Certain embodiments provide a system including a heater. The heater
includes one or more electrical conductors. The heater is configured to
generate a heat output during application of electrical current to the
heater. The heater includes a ferromagnetic material. A conduit at least
partially surrounds the heater. A fluid is located in a space between the
heater and the conduit. The fluid has a higher thermal conductivity than
air at standard temperature and pressure (STP) (0.degree. C. and 101.325
kPa). The system is configured to provide (a) a first heat output below a
selected temperature when time-varying electrical current is applied to
the heater, and (b) a second heat output near or above the selected
temperature when time-varying electrical current is applied to the
heater.