Phonopore and phonoplex F.D.M.telegraph systems used on railways in the late 19th century

The phonopore and the phonoplex were both systems introduced in 1885 for providing additional telegraph channels (or, alternatively, in the phonopore system, a telephone channel) in the frequency range above that of the ordinary d.c. or Morse channels. The phonopore, developed by Charles Langdon-Davies in England, used a special kind of coupling transformer/condenser, which is here analysed but shown to have had few useful special properties, and used a `harmonic¿ telegraph channel, i.e. a keyed oscillatory signal. Edison´s phonoplex used, for the highpass channel, signals derived from the transients produced by an induction coil. Both systems were made commercially, and had a certain success on railways. The Phonopore Syndicate (later Company) had an interesting but not successful history, here partially unravelled; from 1893, its managing director was C.E. Spagnoletti, F.R.S., Past-President of the Society of Telegraph Engineers, and newly-retired telegraph superintendent of the Great Western Railway. Although the company had, by 1895, raised around £100 000 of capital, it was in that year effectively purchased by the New Phonopore Telephone Co. for £2000. The new company continued in existence, with an issued capital reaching no more than about £8000, until it was purchased for £175 in 1916 by the Phonopore Construction Co., which finally ceased operations soon after 1920.