Noises in the Earth

The early telegraph pioneers encountered major problems with cables, so there was a strong incentive for them to minimise the amount of cabling required. Fortunately, the use of an “earth return” meant that one wire (rather than two) was sufficient to carry a telegraph circuit between two locations. A good electrical connection to earth was required at each end of the line to provide the return path for the telegraph circuit, and this could be achieved by connecting to a lump of metal that was buried in damp ground or submerged in a stream.

The idea of an earth return is often surprising to people who have not encountered it before. We tend to expect electrical conductors to be made from metals such as copper, and the surface of the Earth is not noticeably metallic. Certainly, if a copper wire is replaced with an equivalent volume of wet soil, the result would probably turn out to be a rather poor conductor of electricity. However, the Earth is a very large lump of matter, so the return path is made up of a huge number of separate electrical paths; since these paths are connected in parallel, they combine to produce a low resistance path.

The diagram below, which comes from Alexander Graham Bell’s 1876 telephone patent, shows that he anticipated that the telephone would use an earth return in the same way as the telegraph had done (Boxes E and g represent connections to earth at the transmitter and receiver respectively).

Diagram from Bell Telephone Patent

Diagram from Bell Telephone Patent

However, early telephone users found that noise on the line was a major problem. This graphic description comes from a history of the telephone written by Herbert N. Casson in 1910:

“Noises! Such a jangle of meaningless noises had never been heard by human ears. There were spluttering and bubbling, jerking and rasping, whistling and screaming. There were the rustling of leaves, the croaking of frogs, the hissing of steam, and the flapping of birds’ wings. There were clicks from telegraph wires, scraps of talk from other telephones, and curious little squeals that were unlike any known sound. The lines running east and west were noisier than the lines running north and south. The night was noisier than the day, and at the ghostly hour of midnight, for what strange reason no one knows, the babel was at its height.” 

After considerable experimentation, it was found that the noise was caused by the use of an earth return. Since the telephone was an analogue device, it was far more vulnerable than the telegraph to electrical noise. Telephone engineers were eventually forced to the conclusion that, despite the huge additional cost, the only way to build a satisfactory telephone network would be to use conventional wires for both halves of each circuit. 

 

This entry was posted in Telecom Ramblings and tagged , , , , , . Bookmark the permalink.

Comments are closed.