"Reminiscences of an Old Operator" (Part III) by Arthur Leech, Radio Age, November 1924, page 29: "A mass of ships leaving or concentrating on a port like New York create an unimaginable jam, and... having a 120-cycle spark set easy to hear over the coarse 60-cycle notes of the mob, I was a favorite central station during these conditions."
United States. (1917). "Radio Communication": Hearings before the Committee on the Merchant Marine and Fisheries, House of Representatives, Sixty-fourth Congress, second session, on H.R. 19350: A bill to regulate radio communication. January 11 to 26, 1917. Washington: Govt. Print. Off., page 414.