diameter and the section 10 tapers outwardly to an exit end 14 approximately 12 cm diameter there follows a charging section 16 and a focussing section 18 the section 18 is connected to a conical section 20 which decreases to a throat or nozzle 22 at about 1 cm diameter the length of section 10 is about 26 cm the length of section 16 is about 5 cm the length of section 18 is about 21 cm and section 20 is about 12 cm long a minor discharge passage 24 exits axially from the nozzle 22 and an annular major discharge passage 26 exits laterally from the nozzle the nozzle diameter as noted above is at x in fig2 the diameter of the minor discharge passage 24 is about 1 125 cms the major discharge passage 26 has a dimension y fig2 the dimension y decreases radially outward from centre by kr u2212 1 where k is a constant incoming fluid enters inlet 12 and flows through the diffuser section 10 and through the charging section 16 in the section 16 are a plurality of corona wires 30 which charge the particles in the fluid from the charging section 16 the fluid and charged particles flow through the focussing section 18 this section has a plurality of ring electrodes 32 electrodes.