to ordinary convention the retainer ring 23 and insulator ring cover 24 space relatively adjaent discs 12 13 in such way as to provide adequate space for flow of fluid therebetween and also provide for electrical insulation between relatively adjacent discs in order to avoid break down of the insulation due to the aforementioned high stress concentration about the edge 53 of the opening 14 in the disc 12 it is accordingly preferable to provide full insulation about the disc 13 rather than about the disc 12 if the insulation would not break down or if the voltage and stress concentration in particular at the area 53 would not effect break down of the insulation then it would be possible to employ insulation such as the insulator ring cover 24 with respect to the disc 12 instead of with the disc 13 according to the preferred embodiment and best mode of the invention it is desirable to insulate that disc 12 or 13 which is deficient in electrons to minimize insulation break down in this case especially when there is a high electrical stress at the large opening 14 due to electron concentration there such stress the flow of electrons from such area 53 toward the other disc and the direct wiping contact engagement and flow by of fluid with respect to the high stress concentration area 53 and the overall surface of disc 12 maximizes charging efficiency of the fluid another important advantage of insulating the disc 13 especially when the disc 12 has a supply of electrons thereon during use of the mixer charger 10 e g to.