With indirect resistive heating, a crucible holds the evaporant in isolation
from the heater filament. The increased thermal mass and the evaporant/heater
separation stabilizes the evaporation rate. Various iterations of these indirectly
heated devices are called Knudsen cells, K-cells, effusion sources, diffusion
cells, evaporation cells, etc. But distinctions between these names are blurring
as manufacturers explore designs that use features from several devices