Apparatus for Determination of Residual Drag Torque on Small Spinning Spheres in Magnetic Suspension

Abstract

An apparatus for measuring the frequency decay of magnetically suspended freely spinning steel spheres is presented. A residual drag torque, which considerably exceeds the gas friction torque in ultrahigh vacuum, has been determined by means of a new measuring system for small frequency changes, which features high resolution power even at low rotor speed. The decay ratio −ω̇/ω has been recorded for several spheres with different diameters over a frequency range from 0.2 to about 105 Hz. Apart from some characteristic deviations, which have been related to certain experimental conditions, a linear frequency dependence of the decay ratio −ω̇/ω=a0+a1rω has been detected in the described apparatus (r being the sphere radius) with a0≃3×10−9 sec−1 and a1≃3×10−12 m−1. An explanation is given for this type of drag torque. Measurements are reliable within ±6×10−11 sec−1 up to rω≃700 m/sec. Stabilities better than ±5×10−12 sec−1 during one day have been recorded. A long term stability of the latter magnitude is required to verify the existence of an ω6 proportional energy dissipation, which was deduced by Keith in 1963 from relativistic gravitational theories.