A rheometer instrument including a stationary frame, a sleeve suspended from the frame, a bob suspended within the sleeve, a cross-spring pivot suspending the bob from the stationary frame, and a force rebalance system for effecting the rotational displacement of the bob with a torque. The force rebalance system includes an arm attached to the movable portion of the cross-spring pivot, a rotational position sensor in close proximity to the arm for measuring the rotational displacement and angular frequency of the arm, and a force actuator positioned to apply force to the arm. The rheometer instrument may be used to calculate the rheological properties of Newtonian and non-Newtonian fluids.