Residual Gas Effect On Q-factors In Torsion Balance

As a mainly weak force measurement, torsion balance is widely used in gravitational physics,electromagnetism force and material characteristic research. Torsion balance experiment technique has become more developed and perfected in past two hundred years, but still there are many factors effect experiment precision. Residual gas is obvious one of the errors.In rarefied air, the mean free path is larger than the size of test object, it is meaningless to consider boundary. The molecules scatter from object surface meet incidence molecules when moving far away from object surface. So molecules collision effect on Maxwell velocity distribution function is absolutely negligible.From rarefied air theory, we consider incidence flow acting on the surface of test object, which some particles follow mirror reflection and the other particles follow diffusion reflection. We study interaction between gas and surface. We calculate residual gas applied moment on test object of rectangular solid,cylinder and disk. We get a universal expression. At last, we know that the effect on Q is direct ratio to torsion balance frequency and temperature, which is inverse ratio to pressure and the mass of particle. When the pressure is 10-5pa, the period of the pendulum is hundred seconds, residual gas Q-factor is about 10 6 ~108. So we can neglect the residual gas effect.