A furnace for steam reforming a feed stream containing hydrocarbon, preferably methane, having: a combustion chamber, a plurality of reactor tubes arranged in the combustion chamber for accommodating a catalyst and for passing the feed stream through the reactor tubes, and at least one burner which is configured to burn a combustion fuel in the combustion chamber to heat the reactor tubes. In addition at least one voltage source is provided which is connected to the plurality of reactor tubes in such a manner that in each case an electric current which heats the reactor tubes to heat the feedstock is generable in the reactor tubes.