Abstract: A hydraulic cylinder has a piston (13) with a seal (21) in sealed sliding contact within a bore (12) to define a working chamber (16) ahead of the piston for the discharge of pressurised fluid from the bore via an outlet (17) when the piston is undertaking working stroke, an inlet (20) is provided into the bore from an associated reservoir to allow connection between the reservoir and the working chamber when the piston is in a fully retracted state for recuperation of fluid from the reservoir to the working chamber. The connection between the reservoir and working chamber is provided via a sleeve (27) in the bore (12) winch includes axially extending grooves (28), the grooves being closed by the piston seal (21) during a working stroke of the cylinder but the ends (30) of the grooves being open when the piston (13) is in a fully retracted condition within the sleeve thus by-passing the piston sleeve to connect the working chamber and reservoir.

Abstract: A method and apparatus for determining the impedance of the discharge in a plasma reactor system comprising a tuning box having variable capacitors is described. The impedance of the discharge is determined in dependence on a pre-established relationship between the operational positions of the variable capacitors on the one hand and the impedance of the discharge on the other. The ionic current relative to the discharge in the reactor also may be determined by taking into account the high frequency voltage of the reactor.