Sentence:  The efforts and accomplishments fall into three main categories : absolute thermometry based upon the velocity of sound in helium gas , secondary thermometry involving principally studies of the <WSD>behavior</WSD> of germanium resistors , and helium 4 vapor-pressure measurements ( see p. 144 ) . 
Meaning of the word marked with <WSD> tags:  the action or reaction of something (as a machine or substance) under specified circumstances