Earth-return telegraph

Earth-return telegraph is the system whereby the return path for the electric current of a telegraph circuit is provided by connection to the earth through an earth electrode. Using earth return saves a great deal of money on installation costs since it halves the amount of wire that is required, with a corresponding saving on the labour required to string it. The benefits of doing this were not immediately noticed by telegraph pioneers, but it rapidly became the norm after the first earth-return telegraph was put into service by Carl August von Steinheil in 1838.

Earth-return telegraph began to have problems towards the end of the 19th century due to the introduction of electric trams. These seriously disturbed earth-return operation and some circuits were returned to the old metal-conductor return system. At the same time, the rise of telephony, which was even more intolerant to the interference on earth-return systems, started to displace electrical telegraphy altogether, bringing to an end the earth-return technique in telecommunications.

A telegraph line between two telegraph offices, like all electrical circuits, requires two conductors to form a complete circuit. This usually means two distinct metal wires in the circuit, but in the earth-return circuit one of these is replaced by connections to earth (also called ground) to complete the circuit. Connection to earth is made by means of metal plates with a large surface area buried deeply in the ground. These plates could be made of copper or galvanised iron. Other methods include connecting to metal gas or water pipes where these are available, or laying a long wire rope on damp ground. The latter method is not very reliable, but was common in India up to 1868.

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. It is only necessary to ensure that there is good contact with the Earth at the two stations. To do this, the earth plates must be buried deep enough to always be in contact with moist soil. In arid areas this can be problematic. Operators were sometimes instructed to pour water on the earth plates to maintain connection. The plates must also be large enough to pass sufficient current. For the ground circuit to have a conductance as good as the conductor it replaces, the surface area of the plate is made larger than the cross-sectional area of the conductor by the same factor as the resistivity of the ground exceeds the resistivity of copper, or whatever other metal is being used for the wire.

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. This advantage was not so apparent in early telegraph systems which often required multiple signal wires. All of the circuits in such a system could use the same single return conductor (unbalanced lines), so the cost saving would have been minimal. Examples of multiwire systems included Pavel Schilling's experimental system in 1832, which had six signal wires so that the Cyrillic alphabet could be binary coded, and the Cooke and Wheatstone five-needle telegraph in 1837. 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.

The expense of multiwire systems rapidly led to single-signal-wire systems becoming the norm for long-distance telegraph. Around the time earth return was introduced, the two most widely used systems were the Morse system of Samuel Morse (from 1844) and the Cooke and Wheatstone one-needle telegraph (from 1843). A few two-signal-wire systems lingered on; the Cooke and Wheatstone two-needle system used on British railways, and the Foy-Breguet telegraph used in France. With the reduction in the number of signal wires, the cost of the return wire was much more significant, leading to earth return becoming the standard.

Sömmerring's telegraph was an electrochemical, rather than an electromagnetic telegraph and is placed out of chronological order. It is shown here for comparison because it directly inspired Schilling's electromagnetic telegraph, but Schilling used a greatly reduced number of wires.

The first use of an earth return to complete an electric circuit was by William Watson in 1747 excluding experiments using a water return path. 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. Later that year he increased that distance to two miles. One of the first demonstrations of a water-return path was by John Henry Winkler, a professor in Leipzig, who used the River Pleisse in this way in an experiment on 28 July 1746. The first experimenter to test an earth-return circuit with a low-voltage battery rather than a high-voltage friction machine was Basse of Hameln in 1803. These early experiments were not aimed at producing a telegraph, but rather, were designed to determine the speed of electricity. In the event, the transmission of electrical signals proved to be faster than the experimenters were able to measure – indistinguishable from instantaneous.