In 1906, Boltwood returned to Yale as an assistant professor of physics at a time when the newly discovered science of radioactivity was considered both chemistry and physics.  He would eventually become the leading American scientist in the field, and be appointed chair of radiochemistry in 1910, a position that was the first of its kind. He developed a friendship with Ernest Rutherford, whose highly influential thinking played a role in much of Boltwoods work. They communicated primarily through overseas correspondence, with the exception of a short period from 1909-1910 when Rutherford invited Boltwood to join him at University of Manchester in England. Their letters from 1904-1912 were published in 1969 in the book Rutherford and Boltwood: Letters on Radioactivity, which reveals an ongoing conversation as the two scientists work to unravel details of radioactivity and the uranium decay series.'