PURPOSE: To confirm, using a simple and reliable method, a terminal position which an annular piston reaches within an inner space in a friction clutch having an outer member and an inner member.

CONSTITUTION: An inner space 6 having a conical inner peripheral surface is formed between an outer member 4 and an inner member 5 and a conical, axially displaceable annular piston 7 is provided in the inner space 6. The annular piston 7 is axially moved by a pressure medium to produce a radial stress and a frictional coupling between a friction clutch 1 and a shaft 2. To determine the axial position of the annular piston 7, a transmitter cylinder 12 is used and connected via an incompressible medium to a receiver cylinder 16 in the vicinity of the outer surface of the friction clutch 1. The receiver piston 17 of the receiver cylinder 16 has an indicator 20, the position of which is sensed by an externally located sensor 21.