<PICT:0401566/III/1> <PICT:0401566/III/2> <PICT:0401566/III/3> <PICT:0401566/III/4> <PICT:0401566/III/5> <PICT:0401566/III/6> In an electrical precipitator having vertical collecting electrodes of which the upper ends pass through a horizontal header, the electrodes preferably being washed by liquids, means are provided for altering the position of the collecting electrodes in the header both vertically and horizontally. A gas and liquid tight flexible seal, preferably of a thermoplastic material such as asphalt, is provided. This construction enables the levels of the upper ends of the collecting electrodes to be adjusted to obtain an even flow of washing liquid over each electrode. The apparatus comprises a casing 1 containing a series of pipe-type collecting electrodes 5, and corresponding discharge electrodes 6 which are suspended from bus-bars 8 and held in position by a frame 12 and weights 11. The lower ends of the discharge electrodes are covered with shrouds 10 to prevent arcing due to the concentrated field at the lower ends of the pipes 5. The collecting electrodes pass through a top header 13 and are positioned at the bottom by a lattice framework of bars 16, 17 having adjusting screws 18 to position the pipes. The space about the tops of the pipes above header 13 is used as a trough for the wash water which is continuously supplied, and the edges of the pipes, acting as weirs, maintain the water at a definite height, excess flowing down and washing the internal surfaces of the electrodes 5. To simplify the adjustment of the tops of the pipes to a common level, the trough is subdivided by barriers 26, Fig. 3, forming a number of small ponds, to each of which water is supplied by pipe 21 connected to manifolds 23 supplied by vertical pipes 24. The supply of water to each pond is controlled individually by valves 22 and the water is led under the surface of the feedwater standing in the ponds by vertical pipes, Figs. 1 and 4. The collecting electrodes are supported in the header by means of screws 41 projecting through a flange 40 which is attached to the tube 5. The ends of the screws rest on the header plate 13, the flange 40 just fitting in the holes provided in the header. The pipes are first adjusted vertically by the screws 18 in the lattice framework near the bottoms of the pipes, and the top edges 5a then levelled by screws 41, this operation being done for each set of pipes segregated by barriers 26. The joint is sealed with a layer 42, of asphalt and asbestos fibres, covered by a layer 43 of soft asphalt. The top edge and inner surface of the pipes are cleaned. e.g. with caustic soda, and liquid allowed to flow into the trough, final adjustment of the levels of the electrodes being made by screws 41. The supply of water to each trough is preferably identical, giving an even film of water over the electrodes if the number of electrodes in each trough is identical. The supply may be checked by the water control shown in Fig. 6. A three-way valve 53 is provided between the pipes 21 and their control valves 22, thus enabling the water supply, after setting the valve 22, to be diverted to the atmosphere through pipe 54 and measured. An auxiliary water supply through pipe 56 controlled by valve 57 is also provided to flush the electrodes to remove sediment without disturbing the normal water supply. Liquids other than water may be used for washing the electrode surfaces, for example, hydrocarbon oils and other organic liquids possessing a vapour pressure higher than that of water may be used for the primary cleaning of blast-furnace gases. Wetting agents, such as sodium silicate, may also be added. If it is desired to avoid the use of sealing materials, the electrode 49, Fig. 7, may be bolted to the header 46 by flange 45 with a gasket 47 to make a liquid tight joint. Adjustment of the level of the top of the electrode may be made by altering the cap 48, which is a friction fit on the pipe 49. Alternatively the flange 40a, Fig. 9, may have projecting edges 59 which are slightly greater in diameter than the hole in the header 13, and which are forced into position by screws 41b. Other screws 41a, bearing on the header 13, are provided to adjust the level of the top of the electrode. Specification 393,906 is referred to.