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

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 −ω/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 −ω/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 re...