at the negative potential increases with expansion of the sheath and a ratio at two arbitrary points vp 1 and vp 2 can be represented as follows hence so far as both vp 1 and vp 2 at their constant values are imparted to the probe this ratio maintains a constant value to be determined only by the type of gas however when the contaminant film is produced on the surface of the probe the electron current in particular the one flowing through the probe decreases remarkably due to lowering of its electrical conductivity as indicated by ip 1 and ip 2 in fig5 accordingly at the start of the measurement the value of the current ratio of the probes in their clean condition ie vp 1 ii vp 2 is measured and stored and any decrease in the current ratio thereafter is measured whereby it is possible to know the cleanliness of the probes quantitatively no sooner the measuring probes will depart from a certain permissible range of cleanliness as established than is imparted a sufficient quantity of the negative voltage so as to remove the contaminant film by the ion bombardment upon restoration of the current ratio to its initial value as memorized the measurement is resumed by repetition of these operations it becomes possible to perform the continuous measurement of the probes while maintaining their cleanliness in the reactive plasma within a certain definite value at a relatively high pressure under which no sheath producing conditions can be established the current ratio ieo iio becomes dependent on a temperature.