Lead sheathing in the Bell System Technical Journal

After placing the spindles of coils in the various compartments, the case is filled with a moisture proofing compound. By means of a special design of case and cover joint, a double seal is provided to prevent entrance of moisture at this point.

The case design was such as to furnish complete protection of the coils against moisture penetration and adequate mechanical strength for taking up the tension in the cable.

The service record of these loaded submarine cables is excellent, thus demonstrating a satisfactory solution of the many difficult mechanical problems involved.

However, if it were insulated by the methods which have been used in the past in making deep-sea cables, it would lose much of its inductance on laying on account of the effect of the great pressures to which a cable is subjected.

The thickness of gutta-percha must be sufficient to insure the integrity of the insulation at all points.

Conductor joints in a cable are frequently subject to considerable stress and even with the relatively simple joints required for ordinary deep-sea cables trouble is occasionally experienced.

Heretofore submarine cables having waterproof insulation, with the exception of the Catalina Island cables which are insulated with a special rubber mixture, have almost invariably been insulated with gutta percha, or balata, or a mixture of these substances.

It is provided with copper return tapes and also with a thin copper tape under the return tapes for protection against marine organisms.

Corrosion does not appear to be a primary factor affecting the life of aerial cables; failure of these cables occurs usually from intergranular embrittlement and is minimized by the use of alloy sheathing.

It is shown that corrosion of cable sheathing in conduit occurs by means of the operation of small corrosion cells on the surface of the sheath or by the leakage of current from the sheath to ground.

Cable sheathing buried directly in soils is seriously corroded by differential aeration-cell action resulting from physical contact of relatively large soil particles and metal.

In general it is concluded that corrosion of cable sheathing is influenced more by the nature of the environment than by the chemical composition of the metallic material.

The incidence of corrosion of cable sheathing is small owing to the maintenance of non-corrosive chemical and electrical environments in the cable plant.

The property damage resulting from perforation of the sheathing by corrosion and the attending costly interruption of service have served to make the prevention of cable failure a matter of primary concern.

The relatively low incidence of actual corrosion failures can be attributed largely to the vigilance of the electrolysis engineers and the plant forces.

The general adoption of the present standard alloy of lead with 1 per cent antimony in that year has afforded substantial economies and a sheathing of high resistance to fatigue cracking.

Recently, a new development, lead hardened with 0.03–0.04 per cent calcium, has shown in laboratory tests some promise as a cable sheathing material.

There is no evidence that the copper content of this lead has any significant effect upon corrodibility when used in 1 per cent antimony sheathing, although it does appear to be a factor in certain other uses.

Indeed, chemical composition appears to be of lesser importance than environmental influences in the corrosion of cable sheathing.

The prevention of corrosion failures is mainly a matter of providing and maintaining non-corrosive chemical and electrical environments.

While the choice of lead as a cable sheathing material was dictated primarily by physical requirements, notably its adaptability to extrusion, corrosion resistance has been a large factor undoubtedly in its successful use.

It has long been recognized that lead is one of the least corrodible of metals.

Widely used in ancient times for water pipes, roofings, caskets, linings for public baths, etc., many specimens have come down to us in nearly perfect states of preservation.

The Romans, for example, employed lead water pipes in fifteen standard sizes usually ten feet in length and some of these pipes are said to be in use today.

In the form of roofings many examples exist which are five centuries old.

It seems likely that corrosion has been less destructive than war to the original lead roofs of medieval cathedrals and buildings.

Once a protective film has formed on lead the metal may be preserved indefinitely if not physically disturbed.

In the air this film is usually an oxide while in the case of underground burial the film which forms on lead may be a silicate or in some cases merely a film of hydrogen shielded by the presence of soil colloids.

Whether or not a protective film forms depends largely upon the character of the environment to which the metal is exposed.

Under unfavorable conditions, such as exposure to acetic acid vapors, strong alkalies or contact with large soil particles, lead may be readily corroded.

Purity of the metal plays a minor role in corrodibility in the atmosphere although it may affect its behavior in soil waters and other electrolytes.

Corrosion is not a primary factor in the life of aerial cables.

Failure of these cables is usually due to cracking and confined to sections which are subjected to repeated stresses or in some cases to prolonged mechanical vibration.

It is now recognized that the nature of the environment affects the endurance of metals to such stressing and vibration, and the term "corrosion-fatigue" has been applied to the embrittlement and cracking which result from the simultaneous application of tensile and compressive stresses and corrosive media.

The resistance of lead to corrosion-fatigue is lowered, for example, by exposure to the atmosphere.

Evidently the protective oxide coating which forms on lead in the air is not only ineffective in preventing intercrystalline fracture under repeated stressing, but actually constitutes an accelerating factor.

The specific volume of lead oxide is greater than that of the metal from which it is derived and it has been suggested that the presence of the oxide provides some sort of leverage which aids embrittlement.

Intercrystalline corrosion of lead may be produced in the laboratory merely by immersion of the specimen in a solution of nitric acid and lead acetate.

While intergranular corrosion of this type can be produced in lead of high purity, the rate of attack in a given medium is usually a function of the purity of the metal.

Exclusion of the atmosphere or the use of coatings of certain oils or grease have been shown to retard the rate at which lead is embrittled by corrosion-fatigue.

More practical means of minimizing the intergranular failure of cable sheathing lies in modification of the composition of the sheathing.

Alloying with 3 per cent tin, or 1 per cent antimony materially increases the resistance of lead to intercrystalline embrittlement.

The antimony alloy has a considerably greater fatigue resistance than pure lead as measured in a certain type of laboratory fatigue test but decreases in time, or when the alloy is cold worked, owing to an agglomeration of the dispersed antimony particles which occurs particularly in the region near the grain boundaries.

Lead alloyed with 0.04 per cent calcium and suitably age-hardened has been shown in laboratory tests to have a much higher resistance to fatigue failure than the 1 per cent antimony alloy.

Certain ternary alloys of lead containing cadmium are said to possess marked resistance to fatigue failure.

More recently lead containing 0.1 per cent tellurium has been shown to be about 3-fold more resistant than ordinary lead to mechanical vibration.

It should be emphasized that all of these comparisons of fatigue resistance were made in laboratory tests and are not based on field experience.

From the foregoing it will be seen that for the most part the aerial cable plant does not present a serious corrosion problem.

The importance of the unavoidable environmental influences on sheath embrittlement is minimized by the use of lead alloy sheathing together with proper methods of cable suspension.

Other types of lead corrosion are rare in aerial cables.

Recognition of the corrosion hazard involved in such construction is one of the considerations which led to the use of conduit for the housing of even the first cables which were placed underground.

The more recent actual experience of certain small users with soil corrosion has served to confirm the soundness of this practice.

The idea that cable sheathing might be buried safely in direct contact with soils was suggested by the fact that lead had been widely used as water pipes.

Many miles of telephone cables accordingly were laid directly in the earth, notably in Indiana, and frequently with unfortunate results.

In certain sections where it is considered economical to bury cables in the ground, a coating has been devised for the protection of the sheathing against corrosion.

This consists in wrapping the lead-alloy sheathed cable with asphalt-impregnated paper followed by one or more layers of jute impregnated with a preservative compound, and in some cases steel tape armoring over which there is wrapped a final layer of jute.

These points of contact of soil particles and metal become areas of reduced oxygen concentration as compared with surrounding regions of the metal surface which are more freely accessible to the soil atmosphere.

The resulting oxygen concentration cells with a driving force of approximately 100 millivolts provide one of the most important means by which metals corrode in soils.

Silt deposits which sometimes occur on cables in conduit do not give rise to differential aeration action probably because under such circumstances cathodic polarization of the corrosion cells is maintained.

These studies have shown that lead and the lead-antimony alloy are corroded by most soils, but at lower rates than are ferrous metals—losses in weight averaging but 10 per cent and depth of pitting about 25 per cent of those shown by the iron and steel specimens.

After approximately 10 years exposure in 18 soils the lead-antimony alloy was found in the majority of cases to be slightly but definitely more corroded than was commercial lead.

A smaller but more intensive soil corrosion test on lead and certain lead-alloys has been carried on by the Bell Telephone Laboratories in five typical soils in the general vicinities of Lafayette and Monon, Indiana.

The maximum depth of pitting results showed a close correspondence to the losses of weight.

The only specimens perforated by pitting were the lead-tin alloy and these only in the last mentioned soil.

Fig. 1—Comparison of the corrosiveness of five soils toward lead and certain lead-alloys.

It will be seen that of the two variables, soil character and alloy composition, the former is decidedly the more important in its effect upon rate of corrosion.

From a statistical analysis of the data obtained in this test it was concluded that variations in alloy composition within the scope of the test had no significant effect upon the corrosion behavior of the materials.

In other words, the variations observed may be ascribed to chance and there is no indication of a significant difference in the rates of corrosion of lead and lead-antimony alloys when buried directly in the earth.

It is impossible to exclude moisture and soil air or vapors from the conduit.

Surface waters may enter the cable compartments by way of the manholes and soil waters may seep through at duct joints or at small fissures which sometimes develop.

The soil atmosphere tends toward higher concentrations of carbon dioxide and lower oxygen than the outside air; it is often high in humidity resulting in the condensation of drops of moisture on the sheathing.

Muddy soil waters may deposit a layer of silt on the sheathing.

Free lime leached from concrete structures or caustic alkali produced, as indicated in the following paragraph, by the electrolysis of sodium chloride are the principal alkaline constituents.

Even leakage from sewers is sometimes a contaminating influence which induces corrosion.

There is evidence that under this condition the sheathing is less readily corroded by couple action of a miscellaneous character.

At appreciably higher negative potentials alkali or lime salts may be electrolyzed producing thereby caustic alkali or free lime which are corrosive to sheathing.

On the other hand, an electrically positive condition of the cable may be conducive to the ordinary "stray-current" or anodic corrosion.

The mechanism by which cable sheathing corrodes in conduit involves the replacement by the metal of hydrogen or another metal in compounds present in the surrounding environment—a process which has been described in some detail in a previous paper.

Most commonly, the dissolving lead replaces hydrogen from water.

The areas or points on the sheathing at which lead dissolves are the anodes of small corrosion cells, the cathodes of which are the regions at which hydrogen is deposited.

Corrosion cells owing their origin to sheath composition are exemplified by the presence of two metallic phases, one of which is lead and the other either an impurity, such as copper, bismuth, nickel, zinc, etc., or a hardening agent such as tin, calcium, cadmium, or antimony.

Copper and antimony, for example, are cathodic to lead under the prevailing conditions and facilitate the discharge of hydrogen.

The small proportion of cathodic area on the metal surface in both cases, however, will result in high cathodic current densities inducing polarization, and a low rate of corrosion except perhaps in acid solutions where the potential of the lead-hydrogen cell will be increased.

This acceleration in acid solution is borne out by laboratory corrosion tests which show that the rate of corrosion of lead containing 3 per cent tin is about 50 per cent greater and lead containing 1 per cent antimony is about 10 per cent greater than that of soft lead in dilute (0.001 molar) acetic acid.

Antimonial lead is said to corrode more rapidly than pure lead in humic acids.

Relatively larger areas of copper deposited upon the cable by replacement from copper salts have been observed to promote corrosion of the sheathing.

Scraps of copper wire corroded in manholes by saline waters are believed to have been the source of the copper compounds in such cases.

Wiping solder in contact with sheathing at the splicing sleeve provides still another example of a corrosion cell originating from the contact of diverse metals.

Similarly, the 3 per cent tin-lead sheathing in contact with 1 per cent antimony sheath would give rise to a galvanic couple in which the former would be anodic but by smaller values of potential than given above for the solder-sheath couple.

The few cases of corrosion of this type which have been observed, and which have been characterized by pitting of the solder and even of the sleeving (where this was 3 per cent tin) are believed to have arisen in electrolytes somewhat alkaline in nature which contained abnormally low concentrations of film-forming constituents such as silicates, sulfates or organic colloids.

In general, the influence of metallic composition upon corrodibility may be readily detected by measuring the rate of sulfation of the metallic material in sulfuric acid.

From an inspection of these results it appears that the surface reactivity of lead is markedly increased by the presence of impurities or by alloying with small amounts of other metals.

Of the hardening agents chosen for study, tin, tin and cadmium, and calcium exert the smallest influence on rate of sulfation, while antimony, whether used alone or with cadmium, has the most pronounced effect.

Since the environment in which cables are used contains both corrosive and film-forming substances this comparison of rates of sulfation of lead and its alloys is not necessarily a direct indication of the relative rates of corrosion of these materials when used as cable sheathing.

In the case, however, of lead and most lead-alloys suitable for cable sheathing, self-annealing occurs at ordinary atmospheric temperatures, and for this reason it is highly improbable that corrosion is ever initiated as a result of physical condition of the metal.

It is conceivable that scratching or mechanical injury of cable sheathing, such as might occur during installations, could give rise to the familiar metal-metal oxide corrosion cell.

It is of significance that although corrosion readily occurred in this experiment, there was no attack if the scratch were exposed to the atmosphere for two hours before submersion of the specimen.

In other words, the oxide film on lead is readily self-healing and injury to it is unlikely to cause corrosion.

Turning to a consideration of corrosion cells originating from the exposure of the sheathing to an inhomogeneous environment, reference has already been made in discussing soil corrosion to the nature and the importance of oxygen concentration cells, and to the protection which the conduit affords against this hazard.

In the laboratory, lead can be pitted by contact with a glass rod when submerged in a dilute sodium chloride solution.

There are a few instances in which cables in conduit appear to have corroded by means of oxygen concentration cells.

For example, there is evidence that deep pits in sheathing produced by the leakage of stray currents to earth have continued to deepen to the point of perforation of the sheathing, after removal of positive potential conditions.

The bottoms of such pits are less accessible to oxygen and appear in some cases to function as the anodic elements in differential aeration cells.

In this case it seems likely that the naturally protective oxide film on the sheathing was destroyed by the acid following which this region, owing to the exclusion or partial exclusion of oxygen, became anodic to the adjacent areas which were freely accessible to air.

Such a cable may suffer severe corrosion in the region of the dip as a result of the lower oxygen content of the atmosphere in this duct as compared to that prevailing in the other ducts.

An examination of the Rhineland cable which connects Berlin and Cologne has shown that the most extensive corrosion occurred at points where there was an abrupt change in the character of the soil or geological structure.

There is usually sufficient diffusion and circulation of underground waters to equalize ionic concentrations and prevent the development of cells of this type assuming serious proportions.

For example, it has been shown that it is often the ratio of the concentrations of film-forming to corroding substances which determines the character of attack. For high values of this ratio, the metal will be protected; for low values it will be uniformly corroded, but for intermediate values of this ratio, the surface will be only partly protected with the result that corrosion will be localized in the form of destructive pitting.

Of the corroding elements, oxygen is the most important in its effect upon the operation of corrosion cells.

Owing to the high potential required to discharge hydrogen on pure lead (i.e., its high hydrogen over-voltage) these cells tend to cease functioning owing to cathode polarization.

Oxygen, it is obvious, aids corrosion by depolarizing cathodic areas on the surface of the sheath.

That the effect of oxygen is proportional to its partial pressure in the atmosphere has been found in a laboratory study, the results of which are given in Fig. 2.

Fig. 2—Effect of oxygen on corrosion of lead submerged in distilled water.

A number of cases of cable sheath corrosion have been attributed to the presence of nitrates in the duct electrolyte.

Concentrations of nitrate from 20 to 425 parts per million have been found in the electrolyte from locations where failure of the sheathing occurred.

It has been shown that solutions containing a thousand parts of nitrates per million are markedly corrosive to lead.

Often the corroded region of the sheath is black in appearance owing to the presence of loosely adherent, finely divided lead (possibly oxidized) and antimony which accumulates by some sort of undermining action during the rapid attack.

In addition, the high solubility of lead nitrate prevents an appreciable polarization of anodic areas.

Cable sheathing is little affected by the chloride content of most ground waters.

In tests, for example, in which chlorides were added to natural waters, there was no increase in corrosion when the chloride content was of less than 1000 parts per million, a value seldom attained in soil waters.

Even infiltration of sea water does not constitute a corrosion hazard unless the cable is markedly negative to earth.

Indeed, in sea water the corrosion of lead may be retarded by an encrustation of lead chloride which forms on the surface of the metal, as well as by the lower prevailing oxygen content.

Extruded bars of lead and lead containing 1.6 per cent antimony, 60 cm. in length and 2.87 cm. in diameter exposed for four years to tidal action have shown losses in weight of 0.65 per cent and 0.51 per cent, respectively, but doubtless mechanical erosion was an important factor in this rather drastic test.

Laboratory tests made on lead foil in a sodium chloride solution show that the rate of corrosion increases with increasing salt concentration up to a maximum at 1 per cent and that at concentrations of 3 per cent, which corresponds roughly to that of sea water, the rate is markedly less.

The favorable experience with cables submerged in sea water or mixtures of sea water and soil waters indicates that corrosion inhibitive agents exert under these exposures a predominating influence.

The effect of the chloride content of the duct electrolyte must be mainly one of increasing the conductivity, there being insufficient concentrations in relation to the concentration of film-forming substances to produce even pitting or local attack.

A type of cable sheath corrosion of considerable importance is that which is fostered by alkalies.

It is characterized usually by the formation in the region of attack of deep red crystals of lead monoxide or litharge.

The appearance of this red oxide on cable sheathing is a certain indicator of alkali attack.

If detected before failure of the cable, the action can be stopped usually by removing the source of the alkali and thoroughly flushing the cable conduit with water.

At the Panama Canal water seeping through the concrete floors and walls of lock chambers caused serious corrosion of the sheathing of power cables in a short time.

In the same locality a telephone cable in vitrified clay conduit was corroded by the seepage of water through cement sacks used for wrapping the conduit joints.

The use of high alumina cement, which is much less corrosive to cable sheathing, has been proposed.

Another important source of alkali is the electrolysis of sodium chloride or common salt by electrical currents flowing to the cables.

Under these conditions caustic soda is produced at the sheath which is cathodic or negative to earth; hence the terms “cathodic” or “negative” corrosion.

Finally it should be mentioned that the use of calcium chloride on streets for melting snow or laying dust would lead undoubtedly to its coming in contact with the underground cable plant and being converted into corrosive free lime in areas negative to earth.

It has been known since the Middle Ages that lead is corroded by acetic acid.

In the presence of the carbon dioxide of the atmosphere, the corrosion product is the pigment, white lead.

The attack manifests itself by the formation of a white encasement around the globules of moisture on the sheath; at first a mottled effect is produced which in time develops into a heavy white encrustation of the carbonate or basic carbonate of lead.

The early use of wood conduit was attended with occasional cases of acetic acid corrosion and it was found that the wood tar creosote used as the preservative contained this acid.

Properly creosoted this product has not been known in Bell System experience to cause corrosion except when used under such unusual circumstances as close proximity with steam pipes or exposed on viaducts over railroad yards to the heat of locomotive stacks.

The most serious corrosion of cables by acetic acid on record is that which occurred on the Pacific Coast in creosoted Douglas fir conduit a few years ago.

With the expansion of the cable plant into this newly constructed duct system several cases of acetic acid corrosion occurred—most of them within the first 15 months in conduit of recent installation.

The corrosive action was effectively stopped by neutralizing the acid with ammonia gas supplied to the affected conduit in a 2 per cent mixture with air.

The corrosiveness of air laden with acetic acid vapors lies in the persistence of effective non-polarized corrosion cells of constant voltage.

Phenols and other acidic constituents of coal tar pitches have been reported to be corrosive to cable sheathing when in direct contact in the form of protective coatings.

It is well known that lead is markedly corroded in distilled water, and by waters low in hardness and in total solids.

Saturation of distilled water with calcium silicate (soluble to the extent of about 100 parts per million), or with silicic acid derived from a suspension of silica flour, will prevent corrosion of lead.

The corrosiveness of certain natural waters has been greatly reduced by the addition of only 10 parts of sodium silicate (expressed as silicic acid) per million.

The resistance of underground cables to corrosion appears to depend chiefly upon the film-forming action of silicates.

The minimum concentrations required to give protection depend upon the nature and concentrations of the corroding agents which are also present.

The effectiveness of silicates in passivating lead lies in the extremely low solubility of lead silicate.

Consequently silicate ions are precipitated as lead silicate in close contact with the sheath at the anodic areas of the corrosion cells.

As the more anodic regions become polarized in this fashion other areas tend to function as anodes but with the same result until the surface of the sheath becomes entirely covered with an insoluble coating of lead silicate which is impervious to the corrosive elements of the environment.

Chromates and phosphates stand next to silicates in ability to passivate lead, but do not occur in the electrolytes in contact with underground cables.

Sulfates, however, are a common constituent of these environments and in laboratory studies have been shown to be as effective as phosphates.

The passivating effect of sulfates is directly proportional to concentration, 2500 parts per million reducing the rate of corrosion of distilled water about 50 per cent.

Carbonates exert a marked retarding influence on the corrosion of lead.

The water which comes in contact with underground cables always contains carbonate ions derived either from soluble carbonates from the soil or from carbon dioxide of the soil atmosphere.

Pressures of carbon dioxide within this range reduce the rate of corrosion of lead in distilled water about 50 per cent.

It is claimed that high pressures of carbon dioxide, e.g., 6 atmospheres, increases the solvent action of water on lead.

Carbonate equilibria calculations of the system lead carbonate-carbon dioxide-water show that the film of corrosion products which forms on lead in aerated distilled water is a hydrated oxide of lead when the partial pressure of carbon dioxide is less than 10-14 atmospheres.

This means that the effectiveness of soil carbonates in passivating cable sheathing is somewhat reduced by the higher carbon dioxide pressures which obtain underground.

Mention has already been made of the protective influence under certain circumstances of silt or clay deposits on the surface of the sheath.

There are in the underground electrolyte many other substances mainly organic in nature and often colloidal which aid in the preservation of cable sheathing.

The most common kind of cable sheath corrosion, the most destructive and best recognized, is that which occurs when electrical currents flow from the sheath to ground.

In appearance the corroded area may be a clean cut pit or pits, or it may be roughly etched.

When the potential is greater than about 2 volts, a brown colored anodic oxidation product, lead peroxide, may be formed.

Occasionally lead chloride, a white salt, may be formed in the corroded areas under anodic conditions.

The corrosion efficiency of stray current anodic corrosion, i.e., the per cent of the current involved in dissolving lead, will often be appreciably less than 100 per cent.

Cathodic or negative corrosion of cable sheathing, which occurs when current flows from earth to the sheath, has been described already under the discussion of alkaline corrosion.

In this case the sheathing is generally not corroded.

Instances have been observed in which cable sheathing appeared to have corroded from the inside surface. It is believed that the action in these cases was preceded by the occurrence of cracks or fissures in the sheath which admitted moisture and provided electrolytic paths by means of which current flowed from the sheathing to the copper conductors within.

Destruction of cable sheathing by stray electrical currents derived from large-scale galvanic cells has been experienced.

In summarization it may be stated that corrosion is not a primary factor affecting the life of aerial cables.

The tendency of lead to crack as a result of repeated stressing has been minimized by alloying with one per cent of antimony, and aerial cables sheathed with this alloy have shown satisfactory resistance to embrittlement of this character.

When cable sheathing materials are buried in direct contact with soils, serious corrosion develops as a result of differential aeration cell action, and appears to have little or no relation to chemical composition of the metallic material.

In addition to corrosion cells which originate in some inhomogeneity of the environment, such as the partial exclusion of air, corrosion of cable sheathing may occur by means of galvanic cells arising from the presence of metallic impurities or contact with a more noble metal such as copper.

Such constituents as oxygen, nitrates, alkalies, organic acids and chlorides (in low concentrations) facilitate the operation of these cells, thereby increasing the rate of corrosion, whereas silicates, sulfates, carbonates, colloids and certain organic compounds of the soil waters exert a protective action which may retard or prevent corrosion.

From the description of the occurrence and general characteristics of cable sheath corrosion in the present paper it may be concluded that although there are many conditions under which cables may corrode, the actual incidence of corrosion is small owing to the maintenance of non-corrosive chemical and electrical environments in the cable plant.

The discovery of permalloy, a material with very high permeability at low flux densities, together with the invention of means for protecting the loading material from the severe stresses that it would otherwise encounter at the ocean bottom, made it possible to build a cable with many times the band width of corresponding non-loaded cables.

Measurements of impedance both from the ship and from the shore showed the cable to be quite unimpaired both at 2½ miles depth and after recovery and relaying in shallow water.

To exclude water, there is placed over this cylinder an annealed copper tube with water-tight seals at its ends.

It combines a strictly hermetic seal, in which the conductors are brought out through glass, with a plastic seal through which diffusion of water vapor would be extremely slow, should the glass seal fail.

They have also been subjected to repeated bending around a six-foot drum without failure.

Tests of cable-core and joints over a long period of time have shown no observable change under this impressed voltage.

Also, once the cable is laid, it is in a very quiet place, and except in shallow water near shore is not likely to be disturbed.

Experiences of over eighty years since the failure of the first attempt at an Atlantic cable have led to the development of practices which give good assurance of the reliability of cables of simple construction, but when a device such as the proposed repeater is made a part of the structure a new set of hazards is introduced.

The decision to place the repeater on the ocean bottom resulted in special requirements on the structure the first of which is that it be capable of resisting the considerable hydrostatic pressure that is encountered in deep water.

The requirements on flexibility and water-tightness under ocean bottom pressures were the factors of outstanding influence in the design of the repeater housing and of the end seals by means of which the cable enters and leaves the housing.

The repeater housing and core tube are provided with corrosion protective layers and a bedding for the armor wires, the bedding over the core tube being built up in the form of a taper.

The rubber-brass seal, for instance, was tested for penetration of moisture vapor over long periods of time.

Likewise, comprehensive electrical tests had been made to insure that no unexpected effects would be encountered due to the immersion of the repeater in water.

Over the return conductor is wound a teredo tape approximately .003 inch thick with overlap.

Experience with existing cables which land in Havana Harbor indicate that considerable deterioration of armor takes place in this locality and there is also the anchor menace.