are arranged in such a way that one circumferential surface is always tangential to the first electrode 1 and consequently always forms a respective discharge surface 6 or 7 the air exhausting pipe 5 comprises an aluminum or other material shaped member 8 and is open towards the single electrodes 3 4 at the end remote from the single electrodes 3 and 4 the pipe 5 is screwed by means of screws 9 and 10 to a header 11 a plurality of holes 12 in the header 11 and shaped member 8 make it possible for air to pass from the air exhausting pipe 5 into the header 11 two insulators 15 16 made from electrically non conductive material are fixed by means of respective screws 13 14 to the longitudinal sides of the shaped member 8 over the length of the single electrodes 3 4 and between the insulators 15 16 a plurality of spacers 17 are provided having recesses 18 19 on either side for receiving the electrodes 3 4 and projecting between the latter by a web 20 claws 22 23 serve respectively to hold the single electrodes 3 4 in the recesses 18 19 to this end the claws 22 23 are constructed in angular manner and in each case one leg 22a or 23a engages externally with the insulator 14 or 16 whilst the other leg 22b or 23b engages on the electrode 3 or 4 respectively screws 24 serve to hold the.