Sentence:  A study of the <WSD>hydrogen</WSD> line profiles indicates that a measurement of these profiles can be used to calculate a temperature for the arc plasma that is reliable to about * * f percent . 
Meaning of the word marked with <WSD> tags:  a nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universe