Getting close to gravity: developing the experimental techniques to measure the inverse square law of gravity

The torsion balance has a long history of measuring weak forces. Its inherent design results in a very low rotational stiffness and some degree of immunity to tilt and horizontal accelerations coupling into the rotational motion of the suspended mass. Due to, however, the vertical distance of the centre-of-mass from the point of attachment, horizontal accelerations can couple strongly to the simple pendulum mode. This makes control of the torsion bob difficult. Additionally, at some level there will always be some coupling to rotational motion. I present work on experimental techniques to circumvent these issues. This revolves around the Superconducting Torsion Balance (STB) - a novel torsion balance with zero fibre length, and the Air Bearing - a proof-of-concept experiment demonstrating centre-of-buoyancy tuning to decouple horizontal accelerations from the rotational mode. I designed and implemented the capacitive displacement sensor for the STB, where it is projected to give a signal-to-noise ratio greater than 1 for a signal of 2×10−17 N·m within a days integration. I employed finite element analysis of the magnetic forces in the superconducting suspension to demonstrate the feasibility of levitating the test mass. The Air Bearing demonstrates centre-of-buoyancy tuning to a precision of ± 0.3 mm