show a cylindrical pipe piece 1 which as fig2 shows is provided on either side with coupling flanges 2 and which is manufactured from metal or other electrically conductive material an annular recess 3 extending all the way round is arranged in the inner surface of the pipe piece a curved metal measuring electrode 4 extending through an arc of less than 360 u00b0 for instance 270 u00b0 is placed in this annular recess 3 measuring electrode 4 has a smaller width than recess 3 the cylinder encircling measuring electrode 4 also has a smaller external diameter than recess 3 and has a larger internal diameter than the internal diameter of pipe piece 1 measuring electrode 4 can thus be placed concentrically with pipe piece 1 into recess 3 of pipe piece 1 the metal measuring electrode 4 is fixed in this position because it is enclosed by a layer 5 of electrically insulating material such as a plastic for instance a moulding resin this layer 5 fully encloses the metal measuring electrode so that measuring electrode 4 is electrically insulated relative to pipe piece 1 and the interior of pipe piece 1 this measuring electrode functioning as second measuring electrode 4 is connected by means of a connecting wire 6 to a measuring circuit to be elucidated hereinbelow a substantially conical body 10 is further placed in the interior of pipe piece 1 concentrically thereto conical body 10 is.