To prolong the service life of an electrically insulated piston valve by making the potential of a control electrode equal to the potential of the liquid in a liquid outlet.

The control electrode 2 is arranged on a prolonged part 11 of a cylinder 5. The metallic prolonged part 11 is formed to conform with the shape of the control electrode 2. Then, the control electrode 2 is electrically connected with the liquid 8a at the outlet 9. In the case that the prolonged part 11 is not made of metal but made of an electrically insulated material e.g. glass, the control electrode 2 is placed to be electrically connected between the liquid 8a and the electrode 2 and is welded. The shape of the electrode has at least about 3mm radius of curvature. Thus, the control electrode 2 is always at the potential of the outlet side liquid 8a and lowers the electric field intensity on the valve.