Abstract: An electrical control unit operable as a rheostat or a potentiometer comprising an electrical circuit portion of a resistive electrically conducting material formed on a support in a spiral configuration, a spindle mounted for rotation about the center of the spiral, an arm non-rotatably engaged on the spindle but slidable relatively to the spindle radially with respect to the axis thereof, an electrical contact carried on the arm to move radially therewith, to make contact with said circuit portion, and a spiral guide means aligned with the circuit portion spiral and co-operable with the arm to guide said arm radially, so that the electrical contact follows the circuit portion spiral to alter the effective electrical resistance thereof in use.