During World War I, Smyth worked in the Chemical Warfare Service, and at the Aberdeen Proving Ground. After earning his second PhD, he returned to Princeton for the last year of his NRC fellowship. During his early years on the Princeton faculty he lived in the Graduate College west of the main campus. He was appointed an instructor in 1924, an assistant professor in 1925, an associate professor in 1929, and a full professor in 1936. In 1935 he became chairman of the Department of Physics, a position he held until 1949. During 1931–32 he was a Guggenheim Fellow at the University of Göttingen, where he studied the spectra of triatomic molecules, particularly carbon dioxide, with James Franck.
