s2 of sensor element and the equivalent resistance u03b3 of said first electrode plate 2 which inherently functions as an electrode and which assumes a sufficiently low value as compared with the equivalent resistances r s1 and r s2 of the sensor element therefore the substantial equivalent resistance between connection points a and b may be regarded as a parallel resistance of equivalent resistances r s1 and r s2 of the sensor element and this may substantially be considered to be the impedance r s of the sensor element let it now be assumed that there is no influence from the first power source 10 then the output voltage obtainable across the resistor 14 is equ1 where r o is the resistance value of resistor 14 the impedance r s of sensor element 1 can thus be determined by measuring the voltage v s of the second power source 11 and the output voltage v o while the above expression 1 is predicated on the assumption that there is no influence of said first power source 10 the actual requisite for no appearance of an electric signal due to v h in the output voltage v o is that the.