flexible high tension lead 44 connects the strips 271 to the negative terminal of a high voltage source 45 a pair of pulleys 46 is mounted on a transverse shaft 47 at each end of the vessel 40 and a pair of traction wires or cables 48 connected at each end to the electrostatic shields 28 of the end strips 271 are led over the pulleys 46 to form drive means one of the shafts 47 passes through the wall of the vessel 40 and is connected via a variable amplitude oscillatory motion device 49 to an electric motor 50 each of the strips 271 is hollow as shown in fig3 to 5 its interior being filled with cooling water which is supplied through a flexible pipe 52 which enters near one end of the strip the water leaves through a second flexible pipe 51 near the other end of the strip 271 the pipes 51 52 connect the strips 271 in series but the pipes extending between the adjacent strips have been omitted from fig1 for clarity the high tension lead 44 from source 45 is of the co axial cable type the outer conductor being connected to the shield 28 and earthed similar cables 44 connect the strips 271 to one another above the horizontal guide rails 42 a pair of horizontal support rails 53 only one of which is shown are secured to opposite sides of the vessel 40 to support a substrate 31 which is to.