the hollow cylindrical holder members 11 each have a center axle hole 111 a perforated drum 2 is revolvably coupled between the hollow cylindrical holder members 11 the perforated drum 2 comprises two end walls 22 and 23 a perforated cover panel 21 hinged thereto and adapted to close an opening thereof the perforated cover panel 21 has pores 211 the end walls 22 and 23 each have a center coupling hole 221 or 231 respectively coupled to the hollow cylindrical holder members 11 a perforated titanium center contactor or conduit 3 is inserted through the center axle hole 111 of each hollow cylindrical holder member 11 the center contactor or conduit 3 has pores 31 extended through the peripheral wall thereof and two sliding slots 32 longitudinally and bilaterally extended to one end thereof two plate electrodes namely the first plate electrode 25 and the second plate electrode 26 are respectively mounted on the center contactor or conduit 3 and disposed inside the perforated drum 2 the second plate electrode 26 is fixedly fastened to one end wall 23 of the drum 2 the first plate electrode 25 is mounted on a slide 251 which is slidably mounted on the center contactor or conduit 3 the slide 251 comprises a center shaft 252 inserted into the center contactor or conduit 3 a screw hole 253 axially extended through the central axis of the center shaft 252 and two radial ribs 254 radially extended from the periphery of the center shaft 252 at two sides and respectively perpendicularly inserted through the longitudinal sliding slots 32 because the radial ribs 254 of the slide 25 are respectively perpendicularly inserted through the longitudinal sliding slots 32.