Samuel F. B. Morse, Improvement in the Mode of Communicating Information by Signals by the Application of Electro-Magnetism, U.S. Patent 1647, June 20, 1840; see page 4 column 2
See for example, Frank Wood's Telegraph Register, U.S. Patent 338,329, Mar. 23, 1886.
William F. Singer, Electrical Automatic Fire-Alarm System, U.S. Patent 436,640, Sept. 16, 1890; see page 3 line 48
Bernice J. Noyes, Electric Signalling Apparatus, U.S. Patent 534,908, Feb. 26, 1895; see page 1 lines 82-83.