Earth-return telegraph

Connection to earth is made by means of metal plates with a large surface area buried deeply in the ground.

Other methods include connecting to metal gas or water pipes where these are available, or laying a long wire rope on damp ground.

[1] Soil has poor resistivity compared to copper wires, but the Earth is such a large body that it effectively forms a conductor with an enormous cross-sectional area and high conductance.

[4] The advantage of the earth-return system is that it reduces the amount of metal wire that would otherwise be required, a substantial saving on long telegraph lines that may run for hundreds, or even thousands, of miles.

The latter did not require a return conductor at all because the five signal wires were always used in pairs with opposite polarity currents until code points for numerals were added.

Watson, in a demonstration on Shooter's Hill, London, sent an electric current through 2,800 feet of iron wire, insulated with baked wood, with an earth-return path.

[20] One of the first demonstrations of a water-return path was by John Henry Winkler,[note 1] a professor in Leipzig, who used the River Pleisse in this way in an experiment on 28 July 1746.

The demonstration was an attempt to get backing for construction of a New York to Philadelphia line, but the project was unsuccessful (and is unlikely to have worked over a long distance), Dyar was quickly forgotten, and earth return had to be reinvented yet again.

[28] Steinheil realised that the "galvanic excitation" in the earth was not confined to the direct route between the two ends of the telegraph wire, but extended outwards indefinitely.

[29] The use of earth-return circuits rapidly became the norm, helped along by Steinheil declining to patent the idea – he wished to make it freely available as a public service on his part.

[30] However, Samuel Morse was not immediately aware of Steinheil's discovery when he installed the first telegraph line in the United States in 1844 using two copper wires.

The starting and stopping of the trams generated large electromagnetic spikes which overwhelmed code pulses on telegraph lines.

[35] Sensitive instruments like the syphon recorder were used to detect weak signals on long submarine cables, and they were easily disrupted by trams.