a desired position placing the two half tubes in a position of mutual proximity so as to bring into contact the outer periphery of the probe with the inner wall of the outer tube in one embodiment the probe 4 further comprises at least one fastening leg 3 located in a space 20 separating the two half tubes from each other this space 20 takes the form of a longitudinal slot when the two half tubes are in the position of mutual proximity this space exists on both sides of the probe and thus two longitudinal slots separate the two half tubes in the position of mutual proximity this space is wider when the two half tubes are in a position away from one another as diagrammatically shown in the figures the leg 3 is preferably in the form of a rectangular plate but may be of any shape with a relatively small thickness and housed in the space 20 while allowing the two half tubes to get closer to each other as shown by the double dotted arrows in fig3 b the leg 3 is adapted to move longitudinally within the space 20 when the two half tubes are in a position away from each other thereby moving the probe 4 relative to the central conductor 1 the leg thus allows in particular to move the probe when the.