Sentence:  The efforts and accomplishments fall into three main categories : absolute thermometry based upon the velocity of sound in helium gas , <WSD>secondary</WSD> thermometry involving principally studies of the behavior of germanium resistors , and helium 4 vapor-pressure measurements ( see p. 144 ) . 
Meaning of the word marked with <WSD> tags:  being of second rank or importance or value; not direct or immediate