A synergistic biocidal composition comprising a mixture of 2-propenal polymer and 5-chloro-2-methyl-4-isothiazoline-3-one and 2-methyl-4-isothiazoline-3-one. A controlled bias voltage that is applied between the first element of the sensor and a second element of the sensor is then modified to substantially maintain the parasitic DC bias voltage at a desired potential. The channel is formed by filling a pipe line with a packing, and the tip is exposed from the pipe line of the channel. Further, the gate wiring pattern which is constituted of the gate lines GL1, GL2 and the gate-line pull-around lines GLL1, GLL2 and the storage wiring pattern which is constituted of the storage lines STL and the common lines B2, B3 do not cross each other.