Typical apparatus as shown in FIGS. 1 and 12 for indicating the relative positions in a given direction between a reference point (24,59) and a test point (29,68) spaced therefrom and movable relative thereto is useful for measurement of the magnitude and direction of static or dynamic torque in a shaft (8). Overlapping transparent reference (20,60) and test surfaces (25,61), either parallel disks (60,61) or coaxial cylinders (20,25) each having a relevant portion covered with a plurality of equally spaced lines oriented predominantly normal to the given direction and at an angle to each other, produce moire fringes (30) when illuminated (14,15). One of the surfaces (25,61), being flexible, is connected to the shaft (8) so as to flex in response to torque between the reference point (24,59) and the test point (29,68), causing a change in the angle between the lines on the two surfaces (20,25;60,61), and thus of the spacing (D) (FIG. 3) of the moire fringe bands (30), which is optically detected (17) and converted, as shown in FIG. 4, to an electrical signal (42) to control power applied to the shaft (8).