A constant current supply circuit generates a constant current according to a control voltage. 5 to 20% by weight of the ethylene oxide-capped polypropylene oxides have a nominal functionality of four or higher, and the balance of the ethylene oxide-capped polypropylene oxides, but not less than 1. One of the hubs is connected to a hydraulic drive motor and includes a plurality of slots, one of which receives a drive lug on the interior of the brush which is preferably of the type having a core of a thin metal tube.