is arranged parallel to a selected region of this baffleplate with the aforementioned horizontal distance therebetween the third electrode plate 30c too is smaller in surface area than this baffleplate 20b and is arranged parallel to another selected region of the same baffleplate with the aforementioned horizontal distance therebetween as shown in fig4 and 5 these electrode plates 30a 30b and 30c are both mechanically and electrically connected to one another by an assembly of elongate bus bars 38 which are fixed to lower end regions of the electrode plates 30a 30b 30c furthermore each electrode plate 30a 30b 30c is mechanically fastened to the allotted and oppositely located baffleplate 20a or 20b at a plurality of points as illustrated in fig3 but the fastening is made such that each electrode plate is electrically insulated from the opposite baffleplate it is also important to realize the intended horizontal distance between the baffleplate and the electrode plate fastened thereto to satisfy these requirements it is preferable to accomplish the fastening by using an electrically insulating spacer 32 and an additional insulator 36 in combination with an ordinary fastening means 34 such as a rivet or a machine screw at each fastening point although it is also possible to dispose spacers separately from the fastening means 34 fig6 is an illustration of a preferred structure for combining the spacer 32 and the insulator 36 with each rivet 34 used as the fastening means at each fastening point the baffleplate 20b for.