Abstract: The inner spiral wall of each spiral feed duct of a turbine is provided with a slidable extension tongue with an end portion of spiral curvature mounted on a carrier plate for rotation about the machine axis. Movement of the tongue to extend the length of the inner duct wall reduces the exit cross-section of the duct while keeping a spiral shape at the duct end. The tongue may be mounted on a portion of the machine casing that is made integral with the discharge duct that is coaxial with the machine and is mounted axially rotatable on the machine casing. When an axial flow rotor is used, the ducts and tongues have deflecting rims and the tongues have end walls at the rim ends to define the zone of communication between the rotor and each duct. One of the applications of the invention is to exhaust gas driven superchargers for internal combustion engines. A discharge spiral for a rotary pump or compressor may be similarly equipped.