Apparatus for efficient heating of subterranean earth, which includes a heater that is operable within a preselected operating temperature range and a heat transfer means interposed between the heater and a subterranean earth for transferring heat from the heater to the subterranean earth, the heat transfer means configured for down-hole insertion into a well, the heat transfer means including a container and a heat transfer metal within the container, the heat transfer metal characterized by a melting point temperature lower than the preselected operating temperature range and a boiling point temperature higher than the preselected operating temperature range.