Views: 1 Author: Site Editor Publish Time: 2023-04-28 Origin: Site
The 2-wire metallic circuit was developed and patented by Alexander Graham Bell in 1881.
The first telecommunications lines were mostly overhead, placed on telegraph poles (Telegraph Pre-dated Telecommunications) or attached to Fittings on rooftops. These were single grounded wires made of iron or steel. Over time wires were galvanized for corrosion resistance, but corrosion problems were never the less prevalent, and these wires were inherently noisy. Phosphor bronze wires and compound copper steel wires were made in attempts to decrease the noise on lines but with limited success. And whilst the benefits of using copper conducting wire were known, the technology was not available to make a copper wire strong enough particularly for overhead wires.
Fortunately, in 1877, Thomas Doolittle based in Connecticut, developed the process for hard drawn copper wire. He had soft, annealed copper wire drawn through a series of dies in order to increase its tensile strength. The hard drawn copper wire was strong enough for overhead wires, and copper conductors became the medium for carrying all fixed line telephone communications.
Improvements were continually being made to telephone cable, which included the use of more and finer conductors within a single sheathed cable, improved electrostatic performance, and a larger air to paper ratio providing insulation, noise elimination and waterproofing. Some of the cable advances were made possible because of the improvements made in other areas of telephony, such as with transmitters and receivers, coils and switchboards. And then there followed a major development with the loading of the telephone lines. Loading the telephone cables increased transmission efficiencies. In 1912 a new 1% antimony, 99% lead alloy was developed for cable sheaths. This material had good tensile strength, corrosion resistance, and the mechanical properties necessary for both aerial and underground cables. Whilst another major development was the invention of the repeater, which amplified voice signals. Carrier systems or multiplexing meanwhile enabled a single pair of wires to be used for multiple calls.
The end of the 19th and the beginning of the 20th centuries were a time of change and rapid expansion in the telecommunications industry. The telephone went from being a novel item that only a few could afford, to what is a necessity today. Today, most of the improvements in telephone cables have centered around sending more information at a faster rate over the same cable. And this inevitably leads us into the current revolution - in Fiber Optics.