In Magnetorheological Finishing (MRF) a magnetic field is applied to a stream of abrasive magnetorheological fluid, in order that the fluid behaves as the polishing tool. The process may be used to finish the surface of high quality optical lenses. The fluid viscosity is one important parameter the polishing tool characteristic depends on. At the University of Applied Sciences Deggendorf a new viscosity measurement, which uses the inductance of the fluid had been tested. The result of the research is a close relationship between viscosity and inductance. The new viscosity measurement is not an absolute, but a comparative system, based on inductance of the flowing fluid and the fluid age.