having a switch 6 between the electric connection 7 of one of the electrodes 5 while the other electrode 5 has a connection 8 to ground such that with said switch 6 automatically actuated in the closing and opening by a control signal 9 the passage of the current coming from the current source 3 is activated and deactivated this way each electrode 5 always has the same configuration the anode being that through which the current coming from the current source 3 enters and the cathode being that which receives the current after passing through the medium in which the electrodes 5 are found in this configuration it is further envisaged that the other switch 10 for disconnecting the electrode 5 that is connected to ground is included with the goal of preventing in the case of a difference in the ground potential of the filter of application and the ground potential of the sensor a current from flowing between said electrode 5 connected to ground and the metal casing of the filter causing a continual ageing of that electrode 5 connected to ground even if the other electrode 5 is disconnected by the switch 6 the switching bridge 4 may also have a configuration for the operation of the sensor with alternating current according to the example in fig4 having an h.