The object of the invention is to provide a reliable and inexpensive combination of a chamber and a piston to be used in any device where such a combination is needed so that it complies with specific demands towards the operation force for e.g. pumps, specifically manually operated pumps. By a device comprising a chamber and a piston positioned inside the chamber said chamber and said piston relatively movable to each other in a predetermined direction of movement between a first position and a second position of which the cross-section of the chamber in a plane perpendicular to the direction of movement is larger at the first position than at the second position, the change in the cross section of the chamber is essentially continuous between the first position and the second position and the cross-section of the chamber. It is further possible that the piston has a fixed geometrical shape, that the wall of the chamber has different sizes of cross-sections in the direction of the movement and is arranged to adapt itself to the piston. Moreover, both the piston and the wall of the chamber can adapt itself to each other.