A vertical sucker rod string formed of lengths of sucker rods, each having means for attachment at each end thereof to adjacent sucker rods, is provided with electric heating means for maintaining the temperature of the crude oil being pumped to the surface above a preselected level to prevent the congealation of paraffin. An electric heating element coextensive in length with the rod is disposed in a small diamater bore hole in each rod. A solid heat conducting electrical insulation completely fills the bore hole in the rod around the heating element. Couplings are provided between adjacent sucker rods to obtain electrical continuity between the heating element in each of the adjacent rods. A terminal coupling provides a connection between the heating element of the lowermost sucker rod and the sucker rod, whereby the sucker rod string serves as a conductor to complete the circuit for energizing the heating elements.
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