History of the Atlantic Cable & Undersea Communications
Telegraph Construction & Maintenance Co. Ltd.
Cable Sample Case
Photographs of this Telegraph Construction & Maintenance Co. Ltd. sample case were kindly provided by site visitor John Fleetwood.
Unlike most cable sample cases, this one by the Telegraph Construction & Maintenance Co. Ltd. (Telcon), was not made for a specific cable. Designed to be wall mounted, the case shows the range of cables available from Telcon and the components that went into them. It was probably put together for a trade show or exposition in the late 19th century, as the brass tape wrapped around the core did not come into use until 1879 (see below).
The steel wires at the left show the range of diameters of armouring wire used, measured in BWG (Birmingham Wire Gauge), the #14 wire at the top missing.
On the right are four different constructions of “core” (the insulated copper conductor assembly), showing the types and weights available. The copper conductor construction varies from the standard seven-strand (six around one) type at the top to the later ribbon-wrapped conductors below. The numbers alongside each core section refer to the weight in pounds per mile of copper and gutta percha respectively; the samples shown are 130/130, 450/270, 650/375, and 700/360. The more copper (the first number) the better the transmission characteristic, but the higher the cost.
The finished cable samples at the bottom show the cable types available for different applications, and were probably taken from a variety of recently manufactured cables. The letters indicate the type of cable, and the numbers give the number and gauge of the armouring wires.
Types A2 (18/1 and 17/6) and A (10/1) are shore end, heavily armoured against abrasion and ship's anchors and trawls. Of the two conductors in sample A2, one carries the signal, and the other is taken out through the armouring a mile or so out to sea and soldered to the armouring. This forms a solid ground for the equipment at the cable station which allows duplex transmission over the cable (i.e. signalling in both directions at the same time). See the Sea Earths page for further information.
Type O (10/0) is a heavily single-armoured shore end, an uncommon designation.
Type B (two variations, 10/6 and 12/6) is heavy intermediate cable, used next in line from the shore end where there is less chance of damage but heavier armouring is still needed.
Type C (not in this set) would be light intermediate cable, and Types D (19/14) and D1 (18/11) are deep sea, the main run of the cable, where the armouring is more for protection and strength while laying than for resistance to abuse.
The reason for the variations among the same types is that the construction of each cable was tailored for its application. Cable was almost always made to order, and only after a detailed survey of the conditions at the landing points and along the sea bed for the entire route. The nature of the terrain, temperature of the water, and geographical location would dictate the use of different types of cable for optimum results.
Normally the cable would be made and laid under contract by the cable company, and they would guarantee its performance for thirty days after laying; the final payment would not be made until this time. So it was in the company's best interest to make sure the cable was fit for its purpose (as well as ensuring future business).
The core shown above, one of the samples from the case, illustrates the typical construction. A multi-strand copper conductor is used for redundancy—in case of stress on the cable breaking one of the conductors, the continuity would not be lost.
Surrounding the conductor is gutta percha insulation, and around that a protective layer of jute. Next is a thin brass tape, spiral wound to completely enclose the gutta percha. This is known as "teredo tape", which prevents a species of marine boring insect from tunneling through the gutta percha and destroying the insulation of the cable. This was introduced in the late 1870s by Telcon's Chief Engineer, Henry Clifford.
Finally, another layer of jute, this one tarred, helps cushion the core assembly as it is being armoured.
This end view of the Type A2 shore end cable shows further details of the construction. The completed core, as described above, can be seen in the center, with the segmented copper conductor, gutta percha, and teredo tape clearly visible.
In this cable two conductors are used, one being the signal path, and the other a sea earth. Note that the signal conductor is of identical size and construction in all the types of cable supplied for a given route—the variations among the different types are solely in the number and weight of the armouring wires.
Surrounding the cores are further layers of insulating and packing material, including Chatterton's compound and more gutta percha. One or more layers of armouring wire complete the cable, and the assembly is wrapped in tarred jute or hessian to protect the wires during storage and laying, and to make the cable easier to handle.
For an example of the many types of armouring used on a single project, see this page on the 1923 Commercial Cable Company's Atlantic cable, made and laid by Telcon.
Last revised: 24 May, 2015