848,687. Electrostatic precipitation. LUEDER, H. Sept. 6, 1956 [Sept. 6, 1955], No. 27369/56. Class 39(1). Apparatus for imparting uni-polar charges to solid or liquid particles suspended in a gas flowing through a passage, for use in electrical precipitators, comprises a beta-ray emitter in the form of a radioactive elongated member which is mounted across the middle of the passage transversely to the direction to the flow of gas and having a length substantially greater than the radius of influence (as defined) of its beta-radiation, the radiation emitter producing an ionising field around at least the downstream side of the emitter, and means for creating an electrostatic field in the passage in a region extending from the emitter downstream a distance at least equal to the radius of influence of the ionising radiation, the electrostatic field having throughout this region a substantial component in the direction of gas flow which does not change sign. The radioactive elongated member can comprise a strip to one side of which a coating of radioactive material is applied, the coating preferably comprising a metal on which tritium has been adsorbed. The member may emit radiations in all directions. In the theoretical embodiment shown in Fig. 1, the gas carrying the particles to be charged flows in the direction of the arrows through a passage 3 bounded by insulating walls 1 and 2. A radioactive member in the form of a wire 4 emitting beta-radiations in all directions is disposed in the middle of the passage 3. A cylindrical grid 5 surrounds the wire 4 coaxially and has a negative voltage in relation to the wire, the wire 4 and the grid 5 constituting the electrodes of a cylindrical electrostatic field. In the construction shown in Figs. 2 and 3, a gas passage 7 is bounded by two walls 8 and 9 of conducting material and two walls 15 of insulating material, and metal grids 10 and 11 are provided between which a radioactive member 12 is situated. A wide mesh auxiliary electrode or wire coil 14 is provided around the radioactive member 12 at a distance that is small in relation to its radius of influence and has the same potential as the member 12 or a potential which differs from it only slightly in the direction of the potential gradient of the electrostatic field. In the construction shown in Fig. 4, the gas flows through the grid electrode 19 into the passage 16 defined by the walls 17 and 18 of poorly conducting material and leaves through the grid electrode 20, the elongated radioactive member 21 being connected by a wire 22 with a source of D.C. voltage 23. The walls 17 and 18 are provided with narrow conductive bands 24 and 25 connected to an intermediate point 26 of the source 23. The grid electrodes 19 and 20 are connected to the end of the source 23. In the constructions described above a cylindrical electrostatic field is produced whereas in the construction shown in Fig. 5, a homogeneous field is produced. Here the gas passage 27 is defined by walls 28 and 29 of poorly conducting material and two grid electrodes 31 and 32 of opposite potential, the radioactive member 30 being in the form of a narrow band radiating only in the downstream direction and mounted on the electrode 31. The walls 28 and 29 can be of glass or plastic which may be made slightly conductive on the inner surface by coating with a semi-conducting layer, and the electrodes 31 and 32 are in good contact with the walls 28 and 29 by metallising or graphiting the areas of contact. An auxiliary electrode 33 in the form of a grid 33 and a mesh coarser than that of the grid 31 is positioned close to the radioactive member 30 and is given a potential differing only slightly from that of the electrode 31. The down-stream grid electrode can be omitted and the walls extended in the direction of gas flow, and provided with electrodes on the walls. For this purpose parts of an electrode of a separate series-connected separator can be used. Fig. 6 shows a form of radioactive member comprising a spiral 34 wound round a wire 34 and Fig. 7 shows a form comprising a narrow twisted band carrying a radioactive layer 36 only on one side. The radioactive member need not form part of the upstream electrode surface but if there is no electrical connection between them a charge on the radioactive member will occur which will influence the electrostatic field. The radioactive member may be fixed at its ends or provided with holding devices on its upstream side or provided with holding thin wires which run transversely to the gas flow. Two or more of the devices shown in Figs. 3, 4 or 5 can be disposed side by side with one passage wall serving as a partition wall between two passages. In the construction shown in Fig. 8, the gas flows through three annular passages 38, 39 and 40 in the axial direction, the radioactive members 42 being in the form of rings held midway between the passage walls 41. In the construction shown in Fig. 9, a single spiral passage 43 is bounded by walls 44 and contains one radioactive member 45. In the construction shown in Fig. 10, a passage 46 is formed by two parallel flat annular walls 47 and the radioactive member is in the form of a wire ring 48, the gas flowing through the passage in the radial direction either inwards or outwards. Electrode bands 49, 50 and a cylindrical grid electrode 51 are provided. Several passages 46 can be arranged one above the other as shown, and a common outlet 52 provided.