| 摘要 |
Two ionization chambers are formed on the sensor, one including a pair of electrodes and ionizing substance, which are relatively so located that, in the space between the electrodes, the air will be ionized with both positive and negative ions so that the current flowing between the electrodes will be generated by ions of both polarities to form a bipolar ionization chamber; the other has the electrodes and the ionizing substance so located that at least in a portion, preferably a major portion of the space between the electrodes, ions of only a single polarity will occur. The bipolar chamber is shielded from air flow therethrough, the unipolar chamber, however, being exposed to free air flow. Preferably, a third or reference chamber may be combined with the sensor. The unipolar and bipolar chambers are connected to an evaluation circuit which responds when the current between the electrodes in at least one of the unipolar or bipolar chambers drops below a certain threshold value.
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