A resistance testing apparatus makes use of a modular design for cascaded, parallel, bipolar current sources to obviate the need for electromechanical or pneumatic switching systems. The method provides for the removal of heavy sulfur compounds including disulfides and mercaptans from pre-treated olefinic hydrocarbon feedstocks used in cumene synthesis to improve the life and efficiency of catalysts used in the cumene manufacturing process. An engine assembly includes first and second piston assemblies mounted back-to-back and 180\xb0 out of phase.