Experimental apparatus for the elastic torsion of bars — round, tubular, slotted and square sections

A bar clamped between two chucks is twisted by a torque applied through a mass disk and a cable weight. Two angle indicators read the twist at any two points along the bar, letting the student measure $d\varphi/dx$ and extract the shear modulus $G$ and the polar second moment $J_p$ from a single run. A set of four sections — full round bar, closed tube, longitudinally slotted tube, square tube — makes the dramatic contrast between closed and open thin-walled profiles tangible in a single afternoon in the laboratory.