Casson, Herbert N. The History of the Telephone (public domain electronic copy). pp. Chapter IV – via Gutenberg.org. 'At last', said the delighted manager [J.J. Carty, Boston, Mass.], 'we have a perfectly quiet line.'
VLF, LF, and MF communications systems(PDF) (manual). Naval Shore Electronics Criteria. Washington, DC: U.S. Navy, Naval Electronics Systems Command. August 1972. pp. 4.28 –4.30. NAVELEX 0101-113 – via navy-radio.com.