Editing Loading coil (section)

==Submarine cables==
Signal distortion is a particular problem for [[submarine communication cable]]s, partly because their great length allows more distortion to build up, but also because they are more susceptible to distortion than open wires on poles due to the characteristics of the insulating material. Different wavelengths of the signal travel at different velocities in the material causing [[Dispersion (optics)|dispersion]]. It was this problem on the first [[transatlantic telegraph cable]] that motivated Heaviside to study the problem and find the solution.<ref>Griffiths, p. 237</ref> Loading coils solve the dispersion problem, and the first use of them on a submarine cable was in 1906 by [[Siemens and Halske]] in a cable across [[Lake Constance]].<ref>Newell, p. 478</ref>

There are a number of difficulties using loading coils with heavy submarine cables. The bulge of the loading coils could not easily pass through the cable laying apparatus of [[cable ship]]s and the ship had to slow down during the laying of a loading coil.<ref name="Newell, p. 479">Newell, p. 479</ref> Discontinuities where the coils were installed caused stresses in the cable during laying. Without great care, the cable might part and would be difficult to repair. A further problem was that the material science of the time had difficulties sealing the joint between coil and cable against ingress of seawater. When this occurred the cable was ruined.<ref>Britannica, 1911</ref> Continuous loading was developed to overcome these problems, which also has the benefit of not having a cutoff frequency.<ref name="Newell, p. 479"/>

===Krarup cable===
A Danish engineer, [[Carl Emil Krarup]], invented a form of continuously loaded cable which solved the problems of discrete loading coils. Krarup cable has iron wires continuously wound around the central copper conductor with adjacent turns in contact with each other. This cable was the first use of continuous loading on any telecommunication cable.<ref>Kragh, p. 129</ref> In 1902, Krarup both wrote his paper on this subject and saw the installation of the first cable between [[Helsingør]] (Denmark) and [[Helsingborg]] (Sweden).<ref>Huurderman, pp. 321-322</ref>

===Permalloy cable===
[[File:Permalloy loaded cable construction.svg|thumb|right|300px|Permalloy cable construction]]
Even though the Krarup cable added inductance to the line, this was insufficient to meet the Heaviside condition. AT&T searched for a better material with higher [[permeability (electromagnetism)|magnetic permeability]]. In 1914, Gustav Elmen discovered [[permalloy]], a magnetic nickel-iron annealed alloy. In c. 1915, [[Oliver E. Buckley]], [[Harold D. Arnold|H. D. Arnold]], and Elmen, all at [[Bell Labs]], greatly improved transmission speeds by suggesting a method of constructing [[submarine communications cable]] using permalloy tape wrapped around the copper conductors.<ref name=Huurd314>Huurdeman, p.314</ref>

The cable was tested in a trial in Bermuda in 1923. The first permalloy cable placed in service connected New York City and [[Horta (Azores)]] in September 1924.<ref name=Huurd314/> Permalloy cable enabled signalling speed on submarine telegraph cables to be increased to 400 words/min at a time when 40 words/min was considered good.<ref>Huurdeman, p. 308</ref><!-- Huurdeman has apparently read 2000 characters per minute as 2000 words per minute --> The first transatlantic cable achieved only two words/min.<ref>May, pp. 947, 950</ref>

===Mu-metal cable===
[[File:Mu-metal_cable.svg|thumb|left|300px|Mu-metal cable construction]]
[[Mu-metal]] has similar magnetic properties to permalloy but the addition of copper to the alloy increases the ductility and allows the metal to be drawn into wire. Mu-metal cable is easier to construct than permalloy cable, the mu-metal being wound around the core copper conductor in much the same way as the iron wire in Krarup cable. A further advantage with mu-metal cable is that the construction lends itself to a variable loading profile whereby the loading is tapered towards the ends.

Mu-metal was invented in 1923 by the [[Telegraph Construction and Maintenance Company]], London,<ref>Smith, WS, Garnett, HJ, ''New and improved magnetic alloys, and their application in the manufacture of telegraphic and telephonic cables'', Patent GB224972, filed 25 Aug 1923, issued 25 November 1925. patented in the US as US1582353 and US1552769</ref> who made the cable, initially, for the [[Western Union|Western Union Telegraph Co]]. Western Union were in competition with AT&T and the [[Western Electric Company]] who were using permalloy. The patent for permalloy was held by Western Electric which prevented Western Union from using it.<ref name="Green">Green</ref>
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===Patch loading===
Continuous loading of cables is expensive and hence is only done when absolutely necessary. Lumped loading with coils is cheaper but has the disadvantages of difficult seals and a definite cutoff frequency. A compromise scheme is '''patch loading''' whereby the cable is continuously loaded in repeated sections. The intervening sections are left unloaded.<ref>Bakshi & Bakshi, p. 1.55</ref>

===Current practice===
Loaded cable is no longer a useful technology for submarine communication cables, having first been superseded by [[coaxial cable|co-axial cable]] using electrically powered in-line [[repeater]]s and then by [[optical fiber|fibre-optic cable]]. Manufacture of loaded cable declined in the 1930s and was then superseded by other technologies post-[[World War II]].<ref name="Green"/> Loading coils can still be found in some telephone landlines today but new installations use more modern technology.