We present a mechanical rotation sensor consisting of a balance pivoting on a tungsten carbide knife edge. These sensors are important for precision seismic isolation systems, as employed in land-based gravitational wave interferometers and for the new field of rotational seismology. The position sensor used is an air-core linear variable differential transformer with a demonstrated noise floor of 1×10-11 m/√Hz. We describe the instrument construction and demonstrate low noise operation with a noise floor upper bound of 5.7×10-9rad/√Hz at 10 mHz and 6.4×10-10 rad /√Hz at 0.1 Hz. The performance of the knife edge hinge is compatible with a behaviorur free of noise from dislocation self-organized criticality. © 2014 AIP Publishing LLC.