Abstract: A plurality of electrodes are formed, each of which is constituted by a bundle of 50 to 1,000 pieces of stainless steel fibers each having a diameter of 5 to 50.mu.. Each bundle is twisted such that the number n of twist falls within the range of 0.1.ltoreq.n.ltoreq.3. The electrodes are fixed at their base ends to a supportive frame at a right angle to the latter, such that the pitch of the electrodes is within 50 mm and the effective length of each electrode is not smaller than 3 mm. The bundles are released from twisting to obtain a larger width of each electrode at the free end of the latter than at the fixed base end of the same. In one form, the pieces of stainless steel fibers constitute warps. A plurality of groups of wefts are weamed into the warps at a right angle to the latter, such that the distance between the adjacent groups of wefts is not smaller than 3 mm. The wefts and warps are fixed at points of contacts therebetween.