providing an output alarm signal should the current deviate by a predetermined amount from a preset value in the embodiment of fig5 the grounding device 50 at one end of rotor 22 remains unmodified while at the other end thereof means are provided for establishing a resonant excitation frequency more particularly at some ac frequency the current path length through rotor 22 and the stationary assembly 12 will equal one electrical wavelength and the current loop will become resonant this resonant condition can be sensed by observing current and voltage relationships at the driving point and the existence of a rub would cause a change in the distribution of current which would cause a detectable change in the resonant frequency one way of accomplishing this is with the provision of transformer 90 having a secondary winding 91 connected in circuit between the rotor 22 and pedestal structure 30 the primary winding 92 of the transformer is connected to a swept frequency network impedance analyzer 94 which measures the relatinship between current and voltage in the loop current circuit as a function of frequency frequencies of parallel and series loop resonance can be associated with maximum and minimum impedance frequencies respectively a typical swept frequency network impedance analyzer may include a display 95 which will provide a visual.