Hackman, Christine; Sullivan, Donald B. (26 August 1994). "Resource Letter: TFM-1: Time and frequency measurement"(PDF). Time and Frequency Division. National Institute of Standards and Technology, Boulder, Colorado 80303: 1–2. Retrieved 5 January 2016.
patents.google.com
Nicholson, Alexander M. Generating and transmitting electric currentsU.S. patent 2,212,845, filed April 10, 1918, granted August 27, 1940