Introduction: In 1950, Submarine Cables Ltd published a 20-page booklet titled Submarine Telegraph Cable Centenary, commemorating the 100th anniversary of the laying of the first submarine cable from Dover to Calais in 1850. As well as a brief history of significant events in the industry, the booklet included a table of notable technical achievements in cable design.

Ten years later, in 1960, the company produced another booklet, titled One Hundred and Ten Years of Undersea Telecommunications. This updated the technical section of the first publication and added the advances of the 1950s, the most significant one being the introduction of repeaters, which made feasible long-distance undersea telephone cables across the Atlantic and elsewhere.

The introduction from the 1950 booklet and the two sets of technical data are reproduced below, providing an interesting insight into the design changes which improved the performance and longevity of submarine cables from 1850 to 1959.

SALIENT FEATURES IN CABLE DESIGN SINCE 1850

The following information illustrates the various milestones which have marked the development of Submarine Telegraph and Telephone Cables. In order to appreciate the symbols used, the following explanation is made:‑

In early days, most of the progress made in cable design centred round the type and weight of armouring. Iron wires, drawn to one of the standard sizes then available, were used, and the cable armouring was designated by two numbers separated by a stroke (e.g., 12/4). These indicated respectively the number of wires wrapped simultaneously round the cable and the gauge number of the wire. Where double armouring was used, this was expressed by two such symbols (e.g., 14/1, 10/6). Heavy armouring (not necessarily double armouring) is always used for cables near the shore where rocks and tides are encountered, but light armour is used for deep sea work. Armour of intermediate weight is used for " intermediate " conditions of depth. In due course heavily armoured cable (i.e., carrying No. 2 wires or larger) became known as Type A. Intermediate armour (i.e., Nos. 4 or 6 wires) was known as Type B and light armour (i.e., wires of No. 11 to No. 14 gauge) was known as Type C and finally as Type D. A hundred years ago, the steel industry could not give cable manufacturers armour wire of high tensile qualities, but in present times deep sea cable is always armoured with wires having a tensile strength of from 65 to 95 tons per square inch, while low tensile (soft iron) wire of large section having a tensile strength of 35 tons per square inch is used for shallow water cables.

In due course, some cables were sheathed with armour wires each of which was protected with a fabric tape wrapping impregnated with bituminous compound. This was to minimise corrosion of the wires, and such armouring was referred to as "Taped Wires". Helical wrappings of textile material were called servings.

When describing the size of the conductor and insulation, it is now customary to quote two numbers separated by a stroke (e.g., Core 130/130), the two numbers denoting the weights of the copper conductor and the insulation respectively, in lbs. per nautical mile (2,029 yards).

It will be noted that, in recent years, the main developments have been in transmission characteristics rather than in cable armouring.