to the fiber through a twisting torsion wire, and elongation by a moving rack. The load and elongation units are powered separately by two-phase, induction-start, synchronous motors of the type commonly used for constant-speed chart drive in recording instruments. The loading and elongation motions are coordinated by means of a twin photocell activated by a beam of light reflected from a mirror mounted on the torsion wire. The recording chart is mounted on a drum driven by the loading motor; the pen is moved by the elongation motor. By means of a gear shift, the pen displacement may be selected to travel 1, 5, or 10 times the distance over which the fiber elongates. FIG. 1. Schematic diagrazn of mechanical components.