Abstract: A self driven mobile assembly including an ink agitator is supported on the ink fountain of a printing machine by a beam which is adapted to be mounted parallel to the longitudinal axis of the ink fountain so as to provide a guide track for supporting the mobile assembly for linear displacement along the ink fountain with the ink agitator projecting into the ink. The beam has a rack extending parallel to said ink fountain axis, and the mobile assembly includes a reversible motor and drive means connecting the motor both to rotate the agitator and to linearly displace the assembly along the beam by driving a pinion meshing with the rack. The beam also has an exposed bus electrically insulated from the beam and lying parallel to the rack, and the mobile assembly has a pair of members, one mounted for sliding electrical contact along the bus and the other mounted for sliding electrical contact along a portion of the beam separate from the bus for supplying electrical power to the assembly.