or bearing assembly 86 an inboard relatively low friction ball bearing or bearing assembly 88 an outboard field conductor 92 an inboard field conductor 94 and a cartridge housing 96 a cartridge housing 96 which is preferably made of a molded electrically non conductive plastic or phenolic material encloses and supports the ball bearings 86 88 and the field conductors 92 94 except as hereinafter noted the ball bearings 86 88 may be of a conventional design and construction the ball bearings 86 88 include outer races 98 102 respectively inner races 104 106 respectively and a plurality of bearing balls 108 112 respectively which are disposed between and in the annular volume defined between the inner and outer races the bearing balls 108 112 may be maintained between their respective inner and outer races by conventional cages not shown in fig1 4 the ball bearings 86 88 are made of a highly conductive material such as for instance high carbon chromium low carbon steel or a bronze brass alloy it is preferable that approximately one third of the annular volume which as noted is defined by and between the inner and outer race is open that is not occupied by the balls the bearings are packed in an electrically conductive grease that with the balls and cage completely fills the annular volume a preferably useable grease is nyogel 753g manufactured by nye lubricants.