Pioneer in the development of differential analyzers, Hazen collaborated with Vannevar Bush at MIT to construct the first widely practical such machine, built 1928–1931 and comprising six mechanical integrators.
In the same year, Bush described this machine in a journal article as a "continuous integraph". When he published a further article on the device in 1931, he called it a "differential analyzer". In this article, Bush stated that "[the] present device incorporates the same basic idea of interconnection of integrating units as did [Lord Kelvin's]. In detail, however, there is little resemblance to the earlier model." According to his 1970 autobiography, Hazen's collaborator was "unaware of Kelvin's work until after the first differential analyzer was operational."
