Abstract: Apparatus for heating hydrocarbons, and the like, in an underground reservoir for the purpose of lowering the viscosity of the substance in the reservoir and facilitating the flow thereof has a flow channel arrangement disposable in a reservoir of the material to be treated for forming a flow path for a working fluid that is heated by a heating system associated with the flow channel arrangement. The heated working fluid is then discharged from the flow path and into the reservoir of material to be heated for the purpose of lowering the viscosity and improving the flowability of the material in the reservoir.The heating system includes an electrical resistance heating unit comprising an electrical circuit including at least one pair of opposed electrodes electrically connected in series and arranged along and partially forming the flow path for the working fluid, with the portion of the electrical circuit between the electrodes being completed through the working fluid.