Abstract:
Apparatus 30 for detecting the presence of more than a negligible concentration of dust or other particles in a moving stream of air or other gas (31), comprising first (1) and second (2) electrically conductive members spaced apart in a first region (A') of the stream (31), means (32) for providing thereto electrical potentials sufficiently different to provide a potential gradient of at least about 30 kilovolts per centimeter in a limited region adjacent to the surface of one of the conductive members (1 or 2), to provide a corona and thus to produce ions that charge a substantial proportion of any particles in the gas (31) passing through the first region (A'); and passive means (33) in a second region (B") of the stream (31) that is downstream from the first region (A'), for detecting the presence of a detectable concentration of charged particles in the gas (31) while present in the second region (B"). The passive means (33) typically comprises third (3) and fourth (4) electrically conductive members spaced apart in the second region (B"), means (34) for providing thereto electrical potentials of such difference as to provide a potential gradient of substantially less than about 30 kilovolts per centimeter therebetween, and means (35 or 36) for detecting any measurable current flowing to the third member (3) or the fourth member (4).