interfaces of rotating machines in either circumstance system 10 is capable of providing either discrete or continuous information concerning the wear of brush b throughout its useful life in some embodiments of the invention equipment 18 will display visually or otherwise the percentage 34 p 34 of life remaining for brush b such percentage may be obtained at a particular time 34 t 34 by having equipment 18 perform the following calculation equ1 where v ref is the initial steady state voltage present across the rotating component and brush as the length of rod 22 approaches zero thereby signalling the end of the useful life of brush b the value of voltage vs will approach that of v ref making the numerator of the above written equation approximately zero as well when percentage p decreases to zero a length d of brush b continues to cover shunt 30 and prevent exposure of it to the rotating element although distance d may be any selected value preferred embodiments of system 10 have a distance d of approximately one eighth inch fig2 depicts an alternate system 100 of the present invention system 100 is substantially identical to system 10 except that rod 104 does not have uniform electrical resistance throughout its length instead rod 104 is composed of multiple segments 108 any given one 108a having an electrical resistance different from those of its adjacent segments 108b and 108c thus the value of voltage v s measured by equipment 18 will remain approximately constant until.