Sentence:  Carbon resistors and impurity doped germanium resistors have been investigated for use as precision secondary thermometers in the liquid <WSD>helium</WSD> temperature region . 
Meaning of the word marked with <WSD> tags:  a very light colorless element that is one of the six inert gasses; the most difficult gas to liquefy; occurs in economically extractable amounts in certain natural gases (as those found in Texas and Kansas)