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 behavior of germanium <WSD>resistors</WSD> , and helium 4 vapor-pressure measurements ( see p. 144 ) . 
Meaning of the word marked with <WSD> tags:  an electrical device that resists the flow of electrical current