A medium (24 and 124) made up of fossil fuel hydrocarbons (304) is heated by maintaining at least a portion or area of the medium (334) in an alternating current electrical field (36 and 136) provided by a radio frequency signal at a radio frequency that matches a Debye resonance frequency or frequencies of one or more components of the medium (334). As the medium (334), or as at least one individual component of the medium (334) increases in temperature, the frequency of the radio frequency signal is automatically adjusted to track changes in the Debye resonance frequency, which shifts in frequency as the temperature rises. Portions, areas and/or individual chemical compositions of the medium (334) can be heated, by the use of grid electrodes (22, 22, 120, 124), at different rates to assure uniform temperature increases or to achieve a particular desired warming pattern.