The evaporant is placed on/in a refractory metal source through which
a high electrical current flows. The method’s low equipment costs are balanced
by the difficulty of maintaining a constant evaporation rate (when compared
with other techniques) and possible reactions between evaporant and boat
material. These devices are often called thermal sources and are essentially
open heaters frequently without thermal radiation shielding, insulation, or any
tricks to reduce thermal gradients.